UNIT 3: Evolution and Diversity Topic 16 How Populations Evolve
CEB Textbook Chapter 13, pages 242-265 Mastering Biology, Chapter 13
Learning Outcomes After studying this topic you should be able to: • Define and describe the process of natural selection, and explain how this process can lead to evolutionary adaptation. •Compare the ideas of Lamarck, Wallace, and Darwin on the ability of species to change. •Explain how each of the following provides evidence that evolution occurs: the fossil record, biogeography, comparative anatomy, comparative embryology, and molecular biology.
Evolution Videos
Darwin and Natural Selection
Homework • Watch Darwin Videos • Draw a table with the definitions of the following terms: natural selection, evolutionary adaptation and evolution. • Unit Assessment 3: Topic 16 • Mastering Biology Activities: Reconstructing Forelimbs • Evolution Assignment Mastering Biology
• The Genius of Charles Darwin (Pt 1, 2 and 3) – • (Note: Presenter Richard Dawkins is an Atheist..... • BUT is it impossible for someone to agree with the theory of evolution AND be religious? What’s your opinion?) • http://www.youtube.com/watch?v=ptV9sNezEvk • http://www.youtube.com/watch?v=shkWhBVfe3o • http://www.youtube.com/watch?v=cARUZyBJtdY • What Darwin Never Knew (NOVA) • http://www.youtube.com/watch?v=AYBRbCLI4zU
2004: NEW EVIDENCE FOR GLOBAL WARMING Are rising CO2 levels threatening global warming? Most scientists agree that this happens because CO2 traps radiation in the atmosphere. New data gives more support to this explanation. Ice samples have been taken from the Antarctic up to 3 km deep. Air bubbles in the ice have been tested for their CO2 levels. Levels now are the highest recorded. 2003: NEW THEORY FOR START OF LIFE ON EARTH Think life on Earth came from Mars? So do some scientists. But now two of them have come up with a different explanation. They say that evidence beneath the seas can explain how life started on Earth.
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1859: DARWIN BOOK CAUSES ANGRY DEBATE Members of the clergy and scientists are outraged by a new book published today. In On the Origin of Species Charles Darwin explains how he thinks life has developed on Earth. One of his most outrageous claims is that men are descended from apes!
These cartoons were produced in the press and show the strength of feeling about Darwin’s ideas of evolution through natural selection
Theories for change
Dates of theories: • Lamarck (1809) • Cuvier (1825) • Darwin (1844, but not published until 1859)
In the early 19th century: 1. The Church taught that the Bible was true word for word. The wife of the Bishop of Worcester said of Darwin’s ideas: ‘My dear, descended from the apes! Let us hope it is not true, but if it is, let us pray that it will not become generally known.’
2. Almost everyone believed that Earth and all living things had been created in 4004 BC. 3. Scientists had collected lots of evidence of variation in animals and plants. 4. Many people accepted that fossils were the remains of organisms from the past. 5. Scientists saw that different layers of rocks contained different sets of fossils. 6. A few people thought fossils showed that some living things died out and were then replaced by others. 7. Small changes in living things had been observed.
Early Contributions to Evolutionary Thought Jean Léopold Nicolas Frédéric Cuvier (1769 – 1832) French naturalist and zoologist. Cuvier A major figure in natural sciences research in the early 19th century, and was instrumental in establishing the fields of comparative anatomy and paleontology through his work in comparing living animals with fossils.
Early Contributions to Evolutionary Thought Contributors to the development of Darwin’s ideas were: Jean Baptiste de Lamarck (1744-1829) Believed that organisms could pass on traits acquired during their lifetime. Discredited: when the mechanisms of heredity became known. Important: because he was the first to propose that change over time was the result of natural phenomena and not divine intervention.
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Early Contributions to Evolutionary Thought Thomas Malthus (1766-1834) Believed that populations increased in size until checked by the environment, called the ‘struggle for existence’. Charles Lyell (1797-1875) Developed the geological theory of uniformitarianism: the physical features of the earth were the result of slow geological processes that still occur today. Herbert Spenser (1820-1903) Introduced the concept of ‘Survival of the Fittest’.
Explanations for change Person
Explanation they came up with for the data
Lamarck
Evolution – organisms developed new features as a result of an ‘inner urge’ for improvement and they passed the improvements on to their young
Cuvier
Catastrophism – organisms were wiped out by a series of catastrophies. Then God created new, improved versions
Darwin
Evolution by natural selection. All of these theories involved creative thought
Charles Lyell
Herbert Spenser
Explanations for change 2. All the explanations caused arguments. a Round 1: Lamarck vs Cuvier Cuvier won this round. Lamarck’s idea was unpopular. Suggest some reasons why.
…………………………………………………………………………………………… • Cuvier criticized Lamarck’s theory. …………………………………………………………………………………………… • Cuvier was a more influential scientist. • The idea of an ‘inner urge’ was not enough to explain …………………………………………………………………………………………… the appearance or disappearance of characteristics. • Lamarck could not explain how features were passed on. • Evolution went against what was written in the Bible, so Catastrophism was more acceptable at the time. • The accepted time-scale was too short for evolution.
Lamarck Vs Darwin Lamarck proposed that organisms could gradually bring about changes in themselves to suit the environment and, that these changes could be passed on to their offspring.
Creative thought was needed to come up with the explanation? ( or )
Explanations for change
b Round 2: Cuvier vs Darwin
This time many, but not all, important scientists favoured Darwin. Other scientists and some clergymen preferred the explanations of the Bible. Suggest some challenges that people made to each explanation. • Darwin gathered lots of evidence in support of his idea and it did Cuvier: …………………………………………………………………………………
not support Cuvier’s idea. Geologists challenged the idea that there
was no connection between the fossils in successive layers of rock …………………………………………………………………………………
• Darwin had no explanation of how features were passed on. Darwin: ………………………………………………………………………………… • Evolution went against what the Bible said.
• In drawing together the ideas, emphasize that: …………………………………………………………………………………
• different theories can be suggested to explain the same data • the theory that becomes generally accepted at any particular time is the one that: • best fits the data • is not successfully challenged at the time • explains new data
History of Evolutionary Thought Hebert Spencer 1820 - 1903 Proposed concept of the ‘survival of the fittest’
Erasmus Darwin 1731 - 1802 Charles Darwin's grandfather and probably an important influence in developing his thoughts on evolution.
John Baptiste de Lamarck 1744 - 1829 First to publish a reasoned theory of evolution. Proposed idea of use and disuse and inheritance of acquired characteristics.
Rev. Thomas Malthus 1766 - 1834 Wrote: ‘An Essay on the Principles of Population’, attempting to justify the squalid conditions of the poor.
Charles Lyell 1797 - 1875 Major influence on Darwin. Lyell’s work ‘Principles of Geology’ proposed that the earth is very old.
What examples are there that disprove this theory? Alfred Russel Wallace 1823 - 1913 ‘Theory of Natural Selection’
Charles Darwin 1809 - 1882 ‘Theory of Evolution by Natural Selection’
August Weismann 1834 - 1914 Proposed chromosomes as the basis of heredity, demolishing the theory that acquired characteristics could be inherited.
R.A. Fisher 1890-1962 J.B.S. Haldane Sewall Wright 1889-1988
Julian Huxley 1887-1975 Ernst Mayr 1904-2005 T. Dobzhansky
The New Synthesis 1898-1964
Founding of population genetics and mathematical aspects of evolution and genetics.
Gregor Mendel 1822 - 1884 Developed the fundamentals of the genetic basis of inheritance.
Neo-Darwinism: The version of Darwin’s theory refined and developed in the light of modern biological knowledge (especially genetics) in the mid-20th century
1900-1975
Collaborated to formulate the modern theory of evolution, incorporating developments in genetics, paleontology and other branches of biology.
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The Development of Darwin’s Ideas
The Modern Theory of Evolution The modern theory of evolution combines the following ideas: Darwin’s theory of the origin of species by natural selection. with an understanding of genetics (from Mendel). and the chromosomal basis of heredity (from Weismann).
+ Darwin
The first convincing case for evolution, The Origin of Species, was published by Charles Darwin in 1859. In this book, Darwin argued that new species developed from ancestral ones by natural selection. Darwin developed his theory of “survival of the fittest” by building on earlier ideas and supporting his views with a large body of evidence he collected while voyaging extensively on the ship the ‘HMS Beagle’.
