Evolution of Populations

1. Evolution of Populations

2. The Smallest Unit of Evolution • Natural selection acts on individuals, but only populations evolve • Genetic variations contribute to evolution

3. Population genetics • Population genetics • study of how populations change genetically over time • Mendelian genetics with the Darwinian theory • populations as units of evolution

4. Gene Pools and Allele Frequencies • Population • localized group of individuals capable of interbreeding and producing fertile offspring • gene pool • total aggregate of genes in a population at any one time • all gene loci in all individuals of the population

5. The Hardy-Weinberg Theorem • 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 • preservation of genetic variation in a population

6. Hardy-Weinberg Equilibrium • 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

7. Hardy-Weinberg Equilibrium • 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 • And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

9. Generation 1 X CRCR CWCW genotype genotype Plants mate Generation 2 All CRCW (all pink flowers) LE 23-4 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 Generation 4 25% CWCW 25% CRCR 50% CRCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

10. Evolutionary Change • Three major factors alter allele frequencies and bring about most evolutionary change: • Mutations • Natural selection • Nonrandom Mating • Genetic drift • Gene flow

11. Variations that make Natural Selection Possible • Mutation • changes in the nucleotide sequence of DNA • new genes and alleles to arise • Point Mutations • change in one base in a gene • usually harmless • may impact on phenotype

12. Mutations • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Gene duplication is nearly always harmful

13. Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

14. 3 conditions for natural selection to occur and to result in evolutionary change • Variation must exist among individuals in a population • Variation among individuals must result in differences in the number of offspring surviving in the next generation • Variation must be genetically inherited

15. Sexual Recombination • far more important than mutation • produces the genetic differences that make adaptation possible

16. Nonrandom mating • Assortative mating • Phenotypically similar individuals mate • Increases proportion of homozygous individuals • Disassortative mating • Phenotypically different individuals mate • Produces excess of heterozygotes

17. Genetic Drift • The smaller a sample, the greater the chance of deviation from a predicted result • allele frequencies fluctuate unpredictably from one generation to the next • reduces genetic variation through losses of alleles

18. Genetic Drift • The Bottleneck Effect • sudden change in the environment that may drastically reduce the size of a population • gene pool may no longer be reflective of the original population’s gene pool

19. Genetic Drift • The Founder Effect • a few individuals become isolated from a larger population • affects allele frequencies

20. Gene Flow • genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes • gain or loss of alleles • reduce differences between populations over time

21. A Closer Look at Natural Selection • From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations

22. Evolutionary Fitness • Misleading • “struggle for existence” • “survival of the fittest” • Fitness • contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Relative fitness • contribution of a genotype to the next generation, compared with contributions of alternative genotypes for the same locus

23. Directional, Disruptive, and Stabilizing Selection • Selection favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection: • Directional • favors individuals at one end of the phenotypic range • Disruptive • favors individuals at both extremes of the phenotypic range • Stabilizing • favors intermediate variants and acts against extreme phenotypes

25. The Preservation of Genetic Variation • Diploidy • maintains genetic variation in the form of hidden recessive alleles • Balancing selection • natural selection maintains stable frequencies of two or more phenotypic forms

26. Heterozygote Advantage • Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes • Natural selection will tend to maintain two or more alleles at that locus • Sickle cell and malaria

27. Sexual selection • natural selection for mating success • sexual dimorphism • differences between the sexes in secondary sexual characteristics • Intrasexual selection • competition among individuals of one sex for mates of the opposite sex • Intersexual selection • individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex

28. Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations