Population genetics. Population genetics provides a foundation for studying evolution How/Why?

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1 Population genetics 1.Definition of microevolution 2.Conditions for Hardy-Weinberg equilibrium 3.Hardy-Weinberg equation where it comes from and what it means 4.The five conditions for equilibrium in more detail

2 Population genetics Population genetics provides a foundation for studying evolution How/Why? Definition of Microevolution: The change in allele/genotype frequencies in a population over time

3 Population genetics Population genetics provides a foundation for studying evolution How? Definition of Microevolution: The change in allele/genotype frequencies in a population over time A population is the unit of evolution Natural selection acts on the individual

4 Population genetics Population genetics provides a foundation for studying evolution How? Mendel s findings about genetics were a key addition to Darwin s theory of evolution by natural selection Why? Mendel showed that inheritance is particulate, and subsequently it was shown that this type of inheritance can preserve the variation on which natural selection acts.

5 What makes a population evolve? In other words, what makes genotype and allele frequencies in a population change over time?

6 What makes a population evolve? 1. Genetic drift 2. Mutation 3. Natural selection 4. Gene flow (migration) 5. Non-random mating

7 A population won t evolve if there is an infinitely large population (no genetic drift) no change from one allele to the other (no mutation) Equality in genotype viability (no natural selection no migration (no gene flow) Equality in mate choice (random mating) then, the population is said to be in Hardy-Weinberg equilibrium

8 Allele frequencies remain the same generation after generation if only segregation and recombination occur.

9 Here goes a white box with white letters that don t mean Here goes a white box with white letters that don t mean anything.

10 The basis of H-W theorem p is the proportion of allele one q is the proportion of allele two Because the probability of two gametes with allele one joining is p x p, then p 2 = frequency of homozygous individuals for allele one

11 The basis of H-W theorem p is the proportion of allele one q is the proportion of allele two p 2 = frequency of homozygous individuals for allele one Because the probability of two gametes with allele two joining is q x q, then q 2 = frequency of homozygous individuals for allele two

12 The basis of H-W theorem p is the proportion of allele one q is the proportion of allele two p 2 = frequency of homozygous individuals for allele one q 2 = frequency of homozygous individuals for allele two Because the probability of a gamete with allele one joining a gamete with allele two is p x q x 2, then 2pq = frequency of heterozygotes

13 The basis of H-W theorem p is the proportion of allele one q is the proportion of allele two p 2 = frequency of homozygous individuals for allele one q 2 = frequency of homozygous individuals for allele two 2pq = frequency of heterozygotes

14 Derivation of the Hardy-Weinberg equation (p + q) Allele frequencies of male gametes (p + q) = Allele frequencies of female gametes p 2 +2pq+q 2 Genotype frequencies in the next generation p 2 +2pq+q 2 = 1

15 Important to take into account: p IS NOT always the dominant and q the recessive allele p IS NOT always higher than q

16 Genetic drift Online model for Driftworms

17 Genetic drift: Causes of small populations 1. Bottleneck effect An evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing Population size Time

18 1. Bottleneck effect 2. Founder effect When a small part of a population moves to a new locale, the genes of the "founders" of the new society are disproportionately frequent in the resulting population. Example: Ellis-van Creveld syndrome among the Amish culture Genetic drift: Causes of small populations

19 Mutations The only process that gives origin to genetic diversity de novo New genes and new alleles originate only by mutations can change allele frequency Other processes may change the phenotype but not the allele frequency: Crossing over Independent assortment Random fertilization Phenotypic variation results from recombinational shuffling of existing alleles cannot change allele frequency

20 Concept check Of all the mutations that occur, why do only a small fraction become widespread in a gene pool? 2. How does sexual recombination produce variation?

21 Natural selection Some alleles will be preferentially represented in the next generation Traits that result in differential success in reproduction Includes traits that increase survival an individual that lives longer will reproduce more than a shorterlived one And traits that help the individual reproduce more often sexual selection Leads to adaptive variation Genetic variation is the raw material for natural selection Natural selection will act upon the variation that is already there

22 Migration Causes gene flow New alleles enter or leave the population because individuals migrate Will significantly change allele frequency if certain alleles tend to migrate more often than others This is called gene flow

23 Non-random mating Inbreeding Assortative mating: E.g. Snow geese - Birds from white families usually chose white mates, birds from blue families usually chose blue mates, and birds from mixed families chose mates of either color. Both processes tend to increase homozygotes in the population

24 Of the five conditions for H-W equilibrium, which ones change allele fequency, and which ones change phenotypic frequency? Mutation (negligible in most pop) Natural selection Migration Genetic drift Alelle freq Non-random mating Phenot freq