+ Mendel
Alfred Russel Wallace, a young specimen collector working in the East Indies, developed a theory of natural selection independently of Darwin. However, Darwin supported the theory more extensively and receives most of the credit for it.
Weismann
Figure 13.4
The Development of Darwin’s Ideas Darwin’s theory was supported by data collected from: The flora and fauna of South America. These showed different adaptations for diverse environments but were distinct from the European forms. Observations of the fauna of the Galapagos Islands confirming his already formulated ideas from earlier in the trip. He found that most of the Galapagos species are endemic, but resembled species on the South American mainland.
HMS Beagle
Darwin in 1840 North America
Great Britain
Europe
Asia
ATLANTIC OCEAN Africa
Galápagos Islands
PACIFIC OCEAN
Pinta
Marchena
South America
Genovesa Equator
Santiago
Isabela
0 0
Equator
Daphne Islands Pinzón
Fernandina
40 km
Santa Cruz Santa Fe Florenza
San Cristobal
Australia PACIFIC OCEAN
Cape of Good Hope Cape Horn
Española
Tasmania
40 miles
Tierra del Fuego
New Zealand
Fossil finds of extinct species. Evidence from artificial selection.
Figure 13.12
(a) The large ground finch
(b) The warbler finch
(c) The woodpecker finch
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The Concepts of Darwinism Darwin’s view of life was of ‘descent with modification’: descendants of ancestral forms adapted to different environments over a long period of time. The mechanism for adaptation is called ‘natural selection’, and is based on a number of principles:
The Concepts of Darwinism Overproduction: Species produce more young than will survive to reproductive age (they die before they have offspring). Variation: Individuals vary from one another in many characteristics (even siblings differ). Some variations are better suited then others to the conditions of the time. Competition: There is competition among the offspring for resources (food, habitat etc.). Survival of the fittest phenotype: The individuals with the most favorable combinations of characteristics will be most likely to survive and pass their genes on to the next generation. Favorable combinations increase: Each new generation will contain more offspring from individuals with favorable characters than those with unfavorable ones.
Overproduction Variation Competition Survival of the fittest phenotype Favorable combinations increase
Natural Selection
Natural selection
Overproduction
Variation
Populations produce too many young: many must die
Individuals show variation: some variationsare more favorable than others
Natural Selection Natural selection favors the best suited at the time
Inheritance Variations are inherited. The best suited variants leave more offspring.
Figure 13.15-1
The evolution of superbugs?
Insecticide application
Chromosome with gene conferring resistance to pesticide
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Figure 13.15-2
Figure 13.15-3
Insecticide application
Insecticide application
Chromosome with gene conferring resistance to pesticide
Chromosome with gene conferring resistance to pesticide
Survivors Reproduction
Natural selection in rats: warfarin
How do some rats become resistant to warfarin?
These statements describe how the number of warfarin resistant rats may increase in a population. • Warfarin kills most rats.
Click on the links to find out more.
• But a few are resistant to the poison.
• DNA controls the proteins that a cell makes. Remind me about DNA.
• People use warfarin to kill rats.
• DNA is copied when a new cell is made.
• The resistant rats survive the poison. • The resistant rats breed. • They pass on their features to the next generation. • The number of resistant rats increases with each generation.
But there is a big unanswered question:
Sometimes a mistake is made – this is called a mutation.
• How do some rats become resistant to warfarin in the first place?
• Most mutations are harmless, some are harmful. Very rarely mutations may be helpful to an organism.
Tell me about mutations.
What sort of mutations can be helpful? Next
DNA
Mutations • Each gene is the instruction for making one protein.
• DNA molecules are very long. • They have a double helix shape.
• Sometimes a mistake is made when the gene’s DNA is copied.
genes
• The gene may code for a different protein.
• Chromosomes are made of DNA. • Genes are sections of chromosomes. • A gene is the instruction for how to make one type of protein.
• Mutations do happen naturally.
chromosome
DNA
• They can also be caused by some chemicals, and ionizing radiation.
Part of the DNA molecule
Back
Back
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How can mutations be helpful?
List the factors which can combine to produce a new species
• Most mutations do not help the organism. • The different protein that is made cannot do its job well. • But mutations are random – a very small number may help the organism survive in some environments.
• mutation This bacterium is resistant to most antibiotics.
• environmental change • natural selection
• For example, some bacteria have mutations that make them resistant to certain antibiotics. • Sickle-cell anaemia is a serious blood disease. People with two copies of the disease allele can be very ill. But people who carry just one copy of the allele have protection from malaria. This helps them to survive in countries where malaria is common.
A person who is a carrier of the sickle cell allele is protected from malaria.
Back
Who Wants to Live a Million Years? http://science.discovery.com/games-and-interactives/charlesdarwin-game.htm
Homework • Watch Darwin Videos • Draw a table with the definitions of the following terms: natural selection, evolutionary adaptation and evolution. • Unit Assessment 3: Topic 16 • Mastering Biology Activities: Reconstructing Forelimbs
Evolution Videos • The Genius of Charles Darwin (Pt 1, 2 and 3) – VERY GOOD! • http://www.youtube.com/watch?v=ptV9sNezEvk • http://www.youtube.com/watch?v=shkWhBVfe3o • http://www.youtube.com/watch?v=cARUZyBJtdY • What Darwin Never Knew (NOVA) • http://www.youtube.com/watch?v=AYBRbCLI4zU
What is Evolution? Evolution refers to the permanent genetic change (change in gene frequencies) in population of individuals. It does not refer to changes occurring to individuals within their own lifetimes. Populations evolve, not individuals. Microevolution describes the small-scale changes within gene pools over generations. Macroevolution is the term used to describe large scale changes in form, as viewed in the fossil record, involving whole groups of species and genera.
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Evidence for Evolution
Evidence for Evolution Evolutionary theory is now supported by a wealth of observations and experiments
Biogeography: The study of geographic distributions can indicate where species may have originally arisen.
Paleontology: The identification, interpretation and dating of fossils gives us some of the most direct evidence of evolution. Embryology and evolutionary developmental biology: The study of embryonic development in different organisms and its genetic control.
Paleontology Paleontology
Artificial selection: Selective breeding of plants and animals has shown that the phenotypic characteristics of species can change over generations as particular traits are selected in offspring. Biochemistry: Similarities and differences in the biochemical make-up of organisms can closely parallel similarities and differences in appearance.
Comparative anatomy: The study of the morphology of different species.
From gray wolf to Yorkshire terrier: selective breeding can result in phenotypic change
Molecular genetics: Sequencing of DNA and proteins indicates the degree of relatedness between organisms. Comparative anatomy
The Fossil Record
Types of Fossils
The fossil record is a substantial, but incomplete, record of evolutionary history:
Fossils form best when organisms are buried quickly in conditions that slow the process of decay.
Modern species can be traced through fossil relatives to distant origins. Fossil species are often similar to, but usually differ from, today's species. Fossil types often differ between sedimentary rock layers.
These fossil teeth, from Mastodon, an extinct elephant, are similar to the deciduous teeth of modern elephants.
Numerous extinct species are found as fossils. Fossils can be dated to establish their approximate absolute age.
Fossils are most commonly found in sedimentary rock. Mineral-rich hard parts (bones, teeth, shells) may remain as fossils, or minerals dissolved in water, may seep into tissues and replace the organic matter of the organism.
The Archaeopteryx Fossil
Fossils in a Rock Profile
Eight well-preserved fossil specimens have been discovered in finegrained limestone in Germany (dated late Jurassic, about 150 million years ago). Avian Features
Forelimb has three functional fingers with grasping claws.
Vertebrae are almost flatfaced.
Lacks the reductions and fusions present in other birds.
Impressions of feathers attached to the forelimb.
Breastbone is small and lacks a keel.
Belly ribs.
The hind-limb girdle is typical of dinosaurs, although modified. Long, bony tail.
A layer of shell still covers the stone interior of this ammonite
Bird bones preserved in a tar pit
Rates of evolution can vary, with bursts of species formation followed by stable periods.
True teeth set in sockets in the jaws.
Trilobites preserved in sedimentary rock
On rare occasions, fossils retain organic material, as when plant material is compressed between layers of shale or sandstone.
