EVOLUTION: ADAPTATION AND NATURAL SELECTION
Evolution
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A logical consequence of reproduction, heredity, and ecology
The basic observations and questions:
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There are at least 3,000,000 species and much variation between and within species--diversity
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Each species in general shows characteristics that help it live its lifestyle--adaptation
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How and why did so many types of organisms appear?
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How and why is each species so well adapted to its environment?
Hypothesis: Special creation
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Each species is created to match a pattern, called an “archetype”
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Each archetype fits part of a plan
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Variation occurs by random error in reproducing the archetype
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Origin of this hypothesis: Greek philosophers Pythagoras, Plato
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Modern support: Judeo-Christian religions (plan implies a planner)
“New” observations Taxonomy (Linnaeus, Darwin, and others)
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Listed and classified plants and animals
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Showed that species fall into groups sharing characteristics, and that there are well defined hierarchies of relationships --taxa
Paleontology (Wallace, Lamarck and others)
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Study of fossils--signs and remnants of ancient life preserved in rocks
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Showed gradual increase in the number and complexity of species, extinct species, appearance of new species, relationship between ancient and present-day species--history of life
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Wallace: whenever a new species appears, a similar species is already there
Hypothesis: Organic evolution Species appear by evolving from previous species--”descent with modification” Species are selected to fit a particular lifestyle through preferred reproduction, survival Explains: Diversity--one species gives rise to many, each of which give rise to more…”divergence” Adaptation--selection of traits promotes reproduction, survival Taxa--related organisms diverged from a “common ancestor” Fossils, history of life--inferred history represents a real history
Adaptation through natural selection Darwin, Wallace theory:
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Variation within a population occurs randomly
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Traits and their variation are inherited by progeny
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Interaction of organisms with the environment causes a selection of “better adapted” type for survival, reproduction
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Population of progeny tends to resemble the “better adapted” parents
Random mutations provide the basic variation Base substitutions, additions, deletions change amino-acid sequences (leading to changes in protein function) and control of transcription Chromosome deletions, inversions, transversions, duplications change how a gene is expressed Many examples come from the study of Drosophila (fruit flies): e.g., bithorax… and people: e.g., hemophilia (loss of gene for clotting factor), xeroderma pigmentosum (loss of gene for DNA repair)
HbS
(It is more difficult to find random mutations to positive traits, but consider sickle-cell anemia--protection against malaria) How likely are mutations? Depends on the gene and the mutation, But a range is 104-1010 mutations/gene-generation (average 106)
malaria
Other forms of inherited variation occur Recombination through gamete fusion and meiosis increases variety Consider a population of haploid organisms with two genotypes: AB, ab asexual mitosis AB, ab
sexual reproduction: fusion, meiosis AB, ab, Ab, aB
Diploidy increases variety through heterozygosity: heterozygosity gives intermediate phenotypes, “storage” of recessive alleles AABB, AaBB, aaBB, AABb, AaBb, aaBb, AAbb, Aabb, aabb
An agricultural example shows how selection can change a population Corn seeds: protein and oil content per seed varies normally around a mean U. Illinois, 1896-1945: seeds from 200-300 plants selected for high or low protein content or high or low oil content
Selected for: High protein % of seed mass that is protein or oil
High oil
Low protein Low oil generations
Variation in oil and protein was too much, too fast to be provided by mutation--this shows the effect of recombination of previously unexpressed alleles Later experiments showed the reverse selection--allele variation persisted after 80 generations of selection
Selected for: High protein % of seed mass that is protein or oil
High oil
Low protein Low oil generations
Natural selection Favors (in theory):
•Survival to reproductive age (feeding, predator protection, abiotic resistance) •Mating frequency (recognition, reproductive organs) •Fertility (number of young/female) •Embryonic survival (parental protection, coordination of development) Recall the different strategies: high fertility (insects, fish, ash trees) or high survival (mammals, birds, coconuts)
Selection can change allele frequencies in different ways
Stabilizing selection influences human birth weight:
Directional selection produced longhorn cattle in America:
(Longhorns were able better to deter predators)
Disruptive selection results in two populations of West African seedcrackers:
Is evolution occurring in a population? Here is one test: Population genetics A population’s genetic composition can be defined by a set of allele frequencies p = frequency of allele A in population (total number of allele A / sum of all alleles in population) q = frequency of allele a in population p = (2NAA + NAa)/2N
q = (2Naa + NAa)/2N
Where NAA = number of individuals of genotype AA NAa = number of individuals of genotype Aa Naa = number of individuals of genotype aa N = total population
“Hardy-Weinberg equilibrium” shows the genotypes connected with allele frequencies p and q In one generation of random mating and random survival of of the offspring, the population reaches Hardy-Weinberg Equilibrium: NAA = Np2, NAa = 2pq, and Naa = Nq2
Hardy-Weinberg equilibrium applies when a population is NOT evolving
•Mating is random •Population size in large (in small populations, random choice of mates may exclude certain genotypes--called “genetic drift”)
•There is no mutation •There is no gene flow (movement in or out of the population) •Survival is random SO… If a population is NOT in Hardy-Weinberg equilibrium, then it IS evolving at a significant rate through one of these processes
Summary Evolution through random mutation, recombination, and natural selection is the accepted modern explanation for the history and diversity of life Selection works on phenotypes, not genotypes Selection can stabilize, change, or disrupt the composition of a set of alleles Hardy-Weinberg equilibrium describes the allele frequencies of a population that is not evolving; it is a test for evolution in progress
