Genes Within Populations

1. Genes Within Populations Chapter 20

2. Genetic Variation and Evolution • Darwin: Evolution is descent with modification. • Evolution: changes through time. • Species accumulate difference; • Descendants differ from their ancestors; • New species arise from existing ones.

3. Natural selection: proposed by Darwin as the mechanism of evolution. individuals have specific inherited characteristics; they produce more surviving offspring; the population includes more individuals with these specific characteristics; the population evolves and is better adapted to its present environment. Natural selection: mechanism of evolutionary change

4. Darwin’s theory for how long necks evolved in giraffes

5. Natural selection: mechanism of evolutionary change Inheritance of acquired characteristics:Proposed byJean-Baptiste Lamarck. • Individuals passed on physical and behavioral changes to their offspring; • Variation by experience…not genetic; • Darwin’s natural selection: variation a result of preexisting genetic differences.

6. Lamarck’s theory of how giraffes’ long necks evolved.

7. Godfrey H. Hardy: English mathematician.Wilhelm Weinberg: German physician.Concluded that:The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met: Hardy-Weinberg Principle

8. Hardy-Weinberg Principle Five assumptions : • No mutation takes place • No genes are transferred to or from other sources • Random mating is occurring • The population size is very large • No selection occurs

9. Hardy-Weinberg Principle Calculate genotype frequencies with a binomial expansion:(p+q)2 = p2 + 2pq + q2 • p = individuals homozygous for first allele; • 2pq = individuals heterozygous for both alleles; • q = individuals homozygous for second allele; • because there are only two alleles:p plus q must always equal 1 (that is the total population)

10. Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations.

11. Agents of Evolutionary Change A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population. • Mutation:A change in a cell’s DNA. • Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. • Ultimate source of genetic variation.

12. Gene flow:A movement of alleles from one population to another. • Powerful agent of change; • Tends to homogenize allele frequencies.

13. Agents of Evolutionary Change • Nonrandom Mating: mating with specific genotypes: • Shifts genotype frequencies • Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals • Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes

14. Genetic Drift • Genetic drift: Random fluctuation in allele frequencies over time by chance. • important in small populations • founder effect - few individuals found new population (small allelic pool) • bottleneck effect - drastic reduction in population, and gene pool size

15. Genetic Drift: A bottleneck effect

16. Selection • Artificial selection: a breeder selects for desired characteristics.

17. Selection • Natural selection: environmental conditions determine which individuals in a population produce the most offspring. • 3 conditions for natural selection to occur: • Variation must exist among individuals in a population; • Variation among individuals must result in differences in the number of offspring surviving; • Variation must be genetically inherited.

18. Selection Pocket mice from the Tularosa Basin

19. Selection to match climatic conditionsSelection for pesticide resistance

20. Fitness and Its Measurement • Fitness is a combination of: • Survival: how long does an organism live • Mating success: how often it mates • Number of offspring per mating that survive Body size and egg-laying in water striders.

21. Interactions Among Evolutionary Forces • Mutation and genetic drift may counter selection. • The magnitude of drift is inversely related to population size.

22. Interactions Among Evolutionary Forces • Gene flow may promote or constrain evolutionary change: • Spread a beneficial mutation; • Impede adaptation by continual flow of inferior alleles from other populations. • Extent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow: • High in birds & wind-pollinated plants; • Low in sedentary species.

23. Maintenance of Variation • Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population. • Negative frequency-dependent selection: rare phenotypes are favored by selection. • Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population. • Strength of selection changes through time.

24. Maintenance of Variation Positive frequency-dependent selection. Negative frequency - dependent selection

25. Maintenance of Variation • Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time. • Galápagos Islands ground finches • Wet conditions favor big bills (abundant seeds); • Dry conditions favor small bills; • Heterozygotes may exhibit greater fitness than homozygotes.

26. Maintenance of Variation • Homozygous dominant phenotype: no anemia; susceptible to malaria. • Heterozygous phenotype: no anemia; less susceptible to malaria Frequency of sickle cell allele.

27. Maintenance of Variation Disruptive selection: acts to eliminate intermediate types. Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa.

28. Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes Directional selection for negative phototropism in Drosophila.

29. Maintenance of Variation Stabilizing selection: acts to eliminate both extremes. Stabilizing selection for birth weight in humans.

30. Experimental Studies of Natural Selection • In some cases, evolutionary change can occur rapidly; • Evolutionary studies can be devised to test evolutionary hypotheses; • Guppy studies (Poecilia reticulata) in the lab and field • Populations above the waterfalls: low predation • Populations below the waterfalls: high predation

31. Experimental Studies • High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age. • Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.

32. The Limits of Selection • Genes have multiple effects: • Pleiotropy: sets limits on how much a phenotype can be altered. • Evolution requires genetic variation • Thoroughbred horse speed • Compound eyes of insects: same genes affect both eyes • Control of ommatidia number in left and right eye

33. Experimental Studies Selection for increased speed in racehorses is no longer effective.

34. Experimental Studies Phenotypic variation in insect ommatidia.