New fossil types mark changes in the past environmental conditions on the Earth.
Reptilian Features
Fossil fish
The term fossil refers to any parts or impressions of an organism that may survive after its death.
Incomplete fusion of the lower leg bones. Impressions of feathers attached to the tail. LEFT: Archaeopteryx lithographica Found in 1877 near Blumenberg, Germany
Layers of sedimentary rock are arranged in the order in which they were deposited, with the most recent layers nearer the surface.
Most recent sediments
Numerous extinct species Fossil types differ in each sedimentary rock layer
Sedimentary layers can be disturbed by subsequent tectonic activity.
The interpretation of rock layers containing fossils allows us to arrange the fossils in chronological order (order of occurrence), but does not give their absolute date.
Recent fossils are found in recent sediments
New fossil types mark changes in environment
Oldest sediments
Only primitive fossils are found in older sediments
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Radioactive decay of carbon-14
The relative age of fossils is useful, but fossils provide reliable historical data only if we can determine their absolute age.
Electron Spin Resonance
500 000 – 1000
Bone, tooth enamel, cave deposits
Fission Track
1 million – 100 000
Volcanic rock
Obsidian Hydration
800 000 – present
A number of methods are used to date fossils.
Obsidian (volcanic glass)
Amino acid racemization
1 million – 2000
Bone
Thermoluminescence
less than 200 000
Pottery, fired clay, bricks, burned rock
Dating Method
Age Range
(years)
Material Dated
Carbon-14 radioactivity (as % of living organism’s C-14 to C-12 ratio)
Figure 14.15
Dating Fossils
100
75
50
25
0 0
5.6 11.2 16.8 22.4 28.0 33.6 39.2 44.8 50.4
Time (thousands of years)
Uranium/Thorium
Less than 350 000 Bone, tooth dentine
Carbon 14
1000 – 50 000+
Bone, shell, charcoal
Potassium/Argon
10 000 – 100 million
Volcanic rocks
A fossil trilobite, a primitive arthropod that dwelled in the seas of the Devonian period 370 million years ago
How carbon-14 dating is used to determine the vintage of a fossilized clam shell
Carbon-14 in shell
Carbon-14 is taken up by the clam in trace quantities, along with much larger quantities of carbon-12.
After the clam dies, carbon-14 amounts decline due to radioactive decay.
Measuring the ratio of carbon-14 to carbon-12 reveals how many halflife reductions have occurred since the clam’s death.
The History of Life on Earth The history of life is divided up into eons, eras, periods, and epochs: Formation of the earth 4600 mya
Oldest known microfossils Oxygen produced by found in 3500 million year plants accumulates in old chert in Western the atmosphere Australia Precambrian Eon
Millions of years ago
Quaternary
Animation: The Geologic Record
Eras
Right click slide / select “Play” Millions of years ago
Evolutionary History
© 2013 Pearson Education, Inc.
Evolutionary History 2 Bacteria and algae
Bacteria, protists, and fungi have an evolutionary history extending back to the Precambrian. Some invertebrate groups extend back to the Cambrian Period, but land plants only as far back as the Devonian Period.
Land plants
Fungi Sphenophytes (ferns etc) Conifers Cycads Angiosperms
Cnidarians Flatworms
Invertebrates
Based on fossil evidence and radioisotope dating, the evolutionary history of plants, fungi, bacteria, protists, and non-chordate animals can be compiled.
Protists
Molluscks Annelid worms Insecta Crustacea
Tunicates
Similarly, the evolutionary history of chordates can be traced back to the Cambrian, but most animal groups are much more recent than this.
Agnatha (jawless fishes) Sharks and rays Ray finned fishes
Fish
Lungfish
Amphibians
Amphibians
Chelonia (turtles a& tortoises)
Reptiles
Crocodilia Rhyncocephalia (tuatara) Squamata (lizards & snakes)
Birds
Birds
Diplopoda Monotremes Arachnids Echinoderms
Millions of years ago
Mammals
Marsupials Placentals
Millions of years ago
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Figure 14.26a
Figure 14.14
A researcher excavating a fossilized dinosaur skeleton from sandstone
Ancestral mammal
Monotremes (5 species) Extinction of dinosaurs
Reptilian ancestor
A sedimentary fossil formed by minerals replacing the organic matter of a tree
Marsupials (324 species)
A 45-million-year-old insect embedded in amber
Eutherians (5,010 species) 250
200
100 65 50 150 Millions of years ago
0
Trace fossils: footprints, burrows, or other remnants of an ancient organism’s behavior
Tusks of a 23,000-year-old mammoth discovered in Siberian ice
Figure 13.10
Comparative Embryology When we compare the embryonic development of different vertebrates, it is evident that more closely related forms continue to appear similar until a later stage, compared to more distantly related forms. Note that although the early developmental sequences between all vertebrates are similar, phylogeny is not retraced during development.
Developmental Stage
Amphibian
Bird
Monkey
Human
Fertilized egg
Pharyngeal pouches
Late cleavage
Body segment s
Gill slits
Post-anal tail
Limb buds
Chicken embryo
Human embryo
Late fetal
Homologous Structures
Comparative Anatomy The pentadactyl (5 digit) limb found in most vertebrates has the same general bone structure. This similarity of structure is called homology. Homology – Anatomical similarity due to common ancestry
Forelimb
Hind Limb
Humerus (upper arm) Ulna
Femur (thigh)
Fibula
Radius Tibia Carpals (wrist)
Tarsals (ankle)
Metacarpals (palm)
Metatarsals (sole)
Phalanges (fingers)
Phalanges (toes)
Note that forelimbs and hind limbs have different names for equivalent bones.
In many vertebrates, the basic pentadactyl limb has been highly modified to serve specialized locomotory functions.
The same pattern of bones comprising the pentadactyl limb can be seen on each of these examples.
Mole's forelimb
Bird's wing
Such homologies also indicate adaptive radiation, as the basic limb plan has been adapted to meet the needs of different niches.
Dog's front leg
Bat's wing
Seal's flipper
Human arm
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Figure 13.9
Figure 13.17
Common ancestor of lineages to the right
Lungfishes Tetrapods Amniotes
Amphibians
1
Mammals
2
Tetrapod limbs Lizards and snakes
3
Amnion
Crocodiles
4
Cat
Whale
Bat
Analogous Structures
Ostriches 6
Feathers
Hawks and other birds
Analogy in Eye Structure
Not all similarities between species are inherited from a common ancestor. Structures that have the same function in different organisms may come from quite different origins. This phenomenon is termed analogy.
Homologous trait shared by all groups to the right
Birds
5
Human
Fins
Eyes in cephalopods (such as octopus) and Mammalian eye mammals have the same function and are structurally similar, but have evolved from different origins.
Iris Lens Cornea
Flippers Retina
Analogous structures do not imply an evolutionary relationship, but may indicate convergence. Examples: Eye structure in octopus and mammals.
Octopus eye
Retina Iris
Wings
Lens Cornea
Wings in birds and butterflies. Fins in fish and flippers in mammals
Vestigial Organs Organs Vestigial Many organisms have degenerate structures that no longer perform the same function as in other organisms. These organs must have been important in some ancestral form, but became redundant in later species. The wings of kiwi are tiny vestiges and useless. In snakes, one lobe of the lung is vestigial and, in some species, there are also vestiges of the pelvic girdle and hind limbs. The vestigial eyes of burrowing animals are no longer used for vision.
Vestigial Organs in Whales Whales are the descendants of large, four-legged land mammals that took up an aquatic existence some 60 million years ago.
Femur
Pelvis
Over many millions of years, the pelvis and femur of whales have become very small and no longer fulfill a locomotory function. Hindlimb Forelimb
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Cladogram of Whale Ancestors
Whale Ancestors The fossil record exhibiting whale evolution is extensive and well represented by skeletons that show much of their anatomy.
The study of plant and animal distribution is called biogeography.
Black lines represent cladistic relationship (probable relatedness)
Pakicetus (Middle Eocene)
Biogeographical Evidence Lemurs are endemic to the island of Madagascar
The basic principle of biogeography is that each plant and animal species originated only once. The place where this occurred is the centre of origin. The range of a species can be very restricted or, as with humans, almost the whole world (cosmopolitan).
