Evolutionary Change in Populations

1. Evolutionary Change in Populations

2. A population’s gene pool • Includes all the alleles for all the loci present in the population • Diploid organisms have a maximum of two different alleles at each genetic locus • Typically, a single individual therefore has only a small fraction of the alleles present

3. Evolution of populations is best understood in terms of frequencies: • Genotype • Phenotype • Allele

4. Genotype frequencies for all 1000 individuals of a hypothetical population

5. Phenotype frequencies for all 1000 individuals of a hypothetical population

6. Allele frequencies for all 1000 individuals of a hypothetical population

7. Hardy-Weinberg Principle • Explains stability of successive generations in populations at genetic equilibrium • Essential to understanding mechanisms of evolutionary change

8. Genetic equilibrium requires • Random mating • No net mutations • Large population size • No migration • No natural selection

9. Hardy-Weinberg principle • Shows that if population is large, process of inheritance alone does not cause changes in allele frequencies • Explains why dominant alleles are not necessarily more common than recessive alleles

10. Hardy-Weinberg equation • p = frequency of dominant allele • q = frequency of the recessive allele:p + q = 1

11. The genotype frequencies of a population are described by the relationship p2 + 2pq+ q2 = 1 • p2 is frequency of homozygous dominant genotype • 2pq is frequency of heterozygous genotype • q2 is frequency of homozygous recessive genotype

12. (a) Genotype and allele frequencies

13. (b) Segregation of alleles and random fertilization

14. Microevolution • Intergenerational changes in allele or genotype frequencies within a population • Often involves relatively small or minor changes, usually over a few generations

15. Changes in allele frequencies of a population caused by microevolutionary processes: • Nonrandom mating • Mutation • Genetic drift • Gene flow • Natural selection

16. Nonrandom mating • Inbreeding • Inbreeding depression • Assortative mating • Both of these increase frequency of homozygous genotypes

17. Mutation • Source of new alleles • Increases genetic variability acted on by natural selection

18. Genetic drift • Random change in allele frequencies of a small population • Decreases genetic variation within a population • Changes it causes are usually not adaptive

19. Genetic drift • Bottleneck is a sudden decrease in population size caused by adverse environmental factors • Founder effect is genetic drift occurring when a small population colonizes a new area

20. Gene flow • Movement of alleles caused by migration of individuals between populations • Causes changes in allele frequencies

21. Natural selection • Causes changes in allele frequencies leading to adaptation • Operates on an organism’s phenotype • Changes genetic composition of a population favorably for a particular environment

22. Modes of selection • Stabilizing • Favors the mean • Directional • Favors one phenotypic extreme • Disruptive • Favors two or more phenotypic extremes

23. Modes of selection (a) No selection (b) Stabilizing selection

24. Modes of selection (c) Directional selection (d) Disruptive selection

25. Genetic variation in populations caused by • Mutation • Sexual reproduction • Allows new phenotypes

26. Methods of evaluating genetic variation • Genetic polymorphism • Balanced polymorphism • Neutral variation • Geographic variation

28. Balanced polymorphism: two or more alleles persist in a population over many generations • Heterozygote advantage • Frequency-dependent selection

29. Clinal variation in yarrow (Achillea millefolium)