The Evolution of Populations

1. The Evolution of Populations Population Genetics Causes of Microevolution Genetic Variations Natural Selection as the Mechanisms of Adaptive Evolution

2. Population Genetics • Synthesis of evolution and genetics • Emerged as a science in the 1930’s • Important because it reconciled Medelian genetics with Darwinian evolution • Population is the unit of evolution (populations evolve, individuals do not)

3. Key Terms • Population- same species, same place, same time • Species- group of populations that interbreed and produce fertile offspring • Gene pool-all of the genes in a population

4. Hardy-Weinberg Theorem • Nonevolving population • Diploid • Large population • Isolation from other populations • No net mutations • Random mating • No natural selection

5. Using the Hardy-Weinberg Theorem

6. Causes of Microevolution • Small scale evolution represented by a generational shift in a population’s relative allelic frequencies • Genetic drift • Gene flow • Mutation • Nonrandom mating • Natural selection

7. Genetic Drift • Changes in the gene pool due to chance • Bottleneck effect- population drastically reduced by natural disaster • Founder effect-when a few individuals colonize a new habitat

10. Gene Flow • The migration of fertile individuals, or the transfer of gametes between populations

11. Mutations • New mutation that is transmitted in gametes immediately changes the gene pool of a population by substituting one allele with another • Very little quantitative effect on large populations in a single generation

12. Nonrandom Mating • Increases the number of homozygous loci in a population • Inbreeding- individuals usually mate with close neighbors rather than more distant members; reduction of the heterozygotes • Assortive mating-individuals mate with partners that are like themselves in certain phenotypic characters

13. Natural Selection • Variations always exist, some variants leave more offspring that others, differential success in reproduction • Only adaptive form of microevolution

14. Genetic Variations-the Basis of Natural Selection • Polymorphism-two or more contrasting forms (morphs) are present in noticeable frequencies • Geographical variation-differing allelic frequencies in different populations • Cline-a type of geographical variation that is a graded change in some trait along a geographical transect (example: body size of North American mammals increases with increasing latitude)

15. Polymorphism

17. Mutation and Sexual Recombination • Mutations-produce new alleles • Point mutations-single base in DNA • Sexual recombination-produces new combinations with “old” alleles

18. Diploidy and Balanced Polymorphism • The diploid condition “hides” most mutations • Balanced polymorphism-counteracts the forces of natural selection (?) • Preserves variations by heterozygous advantage and frequency-dependent selection • Most variations are neutral

19. Fitness • Relative contribution a an individual makes to the gene pool of the next generation • Relative fitness- the contribution of a genotype compared to the contributions of alternative genotypes for the same locus • Selection coefficient-relative measure of selection against an inferior genotype

20. Patterns of Selection • Stabilizing selection- favors intermediate variants, selects against extremes, reduced phenotypic variations, stable environments • Directional selection- favors variants at one extreme, common to new habitat colonization, changing environment • Diversifying selection- opposite phenotypic extremes are favored, variable environmental conditions

21. Modes or patterns of selection

22. Sexual Selection • Sexual dimorphism- distinction between secondary sexual characteristics, size, color, antlers, manes • Male tends to be “showier”

23. Sexual Selection