Protocetus (Eocene)
Regions that have been separated from the rest of the world for a long time (e.g. Madagascar, Australia, and New Zealand), often have distinctive biota comprising a large number of endemic species (species that are found nowhere else).
Basilosaurus (Late Eocene) Red lines represents fossil record for the genus
Map: University of Texas at Austin (Public Domain image)
Biogeographical Distribution The distribution of species around the world suggests that modern forms evolved from ancestral populations and spread out (radiated) out into new environments. Good examples are found on islands offshore from large continental land masses:
Galapagos Islands The Galapagos Islands have species very similar to, but distinct from, the South American mainland. Ancestral forms probably migrated to the islands from the mainland in the past.
Galapagos Islands The giant tortoises are among the most well known of the Galapagos fauna
Cape Verde Islands Tristan da Cunha
Island Colonizers Land mammals rarely colonize islands. A high metabolic rate requires much food and water. Mammals cannot sustain themselves on long sea journeys.
Small birds, bats, and insects are blown to islands by accident. They must adapt to life there or perish.
Blown by strong winds
Reptiles probably reach distant islands by floating in driftwood. A low metabolic rate enables them to survive long periods without food and water.
Figure 13.8
Active flight
Seabirds fly to and from islands with relative ease. Some adapt to life on land, (e.g. the flightless cormorant in the Galapagos Islands). Others, may treat the island as a stopping place (e.g. the frigate bird).
Australia Common ringtail possum
Koala
Oceanic island
Rafting on drifting vegetation Amphibians cannot live away from fresh water. They seldom reach offshore islands unless that island is a continental remnant.
Planktonic larvae
Swimming
Sea mammals have little difficulty in reaching islands (e.g. seals, sea lions). They do not colonize the interior of islands.
Common wombat Deep ocean
Crustacean larvae drift to islands (e.g. crabs). Some crabs have adapted to an island niche.
Red kangaroo
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Molecular Biology
DNA Hybridization Method
One way to reconstruct the evolutionary history of a species is using DNA hybridization.
DNA is isolated from blood samples from each species:
Stork
In this technique, the DNA from different species is ‘unzipped’ and recombined to form hybrid DNA.
The greater the similarity in the DNA base sequences, the stronger the attraction between the two strands and the harder it is to separate them again.
Heat can be used to separate the hybridized strands. The amount of heat required to do this is a measure of how similar the two DNA strands are (% bonding).
Extract human DNA
Unzip the DNA using heat (both human and chimpanzee DNA unwinds at 86°C)
A crude measure of DNA relatedness can be achieved by measuring how hard it is to separate the hybrid DNA.
EXAMPLE: The relationships among the New World vultures and storks has been determined on the basis of DNA hybridization.
Extract chimpanzee DNA
Mix strands to form a hybrid
This is done by finding the temperature at which it unzips into single strands again (in this case it would be 83.6°C).
New World vulture
Some of the opposing bases in the DNA sequence do not match
Figure 13.11
DNA Sequencing Recent advanced techniques have enabled the sequence of DNA in different species to be determined. Species thought to be closely related on the basis of other evidence, were found to have a greater percentage of DNA sequences in common.
92%
100%
Human
Gorilla Orangutan
Gibbon
An interesting finding was that the DNA of humans and chimpanzees is more closely matched than that of chimpanzees and gorillas.
Old World monkey
Amino Acid Sequencing
Artificial Selection in Dogs
Squirrel monkey
Gibbon
Dogs were probably first domesticated at least 14 000 years ago from a gray wolf ancestor. Some 400 breeds have been bred from this single wild species as a result of selective breeding by humans.
Rhesus monkey
Primate
No. of amino acids different from humans
Chimpanzee
Identical
Gorilla
1
104
Position of changed amino acids –
Gibbon
3
80 87 125
Rhesus monkey
8
9 13 33 50 76 87 104 125
Squirrel monkey
9
5 6 9 21 22 56 76 87 125
Gorilla
Chimpanzee
96%
Chimpanzee
Humans and chimpanzees have a 97.6% similarity in their DNA sequences and are very closely related.
Amino acid differences for beta-hemoglobin in primates compared to the human sequence:
Percent of selected DNA sequences that match a chimpanzee’s DNA
Primate
The 'position of changed amino acids' is the point in the protein, composed of 146 amino acids, at which a different amino acid occurs.
Example: The staffordshire bull terrier was produced by breeding bulldogs and terriers. From each litter, breeders selected pups with the characteristics they desired. Bulldog
Gray wolf
Terrier Staffordshire bull terrier Staffordshire bull terriers combine characteristics of both bulldogs and terriers
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Artificial Selection in Dogs The gray wolf is distributed throughout Europe, North America and Asia. Amongst this species, there is a lot of phenotypic variation.
Selective Breeding or Artificial Selection
Selection is based on both physical and behavioral characteristics. In this way, different breeds have been suited to different tasks. Five ancient dog breeds are recognized, from which all other breeds are thought to have descended by artificial selection.
Mastiff-type Originally from Tibet, this breed dates back to the Stone Age
Pointer-type Bred for the purpose of hunting small game.
Greyhound One of the oldest breeds, originating the Middle East.
Sheepdog Originated in Europe and bred for stock protection.
Wolf-type Developed in snowcovered habitats in Alaska, northern Europe, and Siberia.
Grey wolves are the ancestors of all dogs.
• Salukis are thought to be one of the oldest domesticated dog breeds.
• Several breeds of dog lived with the ancient Greeks and Romans.
• Pictures of them are carved in Ancient Egyptian tombs.
• These included the greyhound, mastiff, and bloodhound.
• In the 1800s dalmations were trained to run next to horse carriages. • They guarded the horses from other dogs.
There are over 400 different breeds of domestic dog.
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Artificial selection vs Natural selection
SUMMARY EVIDENCE OF EVOLUTION Evolution leaves observable signs. Five of the many lines of evidence in support of evolution: 1. 2. 3. 4. 5.
the fossil record, biogeography, comparative anatomy, comparative embryology, and molecular biology.
© 2013 Pearson Education, Inc.
Activity – Process of Science Complete 1) What are the Patterns of Antibiotic resistance 2) How Do Environmental Changes Affect a Population?
More Evolution Videos (Useful) • Crash Course in Biology – Natural Selection • http://www.youtube.com/watch?v=aTftyFboC_M&list=PL5C9 56FAA7ADD146E • Crash Course in Biology – Comparative Anatomy • http://www.youtube.com/watch?v=7ABSjKS0hic
© 2013 Pearson Education, Inc.
UNIT 3: Evolution and Diversity Topic 17 Microevolution
CEB Textbook Chapter 13, pages 256-262 Mastering Biology, Chapter 13
Learning Outcomes After studying this topic you should be able to: •Define a population, describe its properties, and explain why a population is the smallest unit of evolution. •Define microevolution. •Explain the three mechanisms of microevolution: Genetic drift, gene flow and mutations.
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Who Evolves in Microevolution?
What is Microevolution? Microevolution describes the small-scale changes within gene pools over generations.
The smallest biological unit that can evolve is the POPULATION Individuals do not evolve – populations evolve.
Gene Pool
Populations From a population genetics viewpoint: A population comprises the total number of one species in a particular area. All members of a population have the potential to interact with each other. This includes breeding. he same species.
Continuous distribution Example: human population, Arctic tundra plant species
AA
A gene pool is defined as the sum total of all the genes/allelles present in a population at any one time. Evolution is a change over time in the gene pool of a species as more fit individuals are selected for leading to those alleles building up in the gene pool complement of the population.
AA
Aa
aa Aa AA AA
aa
Aa AA
Aa
A gene pool made up of 16 individuals
Example: Some frog species
Gene Pool
Changing Allele Frequencies Geographic boundary of the gene pool
Emigration
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Immigration AA
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Fragmented distribution
Individual is homozygous recessive (aa)
aa
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A’A AA
Boundary of gene pool
Natural selection Aa
Mutation
Individual is homozygous dominant (AA)
aa
aa AA
Aa aa Aa Aa
Mate selection (nonrandom mating)
AA AA
Gene flow
Geographical barrier
Aa
aa Aa
Aa
Aa Aa
Individual is heterozygous (Aa)
Aa
AA
A gene pool made up of 16 individual organisms with gene A, and where gene A has two alleles
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Aa aa
Genetic drift
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Activity – Process of Science Complete 1) How Do Environmental Changes Affect a Population?
