Population Genetics

1. Population Genetics

2. Natural selection acts on individuals • On phenotypes • only populations evolve • Genetic variations contribute to evolution • Microevolution • change in the genetic makeup of a population from generation to generation Overview: The Smallest Unit of Evolution

3. Population genetics • study of how populations change genetically over time • integrates Mendelian genetics with the Darwinian theory of evolution by natural selection The Modern Synthesis

4. gene pool is the total aggregate of genes in a population at any one time • gene pool consists of all gene loci in all individuals of the population Gene Pools and Allele Frequencies

5. MAP AREA CANADA ALASKA Beaufort Sea Porcupine herd range NORTHWEST TERRITORIES Fairbanks Fortymile herd range Whitehorse LE 23-3 ALASKA YUKON

6. describes a population that is not evolving • frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation • provided that only Mendelian segregation and recombination of alleles are at work • Mendelian inheritance preserves genetic variation in a population The Hardy-Weinberg Theorem

7. Generation 1 X CRCR CWCW genotype genotype Plants mate Generation 2 All CRCW (all pink flowers) 50% CW 50% CR gametes gametes come together at random Generation 3 25% CRCR 50% CRCW 25% CWCW 50% CR 50% CW gametes gametes come together at random LE 23-4 Generation 4 25% CWCW 25% CRCR 50% CRCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

8. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then • p2 + 2pq + q2 = 1 • p2 and q2represent the frequencies of the homozygous genotypes • 2pqrepresents the frequency of the heterozygous genotype • p + q = 1 • p represents the frequency of one allele (dominant) • q represents the frequency of the other allele (recessive)

9. The five conditions for non-evolving populations are rarely met in nature: • Extremely large population size • No gene flow • No mutations • Random mating • No natural selection

10. We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease Population Genetics and Human Health

11. Mutation • changes in the nucleotide sequence of DNA • cause new genes and alleles to arise • point mutation is a change in one base in a gene • usually harmless but may have significant impact on phenotype • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Gene duplication is nearly always harmful • Mutation rates are low in animals and plants • one mutation in every 100,000 genes per generation • Mutations are more rapid in microorganisms • sexual recombination • far more important than mutation in producing the genetic differences that make adaptation possible variation that makes evolution possible

12. Deletion Duplication Inversion Translocation

13. Three major factors alter allele frequencies and bring about most evolutionary change: • Natural selection • Genetic drift • Gene flow a population’s genetic composition

14. CWCW CRCR CRCR CRCR CRCR CRCW CRCW CRCR CRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCR CRCR CRCR CWCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CWCW CRCR CRCW CRCR CRCR CRCR CRCW CRCW CRCR CRCW LE 23-7 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Generation 3 p = 1.0 q = 0.0 Generation 2 p = 0.5 q = 0.5

15. Original population Bottlenecking event Surviving population LE 23-8

16. Genetic variation occurs in individuals in populations of all species It is not always heritable Genetic Variation

17. Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white LE 23-9

18. Phenotypic polymorphism • a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable • Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups Polymorphism

19. Heights of yarrow plants grown in common garden 100 Mean height (cm) 50 0 3,000 LE 23-11 Altitude (m) 2,000 Sierra Nevada Range Great Basin Plateau 1,000 0 Seed collection sites

20. Selection favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection: • Directional • Disruptive • Stabilizing Selection

21. Directional selection • favors individuals at one end of the phenotypic range • Disruptive selection • favors individuals at both extremes of the phenotypic range • Stabilizing selection • favors intermediate variants and acts against extreme phenotypes

22. Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population LE 23-12 Directional selection Disruptive selection Stabilizing selection

23. Diploidy • maintains genetic variation in the form of hidden recessive alleles • Balancing selection • occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population • leads to a state called balanced polymorphism • Heterozygote Advantage • maintains two or more alleles at that locus • Ex. sickle-cell allele • causes mutations in hemoglobin • also confers malaria resistance The Preservation of Genetic Variation

24. Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% LE 23-13 Distribution of malaria caused by Plasmodium falciparum (a protozoan) 7.5–10.0% 10.0–12.5% >12.5%

25. In frequency-dependent selection • fitness of any morph declines if it becomes too common in the population

26. On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). Parental population sample 0.6 Experimental group sample 0.5 LE 23-14 Phenotypic variation 0.4 Frequency- independent control 0.3 0.2 0 20 40 60 80 100 Generation number Plain background Patterned background

27. Sexual selection • natural selection for mating success • can result in sexual dimorphism • marked differences between the sexes in secondary sexual characteristics • Intrasexualselection • competition among individuals of one sex for mates of the opposite sex • Intersexual selection • occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex • Selection may depend on the showiness of the male’s appearance Sexual Selection