Population genetics reconciled Darwin and Mendel's ideas by showing how natural selection could act on variation present in populations. The Hardy-Weinberg theorem describes genetic equilibrium in a population where allele frequencies remain constant between generations unless disrupted by factors like genetic drift, migration, non-random mating, mutation, or natural selection. These disruptions to equilibrium allow for microevolution and populations to change over time through natural selection acting on genetic variation.

1. Population Genetics Reconciling Darwin & Mendel

2. 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?

3. 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

4. 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

5. 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

6. 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

7. 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%

8. 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 2 heterozygote combinations: aA & Aa sperm or egg

9. 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: [0.64 + (0.32  2)] = 0.8 a: [0.04 + (0.32  2)] = 0.2 The gene pool is at equilibrium This is called Hardy-Weinberg equilibrium

10. 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 = 2 pq Therefore: p 2 + 2 pq + q 2 = 1

11. 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

12. 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

13. 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

14. 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

15. 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

16. Illustrating Genetic Drift

17. The Bottleneck Effect

18. 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

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

20. Isolation

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

22. 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