The gene pool © 2013 Pearson Education, Inc.
Three Mechanisms of Microevolution
Mutations
1) Mutations 2) Gene Flow 3) Genetic Drift
AA
New recessive allele
Aa
AA AA a’a
In the graph below, a mutation creates a new recessive allele: a' The frequency of this new allele increases when environmental conditions change, giving it a competitive advantage over the other recessive allele: a
Aa
aa
Aa Aa
Aa
Aa Aa
aa
Aa
AA
aa
AA
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Environmental conditions change Allele frequency
Mutations are the source of all new alleles. Mutations can therefore change the frequency of existing alleles by competing with them. Recurrent spontaneous mutations may become common in a population if they are not harmful and are not eliminated.
Mutation causes the formation of a new recessive allele
Generations
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Gene flow is the movement of genes into or out of a population (immigration and emigration). A population may gain or lose alleles through gene flow. Gene flow tends to reduce the differences between populations because the gene pools become more similar.
Gene Flow
Genetic DriftDrift Barriers to gene flow
AA AA
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Population A
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No gene flow
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Population B Population C AA AA
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Migration into and out of population B AA
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Population A
aa
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Gene flow
aa Aa
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Genetic Drift = Random changes in the allele frequencies in a population
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aa
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AA aa
Population B
Aa
For various reasons, not all individuals will be able to contribute their genes to the next generation. As a result, random changes occur in allele frequencies in all populations. Genetic drift is often a feature of small populations that become isolated from the larger population gene pool, as with island colonizers (right).
Population C
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Allele Frequencies and Population Size
Genetic Drift: Generation 1 In the following hypothetical example, the allele frequencies in the gene pool of a small population are recorded over three generations.
Aa
The allele frequencies of large populations are more stable because there is a greater reservoir of variability and they are less affected by changes involving only a few individuals.
Aa
aa
Aa Aa
Small population Cheetahs have a small population with very restricted genetic diversity
Small populations have fewer alleles to begin with and so the severity and speed of changes in allele frequencies are greater. Endangered species with very low population numbers or restricted distributions may be subjected to severe and rapid allele changes.
AA
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aa AA
AA AA
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The effect this had on the gene pool was to reduce the frequency of the dominant allele from 53% to 50%.
Aa
Aa
aa
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aa
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aa
An example may be the sparsely distributed individuals of the Siberian tiger population.
AA
AA
Fail to locate a mate
Genetic Drift: Generation 3
A = 15 (50%) a = 15 (50%)
Aa
AA
Aa
Aa
The change in allele frequencies is directionless; there is no selection pressure operating on the alleles. A = 13 (43%) a = 17 (57%)
aa
AA
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Aa
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aa
AA
Aa
Aa
Aa
aa
Killed in a rock fall
Generation 3: In another chance event, a dark beetle was blown out to sea by the strong winds during a cyclone. The effect on the gene pool was to further reduce the frequency of the dominant
Fail to locate a mate due to low population density
Genetic Drift in Populations Large gene pool
The changes in allele frequencies as a result of random genetic drift can be modelled in a computer simulation. The breeding populations vary from 2000 (top) to 20 (bottom). Each simulation runs for 140 generations.
Aa
Large population
Genetic Drift: Generation 2
Two dark beetles were accidentally killed in a rock fall. This could equally have killed any beetle; it was not a test of the ‘fitness’ of the phenotype.
AA
This factor alone prevented them from contributing their genes to the next generation.
AA Aa Aa
With the random loss of alleles carried by these individuals, the allele frequency changes from one generation to the next. Generation 2: Another two beetles fail to breed because they could not find a mate in the dispersed population.
A = 16 (53%) a = 14 (47%)
Generation 1: As a result of the sparse distribution of the population, two beetles fail to locate a mate.
Breeding population = 2000 Fluctuations are minimal because large numbers of individuals buffer the population against large changes in allele frequencies.
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Killed in a cyclone
The Bottleneck Effect Populations may be reduced to low numbers through periods of:
Small gene pool Breeding population = 200 Fluctuations are more severe because random changes in a few alleles cause a greater percentage change in allele frequencies.
Very small gene pool Allele lost from the gene pool
Breeding population = 20 Fluctuations are so extreme that the allele may become fixed (100%) or lost altogether (0%)
Seasonal climatic change
Heavy predation or disease
Catastrophic events (e.g. flood, volcanic eruptions, landslide)
As a result, only a small number of individuals remain in the gene pool to contribute their genes to the next generation. The small sample that survives will often not be representative of the original, larger gene pool, and the resulting allele frequencies may be severely altered.
In addition to this ‘bottleneck’ effect, the small surviving population is often affected by inbreeding and genetic drift.
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Population Bottlenecks
Population Bottlenecks Large, genetically diverse population
The original gene pool is made up of the offspring of many lineages (family groups and sub-populations) aa
aa
AA
Genetic Extinction event such bottleneck as a volcanic eruption
All present day descendants of the original gene pool trace their ancestry back to lineage B and therefore retain only a small sample of genes present in the original gene pool
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AA
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Population grows to a large size again, but has lost much of its genetic diversity AA
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Population bottleneck: the population nearly becomes extinct as numbers plummet
Population numbers
Only two descendants of lineage B survive the extinction event
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Population reduced to a very low number with consequent loss of alleles
Time
Genetic Bottlenecks & the Cheetah Population The world population of cheetahs has declined in recent years to fewer than 20 000. Recent genetic analyses has found that the total cheetah population has very little genetic diversity. Cheetahs appear to have narrowly escaped extinction at the end of the last ice age: 1020 000 years ago. All modern cheetahs may have arisen from a single surviving litter, accounting for the lack of diversity.
Genetic Diversity in Cheetahs The lack of genetic variation has led to: sperm abnormalities decreased fecundity high cub mortality sensitivity to disease
Since the genetic bottleneck, there has been insufficient time for random mutations to produce new genetic variation.
At this time, 75% of all large mammals perished (including mammoths, cave bears, and saber-toothed cats).
Figure 13.24-1
Original population
Figure 13.24-2
Original population
Bottleneck event
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Figure 13.24-3
The Founder Effect Occasionally, a small number of individuals may migrate away or become isolated from their original larger population. This colonizing or founder population will have a small and probably non-representative sample of alleles from the parent population’s gene pool. Original population
Bottleneck event
The Founder Effect Small founder populations are subject to the effects of random genetic drift. The founder effect is typically seen in the populations of islands which are colonized by individuals from mainland populations. Often these species have low or limited mobility; their dispersal is often dependent on prevailing winds (e.g. butterflies and other insects, reptiles, and small birds).
Natural selection acts on phenotype
Offshore islands can provide an environment in which founder populations can evolve in isolation from the parental population.
As a consequence of this founder effect, the colonizing population may evolve in a different direction than the parent population.
Surviving population
Island population
The Founder Effect
The marine iguana of the Galapagos has evolved in an isolated island habitat
Colonizing island population
Aa AA AA
AA
AA Aa Aa Aa
Colonization
Mainland population
In this hypothetical population of beetles, a small, randomly selected group is blown offshore to a neighboring island where they establish a breeding population.
This population may not have the same allele frequencies as the mainland population
Some individuals from the mainland population are carried at random to the offshore island by natural forces such as strong winds
Mainland population
Aa AA Aa AA
AA
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aa
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AA Aa aa
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AA Aa aa
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EXAMPLE OF NATURAL SELECTION: Gene pool of grey and white alleles
• Natural selection therefore changes the composition of a gene pool and increases the probability that favourable alleles will come together in the same individual.
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Environment is the Selective Pressure The environment is never constant in different parts of the world, so natural selection acts on different characteristics, depending on where the selection is taking place
• Disruptive Selection Environment selects against intermediate phenotype, allowing both extremes to become more prevalent.
Types of natural selection • Directional Selection Environment selects against one phenotypic extreme, allowing the other to become more prevalent. English peppered moth. Gene pool changed dramatically in 50 generations.
• Stabilizing Selection Environment selects against two extreme phenotypes, allowing the intermediates to become more prevalent.
