Population Genetics Reconciling Darwin & Mendel

Darwin Darwin’s main idea (evolution), was accepted But not the mechanism (natural selection) –Scientists did not understand Darwin’s mechanism because there was no understanding of genetics Even once scientists grasped Mendel, genetics was viewed as an either/or –didn’t understand many traits are polygenic So how do you get the variation on which selection works?

Ideas About Evolution Orthogenesis –1920’s –saw evolution as a predictable progression to more & more elite forms of life Population Genetics –1930’s –reconciled Darwin & Mendel

Genetics of Populations Population –a localized group of individuals belonging to same species –The definition of a species not always clear Gene pool = The total genes in a population Evolution on the smallest scale occurs when the relative frequency of alleles in a population changes over a succession of generations = microevolution

Genetics of a Non-evolving Population The gene pool is in stasis This is described by Hardy-Weinberg Theorem: The frequencies of alleles in a population’s gene pool remain constant over the generations unless acted on by agents other than sexual recombination i.e. shuffling the deck has no effect on the overall genetic make-up of the population

The Hardy-Weinberg Theorem Example In pink flowers (A), is dominant over white flowers (a) 2 alleles for at this locus Sample 500 plants: 20 white flowers (aa) 480 pink [320 (AA); 160 (Aa)] Therefore there are 1000 genes for flower color in the population

Example (Continued) The dominant allele accounts for 800 of these: –[(320 x 2) + (160 x 1)] Therefore: –the frequency of A in the population = 80% –the frequency of a = 20%

Predicting Change How will genetic recombination during sexual reproduction affect the frequencies in the next generation? If mating is random: –the probability of picking 2 AA = (0.8 x 0.8) =.64 –the probability of picking 2 aa = (0.2 x 0.2) =.04 –and of heterozygotes = 2(0.8 x 0.2) = 0.32 There are 2 heterozygote combinations: aA & Aa sperm or egg

Hardy-Weinberg Equilibrium This shows that the alleles are present in the gene pool in the same frequencies as they were in the previous generation A: [ (0.32  2)] = 0.8 a: [ (0.32  2)] = 0.2 The gene pool is at equilibrium This is called Hardy-Weinberg equilibrium

The Hardy-Weinberg Equation This example is the simplest case: –2 alleles, one is dominant For this case: –if p = frequency of one allele –q the frequency of the other Then: p + q = 1 probability of AA = p 2 probability of aa = q 2 probability of Aa = 2pq Therefore: p 2 + 2pq + q 2 = 1

Uses of Hardy-Weinberg Thus you can calculate the frequency of a gene in a population if you know the frequency of the genotypes This is important in genetic disease counseling

Relevance to Evolution A population at genetic equilibrium does not evolve Hardy-Weinberg tells us what to expect in non-evolving populations Therefore it is a baseline for comparing actual populations where gene pools may be changing. Can determine if the population is evolving

Genetic Equilibrium Hardy-Weinberg equilibrium is maintained only if the population meets all 5 of the following criteria: –Very large population size –Isolation from other populations migration can effect the gene pool –No net mutations –Random matings –No natural selection no difference in reproductive success) Describes an ideal that never exists in nature

Altering Genetic Equilibrium For evolution to take place something must upset the genetic equilibrium of the population: Factors that change genetic equilibrium are: –Genetic drift –Migration (Gene flow) –Non-randon mating (Isolation) –Mutation –Natural selection

Genetic Drift Changes in gene frequency of a very small population due to chance Controlled by the laws of probability & chance Bottleneck effect –Chance sampling error due to small population Founder’s effect –a few individuals colonize a remote spot –causes drift

Illustrating Genetic Drift

The Bottleneck Effect

Gene Flow (Migration) Movement of organisms into or out of a population Takes their genes out of the gene pool Most populations are not completely closed –gain & lose alleles

Non-random Mating More apt to mate with close neighbors Promotes inbreeding Assortive mating –seek mate like self (i.e. size)

Isolation

Mutation A change in a gene An alteration of DNA The original source of variation Raw material on which natural selection works

Natural Selection If one type produces more offspring than another, upsets the balance of equilibrium There are three types of natural selection: –Stabilizing Selection –Disruptive Selection –Directional Selection