Sickle cell anaemia As an example of natural selection: Sickle cell anemia is an inheritable disease that causes red blood cells to form a sickle shape that is inefficient at carrying oxygen Sickle cell allele is recessive Homozygous recessive condition is detrimental to health Heterozygous condition has minor affect on health
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Heterozygous Advantage If individuals who are heterozygous for a particular gene have greater fitness than homozygotes, natural selection will tend to maintain the two alleles.
Malaria • Heterozygotes have a protection against malaria
•In America – Homozygous recessive: selected against Heterozyogous: Slightly less fit than Homozygous Dominant
• In areas where malaria is a major killer, heterozygotes are selected for.
•In Africa – Homozygous recessive: selected against
• This leads to the recessive allele being maintained in those populations
Heterozygous: More fit than Homozygous Dominant
Artificial selection – A Form of Microevolution
Artificial selection (selective breeding) The ability of people to control the breeding of domesticated animals and crop plants has resulted in a astounding range of phenotypic variation over relatively short time periods It is people that is the selective force rather than the environment!
Domestication of animals What characteristics impacted what animals were domesticated? • • • • • •
Artificial selection • Artificial selection involves breeding from individuals with the most desirable phenotypes. The aim of this is to alter the average phenotype within the species. • In this way the gene pool gradually changes • Artificial selection is a form of directional selection and depends on the presence of genetic variability
Use of animal – food, milk, wool, leather, work Breeding – need to be able to breed in captivity Disposition – ability to be domesticated Social structure – dominance hierarchies, herds Growth rate – fast growth rate more beneficial Tendency to panic – slower less nervous = easier to catch
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Hunting large game dog
Example of the domestic dog • 400 different breeds • One species – Canis familaris - different species can interbreed = Xs • Descended from the grey wolf over 15,000years ago
• Good sense of smell (tracking) • Fearless • Aggressive • Strong bite • Strong neck muscles
Game fowl hunting • Excellent sense of smell (detection) • Good eyesight • Understanding of need to hold, point, retrieve • Obedience/ self-control (not eating or mauling prey)
Stock control • Must not regard stock as prey – low aggression • Obedience • Ability to anticipate behaviour of stock • Ability to control stock with bark and body language • Ability to protect stock from predators
Family pet • • • •
Low level aggression Playful attributes Friendly disposition Obedience?
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Guard dog • Aggressive to strangers • Excellent hearing and smell • Alert to the arrival of intruders • Bark response • Size?
Jack Russels
• The Jack Russell Terrier was breed to hunt the red fox, who live in small underground dens. Traits selected for in the breeding of JRTs are size - must be small enough to get to its quarry. Vocal – the hunt requires a dog that will bark at prey so it can be located underground and be dug out if necessary. High intelligence, highenergy dogs – requirements of a working dog which must problem-solve in the field and work tirelessly against often formidable quarry. • However the selected traits for the breed mean they can also be problematic pets. They may exhibit unmanageable behaviour, including excessive barking, escaping from the yard, or digging.
• Breed to chase small furry animals, so can tend to be cat aggressive • Some JRT's exhibit a Napoleon complex regarding larger canines that can get them into dangerous situations. Their fearlessness can scare off a larger animal, but their apparent unawareness of their small size can lead to a lopsided fight with larger dogs if not kept in check.
Artificial selection vs Natural selction
Domesticating foxes? - http://www.youtube.com/watch?v=-L58NPPQ5eI
Artificial selection in dog breeding
Artificial Selection in Brassica Different parts of the wild brassica have been developed by human selection to produce at least six distinctly different vegetables.
Cauliflowe r (flower)
Cabbage (terminal buds)
All these vegetables form a single species and will interbreed if allowed to flower. Example: The new “broccoflower” is a cross between broccoli and cauliflower.
Pedigree Dogs Exposed - http://www.youtube.com/watch?v=yZMegQH1SPg Secret Life of Dogs - http://www.youtube.com/watch?v=5h8lWBd1hmE
Broccoli (inflorescence)
Brussels sprout (lateral buds)
Kale (leaf)
Wild Form Brassica oleracea
Kohlrabi (stem)
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Homework • Unit Assessment 3 Topic 17 • Mastering Biology Activities: Genetic Variation From Sexual Recombination, Causes of Evolutionary Change, Mechanisms of Evolution
• Complete Bioflix study sheet: Mechanisms of Evolution • Complete Evolution Assignment on Mastering Biology – Due first lesson back after break.
Evolution Videos • Crash Course in Biology – Evolution • http://www.youtube.com/watch?v=P3GagfbA2vo
Key Words • • • • • • • • • • •
Natural Selection Gene Pool Allelle Frequencies Population Gene Flow Bottleneck Effect Mutations Founder Effect Artificial Selection Microevolution Directional, Stabilizing or Disruptive Selection
UNIT 3: Evolution and Diversity Topic 18 Macroevolution
CEB Textbook Chapter 13, pages 256-262 Mastering Biology, Chapter 13
Learning Outcomes After studying this topic you should be able to: •Define macroevolution and explain what differentiates it from microevolution. •Define and explain the biological species concept. Describe and explain the two types of reproductive isolating mechanisms: prezygotic and post-zygotic. •Define and describe the differences between: allopatric and sympatric speciation.
What is Macroevolution? Macroevolution is the term used to describe large scale changes in form, as viewed in the fossil record, involving whole groups of species and genera.
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Macroevolution Macroevolution refers to evolutionary changes above the level of the species: changes in genera or orders. Macroevolution is concerned with changes in the kinds of species over evolutionary time and includes: The origin of unusual features (evolutionary novelties). The origin of evolutionary trends (e.g. increased brain size in primates). Adaptive radiation (a form of divergent evolution). Extinction.
Example of an evolutionary trend: brain size in hominids Increasing Brain Size
Animation: Macroevolution H. erectus 1100 ml
H. sapiens 1450 ml
Micro- vs Macroevolution The mechanisms of gene pool change and natural selection represent the modern synthesis of evolution. The gradualist view is that, over long periods of time (millions of years), microevolutionary processes are sufficient to account for the origin of new genera, families, orders and phyla. The punctuated equilibrium view is that most morphological change occurs during abrupt speciation events and, once in existence, species then change very little.
The debate is not about the fact of evolution; only about the relative importance of different evolutionary mechanisms.
The Biological Species Concept Species Species are often composed of different populations (often in different habitats) that are quite distinct. These are often called subspecies, races, and varieties depending on the degree of reproductive isolation. There are up to 20 000 species of butterfly; they are often very different in appearance and do not interbreed.
Right click slide / select “Play” © 2013 Pearson Education, Inc.
The Biological Species Concept – Species is a Latin word meaning • “kind” or • “appearance.”
Species are recognized on the basis of their morphology (size, shape, and appearance) and, more recently, by genetic analysis. A biological species is: a group of interbreeding (or potentially interbreeding) individuals, reproductively isolated from other such groups. These are often called subspecies, races, and varieties depending on the degree of reproductive isolation.
Species The boundaries of a species gene pool can be sometimes unclear, such as the genus to which all dogs, wolves, and related species belong: Coyote–red wolf hybrids
Coyote Canis latrans
Red wolf Canis rufus Interbreedin g
Interbreeding Domestic dog Canis familiaris
Dingo Canis familiaris dingo
Side-striped jackal Canis adjustus
Interbreedin g
Black-backed jackal Canis mesomelas
Gray wolf Canis lupus No interbreeding
H. habilis 575 ml
No interbreeding
A. afarensis 440 ml
Golden jackal Canis aureus
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Figure 14.2a
Figure 14.2b
Similarity between different species
Reproductive Isolating Mechanisms Reproductive isolating mechanisms (RIMs) prevent successful breeding between different species. They are barriers to gene flow. A single barrier may not completely isolate a gene pool, but most species have more than one isolating mechanism operating to maintain a distinct gene pool. Geographical barriers prevent species interbreeding but are not considered to be RIMs because they are not operating through the organisms themselves.
Diversity within one species
Geographical Barriers Geographical barriers isolate species and prevent interbreeding. Geographical barriers include mountains, rivers, and oceans. Geographical features that may be barriers to some species may not be barriers to others. In the USA, two species of antelope squirrels occupy different ranges either side of the Grand Canyon. Their separation is both geographical and ecological. They are separated by the canyon and by the different habitat preferences in the regions they occupy.
Reproductive Isolating Mechanisms Reproductive isolating mechanisms can be categorized according to when and how they operate: Prezygotic (pre-fertilization) mechanisms include: habitat preference behavioral incompatibility structural incompatibility physiological incompatibility Postzygotic (post-fertilization) mechanisms include: zygote mortality poor hybrid fitness hybrid sterility
Although they are in the same region, the white tailed antelope squirrel inhabits desert to the north of the canyon, while Harris’s antelope squirrel (above) has a more limited range to the south.
Prezygotic Isolating Mechanisms Prezygotic isolating mechanisms act before fertilization to prevent successful reproduction or mating. 1) Ecological or habitat: Different species may occupy different habitats within the same geographical area, e.g. desert and coastal species, ground or tree dwelling. In New Zealand, Hochstetter’s and Archey’s frogs occur in the same relatively restricted region but occupy different habitats within that range.
Archey’s frog (top) has no webbing between the toes and is found in forested areas away from water. Hochstetter's frog (bottom) has partial toe webbing and can be found in stream margins.
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Prezygotic Isolating Mechanisms
Prezygotic Isolating Mechanisms Behavioral: Peacock
Species may have specific calls, rituals, postures etc. that enable them to recognize potential mates (many bird species have elaborate behaviors).
Breeding season for species A
Temporal (time-based): Species may have different activity patterns; they may be nocturnal or diurnal, or breed at different seasons. In this hypothetical example, the two species of butterfly will never mate because they are sexually active at different times of the year.
Structural:
Breeding season for species B
Insects have very specific copulatory organs which act like a lock and key
For successful mating, species must have compatible copulatory apparatuses, appearance, and chemical make-up (odor, chemical attractants).
Gamete mortality:
Egg
If sperm and egg fail to unite, fertilization will be unsuccessful.
Attempted fertilization
Sperm
Figure 14.4
Postzygotic IsolatingMechanisms PREZYGOTIC BARRIERS Temporal Isolation
Behavioral Isolation
Postzygotic isolating mechanisms act after fertilization to prevent successful reproduction. Hybrid inviability:
Habitat Isolation
Mechanical Isolation
The fertilized egg may fail to develop properly
Gametic Isolation
Species A
F1
X
Hybrid AB
Reduced viability
F2
Species B
X Hybrid AB Reduced viability
Hybrid AB Non-viable or sterile
Fewer young may be produced and they may have a low viability (survivability).
Hybrid sterility: The hybrid of two species may be viable but sterile, unable to breed (e.g. the mule).
Hybrid breakdown: The first generation may be fertile but subsequent generations are infertile or nonviable.
This mule is a hybrid between a horse and a donkey
Figure 14.5
Hybrids in the Horse Family Zebra stallion (2n = 44)
Sterile hybrids are common among the horse family. The chromosomes of the zebra and donkey parents differ in number and structure, producing a sterile zebronkey.
POSTZYGOTIC BARRIERS
Donkey mare (2n = 62)
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Horse
X ‘Zebronkey’
Donkey
offspring (2n = 53) Chromosomes contributed by zebra father
Y
Chromosomes contributed by donkey mother
Mule
X
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Speciation
Types of Speciation
Speciation refers to the process by which new species are formed. Speciation occurs when gene flow has ceased between populations where it previously existed. Speciation is brought about by the development of reproductive isolating mechanisms which maintain the integrity of the new gene pool.
Different species of swallowtail butterflies in the genus Papilio
Several models have been proposed to account for new species among sexually reproducing organisms: Allopatric speciation: Populations become geographically separated, each being subjected to different natural selection pressures, and finally establishing reproductive isolating mechanisms. Sympatric speciation: A population forms a new species within the same area as the parent species.
Figure 14.6
Allopatric Speciation STAGE 1: Moving into new environments The parent population expands its range and occupies new parts of the environment. Expansion of the range may be due to competition.
Allopatric speciation
The population has a common gene pool with regular gene flow (any individual has potential access to all members of the opposite sex for the purpose of mating).
Sympatric speciation
Allopatric Speciation
Allopatric Speciation
STAGE 2: Geographical isolation Gradual formation of physical barriers may isolate parts of the population at the extremes of the species range As a consequence, gene flow between these isolated populations is prevented or becomes rare. Agents causing geographical isolation include: continental drift, climatic change, and changes in sea level (due to ice ages).
Mountain barrier prevents gene flow
STAGE 3: Formation of a subspecies
River barrier prevents gene flow
Isolated Population A
The isolated populations may be subjected to quite different selection pressures. Isolated Population B
Some natural variation exists in each population Isolated Population C
Parent population
These selection pressures will favor those individuals with traits suited to each environment. Allele frequencies for certain genes change and the populations take on the status of a subspecies (reproductive isolation is not yet established).
Wetter climate
Sub-species A
Cooler climate
Sub-species A
Drier climate
Sub-species C
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Figure 14.7
Allopatric Speciation STAGE 4: Reproductive isolation Each separated subspecies undergoes changes in its genetic makeup and behavior. This will prevent mating with individuals from other populations. Each subspecies’ gene pool becomes reproductively isolated from the others and they attain species status. Even if geographical barriers are removed to allow mixing of the populations, genetic isolation is complete.
Sympatric species
Species A
Ammospermophilus harrisii
Ammospermophilus leucurus
Species B
Mountain barrier remains
River barrier removed
Allopatric species Species A
Sympatric species: Closely related species with overlapping distribution Allopatric species: Closely related species still geographically separated
Figure 14.8
Figure 14.10
Populations become allopatric
Populations become sympatric Populations interbreed Gene pools merge: No speciation
Punctuated pattern Time
Geographic barrier
Populations cannot interbreed Reproductive isolation: Speciation has occurred
Gradual pattern
Time
Sympatric Speciation Sympatric speciation: A new species within the same area as the parent species. There is no geographical separation between the speciating populations.
Wild Einkorn
All individuals are, in theory, able to meet each other during the speciation process.
Sympatric speciation is rarer than allopatric speciation among animals, but it is probably a major cause of speciation among plants!
Sympatric speciation may ocur through: A change in host preference, food preference or habitat preference.
© 2013 Pearson Education, Inc.
Animation: Allometric Growth
The partitioning of an essential but limiting resource.
Right click slide / select “Play”
Instant speciation as a result of polyploidy (particularly among plants, as in the evolution of wheat).
Common Wheat
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A change in habitat preference:
Sympatric Speciation
An insect forced to lays its eggs on an unfamiliar plant species may give rise to a new population of flies isolated from the original population
It is not uncommon for some insect species to be conditioned to lay eggs on the plant species on which they themselves were reared. If the normally preferred plant species is unavailable, then the insect may be forced to choose another species to lay eggs on. A few eggs surviving on this new plant will give rise to a new population with a new plant species preference.
Original host plant species
Sympatric Speciation Polyploidy involves the multiplication of whole sets of chromosomes (each set being the haploid number N). Polyploids occur frequently in plants and in some animal groups such as rotifers and earthworms. When such individuals spontaneously arise, they are instantly reproductively isolated from their parent population. As many as 80% of flowering plant species may have originated as polyploids. Different species of Chrysanthemum (right) have arisen as a result of polyploidy. They have chromosome numbers (2n) that are multiples of 18: 2n = 18, 36, 54, 72, and 90.
Allopatric speciation
New host plant species
Establishing reproductive isolation:
Each host plant will attract flies that were reared on that plant where they will mate with other flies with a similar preference
If mating and rearing of offspring takes place entirely within the habitat, then the population will become reproductively isolated. Further differentiation of the two populations is likely as each becomes increasingly adapted to their respective habitats.
No gene flow
Gene flow
Original host plant species
Ultimately, the two groups will diverge to be recognized as separate species.
New host plant species
Stages in Species Formation Homogeneous Ancestral Population
Different types of isolating mechanisms operate and different amounts of gene flow take place as two populations diverge to form new species.
Population splits Population A Geographic isolation
Population B Gene flow common
Race A Prezygotic isolation
Gene flow uncommon
Species A
Prezygotic isolation Subspecies B
Subspecies A Postzygotic isolation
Geographic isolation Race B
Evolutionary Development
Sympatric Speciation
Gene flow very rare No gene flow
Postzygotic isolation Species B
Sympatric speciation
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Summary: Forces Operating in Evolution
Speciation summary
UV Light
Various “forces” or phenomenon have a part to play in the evolutionary process: At the molecular level: Point mutations Control of gene expression Rate of protein synthesis
Forces Operating in Evolution
Forces Operating in Evolution At the organism level:
At the chromosomal level:
Environmental modification of phenotype
Crossing over
Reproductive success
Block mutations
Selection pressures
Polyploidy
'Fitness' of the phenotype
Aneuploidy
Egg
Independent assortment Recombination Sperm
Forces Operating in Evolution At the population level:
Forces Operating in Evolution
AA Aa AA
Genetic drift and population size
AA
At the species level: Geographical barriers
Aa Aa
Natural selection altering gene frequencies aa
Aa
Mate selection Intraspecific competition
Reproductive isolation (prezygotic and postzygotic)
aa
Selection pressures
aa Aa Aa
AA
Interspecific competition
AA
Founder effect Immigration/emigration (gene flow)
aa
AA
Population bottlenecks
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Activity – Process of Science Complete 1) How Do New Species Arise by Genetic Isolation?
Homework • Unit Assessment 3 Topic 18 • Mastering Biology Activities: Polyploid Plants, A Scrolling Geologic Record
• Complete Evolution Assignment on Mastering Biology – Due first lesson back after break. • Watch Crash Course Biology: Speciation © 2013 Pearson Education, Inc.
Evolution Videos • Crash Course in Biology – Speciation • http://www.youtube.com/watch?v=2oKlKmrbLoU
Key Words • • • • •
Speciation Allopatric Speciation Sympatric Speciation Prezygotic Barrier Postzygotic Barrier
Starter
What do we know about human evolution? (For your own interest: Will not be assessed)
Watch the introductory ‘Prologue’ clip from www.becominghuman.org Note: •
new observations may or may not support the current explanation
•
if they do not support it, the explanation may need to be reconsidered
•
our understanding of human evolution is still developing
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What to do…
Review Presentation Evidence for human evolution and answer the following questions: a. Is there any evidence that humans evolved in a similar way to other animals? b. What sort of evidence should we look for? There are still problems with our interpretation of the human evolution story… a. We can never know whether what we call different species were different. Why? b. We can see variation between the bones – but there is lots of variation within our species, Homo sapiens, today. Give some examples of such variation. c. The number of specimens found is too small to provide conclusive evidence. Why is this an issue?
Feature
Gorillas
Human beings
• Human beings share many features with them. • Humans are NOT descended from modern apes. • But we do share a common ancestor.
human beings
Chimpanzees
Head hair
short
long
short
Calf muscle
small
large
small
thin
fat
thin
Arms vs legs
shorter legs
shorter arms
shorter legs
Canine teeth
large
small
large
Thumbs
long
long
short
Chromosomes
48
46
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Buttocks
• Chimps and gorillas are apes.
chimps or gorillas? chimps or gorillas?
• It’s not a trick question! • So far we haven’t found enough evidence to decide.
From all this evidence, do you think human beings are closest to chimps or gorillas?
• We know that ape-like animals were living in Africa over 20 million years ago. • The evidence: - scientists have found skulls with ape-like features - they can date the fossil apes.
• But there is enough evidence to say that humans and apes share the same ancestor.
• These early apes share some features with living apes: - no tail - shoulder blades at the back of the body • But they do also have some differences.
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human beings
human beings
chimpanzees and gorillas
?
orang-utans gibbons
chimpanzees and gorillas
fossils
Scientists use the evidence to work out how living apes are related to fossil apes.
Do human beings have any closer relatives in the fossils?
sinus (spaces inside skull) eye socket broad nose
• Australopithecines lived in Africa 1.5 to 4 million years ago.
modern human
• Lucy – the most complete Australopithecine skeleton found.
jaw more like human than chimpanzee
A. africanus
chimpanzee
• Australopithecines share some features with human beings: - eye sockets are wide and set apart
• So is Lucy more closely related to us or to living apes?
- broad nose - sinus inside front of skull
sinus (spaces inside skull) eye socket broad nose
modern human
jaw more like human than chimpanzee
A. africanus
•
Chimps and gorillas also have these features. But other apes don’t.
•
So are Australopithecines more closely related to:
(a) human beings? or (b) chimps and gorillas?
chimpanzee
• In 1978 scientists found the evidence to answer this question. • Evidence suggests that these footprints were made in Africa by Australopithecines. • They walked on two legs.
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human beings Australopithecines
chimpanzees and gorillas
• But scientists think that we have even closer fossil relatives. • Habilines lived in Africa 1.6 to 2 million years ago.
So Australopithecines were more like human beings than chimps and gorillas.
Species Human beings
• Fossils showed that their spines were joined to the middle of their skull, so Habilines walked upright.
Brain size (ml)
human beings
1400
Australopithecines
500
Habilines
650
• We have more evidence about Habilines. They had much bigger brains than Australopithecines like Lucy. • We also know that they made tools. • So the evidence tells us that Habilines are more closely related to modern humans than Austalopithecines.
habilines
Australopithecines
• Habilines were probably the first animals on Earth to make tools. • Tool making is a very important feature of human beings. • So scientists think Habilines were the first early humans. • They are called Homo habilis.
modern humans Species
Brain size (ml)
Human beings
1400
Australopithecines
500
Habilines
650
Homo erectus
900
• Fossils of other early humans have also been found. • Homo erectus lived in Africa 1.5 million years ago.
Homo erectus
Habilines (Homo habilis)
• Their large brains mean that Homo erectus are more closely related to modern humans. • Scientists have also found evidence that they were able to make fire.
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• Homo erectus were also the first early humans to leave Africa.
• But Homo erectus are not quite the same as modern humans. For example, their skulls have a thick, straight brow ridge.
• Their skeletons have been found in Asia and Europe.
• So scientists think that we must have at least one more recent ancestor.
modern humans (Homo sapiens)
Homo erectus
Habilines
• The search goes back to Africa. We know that not all Homo erectus left when they first moved out of Africa. • Those that stayed carried on evolving into modern humans. • We know this because skulls shaped more like a modern human have been found in Africa. This one from Ethiopia is only 160 000 years old.
• By 40 000 years ago modern humans had spread across the world. • Evidence like cave paintings and tools tells us where and how they lived.
• Modern humans are called Homo sapiens. • They left Africa about 120 000 years ago. • Homo sapiens fossils this old have been found in Israel.
• These modern humans were hunters and farmers. • The symbols in their paintings tell us that they had language. • They also had ceremonies like burials.
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modern humans
What to do…
Review Presentation Evidence for human evolution and answer the following questions:
early humans
a. Is there any evidence that humans evolved in a similar way to other animals?
Australopithecines living apes, like chimps and gorillas
Summary: • Different groups of humans evolved from a common ancestor. • All but one of these groups died out. • Only Homo sapiens (modern humans) survived. • Modern humans evolved in Africa.
•
Explore these
•
Science Museum, London, Evolution of language:
•
Hunterian Museum, University of Glasgow, illustrates the human evolution story with images of its exhibits and brief text passages:
•
Institute of Human Origins, Arizona State University, Becoming Human, broadband documentary: www.becominghuman.org/
•
US Public Broadcast Service hosts a large, attractive site with masses of information on aspects of evolution: www.pbs.org/wgbh/evolution/
b. What sort of evidence should we look for? There are still problems with our interpretation of the human evolution story… a. We can never know whether what we call different species were different. Why? b. We can see variation between the bones – but there is lots of variation within our species, Homo sapiens, today. Give some examples of such variation. c. The number of specimens found is too small to provide conclusive evidence. Why is this an issue?
www.sciencemuseum.org.uk/exhibitions/brain/256.asp
www.hunterian.gla.ac.uk/museum/hominid/hominid.html
including: Is love in our DNA? Has evolution shaped human beings’ choice of mates? Higher level, useful case study option: www.pbs.org/wgbh/evolution/sex/love/index.html •
Smithsonian Institute Human Origins exhibit is more appropriate for teachers’ information: www.mnh.si.edu/anthro/humanorigins/index.htm
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