Evolution of Populations

1. Evolution of Populations • Population = A localized group of organisms which belong to the same species. • Species = Actual or potentially interbreeding natural populations which are reproductively isolated from other groups. • Gene Pool = The total aggregate of genes in a population

2. Hardy-Weinberg Theorem • In the absence of other factors, the segregation and recombination of alleles during meiosis and fertilization will not alter the genetic makeup of a population. • Describes a non-evolving population • p2 + 2pq + q2 = 1 • Where p = frequency of A allele q = frequency of a allele

3. The Hardy-Weinberg theorem

4. The Hardy-Weinberg theorem

5. Question 2 A (p) = 0.7 b) If the frequency of the homozygous dominant genotype is 0.49, what is the frequency of the dominant allele. Let the dominant genotype be represented by AA…. AA = 0.49 … (A2 = 0.49)same as (p2 = 0.49) So what is the frequency of A (p) ??

6. Question 2 A = 0.4 c) If the frequency of the homozygous recessive genotype is 0.36, what is the frequency of the dominant allele. Let the recessive genotype be represented by aa…. aa = 0.36 … (a2 = 0.36) So …a = 0.6 Then what is A ?

7. Question 2 d) If the frequency of the homozygous dominant genotype is 0.49, what is the frequency of the homozygous recessive genotype. Let the dominant genotype be represented by AA…. AA = 0.49 … (A2 = 0.49) Then A = 0.7 so…a must = 0.3 Therefore aa = 0.3x0.3 = 0.09

8. Hardy-Weinberg Theorem • Conditions needed for H-W to work • Very large population size • Isolation from other populations • No mutations • Random mating • No natural selection Hardy-Weinberg animation

9. Microevolution

10. Microevolution • MICROEVOLUTION = Small scale evolutionary change represented by a generation-to-generation change in a population’s allele or genotype frequencies

11. Microevolution • Genetic drift • Gene flow • Mutation • Nonrandom mating • Natural selection Natural selection is adaptive. It leads to accumulation of favorable adaptations in a population

12. Genetic Drift • Changes in the gene pool of a small population due to chance • The larger the population…the less important is genetic drift • Most (but not all) natural populations are so large that the effect of genetic drift is negligible • Reduces overall genetic variability

13. Genetic drift

14. Genetic Drift - Bottleneck Effect • Drastic reduction in population size by some natural disaster which kills organisms nonselectively • The smaller population is now unlikely to represent the genetic makeup (diversity) of the original population • Some alleles will be overrepresented, others will be absent. • Example: hunting to near extinction

15. The bottleneck effect: an analogy

16. Cheetahs, the bottleneck effect (Very low genetic diversity)

17. Founder Effect • When a few individuals colonize a new habitat. • The smaller the population size … the less likely the genetic makeup of the colonists will represent the gene pool of the large population that they left

18. Gene Flow • Migration of fertile individuals or transfer of gametes (wind blown pollen for example) between populations • Extensive gene flow can eventually group neighboring populations into a single population

19. Gene flow and human evolution

20. Mutations • Have very little immediate effect on a large population • Important to evolution since it is the original source of genetic variation which is the raw material for natural selection

21. Nonrandom Mating • Inbreeding • Assortive mating. Individuals mate with partners that are like themselves in phenotypic characters

22. Natural Selection • In any sexually reproducing population, variation among individuals exists (genetic variation) and some variants leave more offspring than others • Natural selection is the differential success in reproduction • Natural selection is the only cause of microevolution that is ADAPTIVE, since it accumulates and maintains favorable genotypes

23. Review of Microevolutionary changes Animation Link

24. Genetic Variation Review of genetic variation from sexual reproduction

25. Genetic Variation • Polygenic characters • Multiple loci involved-vary quantitatively (many intermediate phenotypes) within a population. Example = height • Discrete characters • Determined by a single locus • Polymorphism - when 2 or more forms of a discrete character are well represented in a population

26. Polymorphism in garter snakes

27. Genetic Variation • Geographic variation • This variation in alleles exists among populations of most species • Cline – a type of geographical variation that shows a graded change in some trait along a geographical feature (such as elevation)

28. Clinal variation in a plant

29. Genetic Variation • Generation of variation • Mutation • Point mutation – involves a single base pair in DNA • Chromosomal mutation – usually effect many gene locus and are almost always deleterious • Recombination • Nearly all genetic variation in a population results from new combinations of already existing alleles !

30. Genetic Variation • Maintenance of variation (how is it preserved?) • Diploid state hides some genetic variation from selection by the presence of recessive alleles in heterozygotes

31. Genetic Variation • Maintenance of variation (how is it preserved?) • Balanced Polymorphism = the ability of natural selection to maintain diversity in a population • Heterozygote advantage • Frequency dependent selection

32. Frequency-dependent selection in a host-parasite relationship

33. Frequency-dependent selection Polymorphism in sneetches (star-bellied and plain bellied forms) - a classic tale of frequency-dependent selection

34. Genetic Variation • Neutral Variation • Variations that confer no selective advantage/disadvantage

35. Natural Selection is Adaptive Fitness • Measured by the relative contribution an individual makes to the gene pool of the next generation Selection acts on phenotypes and can only act indirectly on genotypes

36. Modes of selection

37. Modes of Natural Selection • Stabilizing selection Favors intermediate variants by selecting against extreme phenotypes • Directional selection Favors variants to one extreme • Diversifying selection Opposite phenotypic extremes are favored over intermediate phenotypes

38.   Modes of selection

39. Sexual Dimorphism (Male peacock) Distinction between secondary sexual characteristics of males and females

40. Sexual selection and the evolution of male appearance

41. Evolution does not fashion perfect organisms • Adaptations are often compromises. An organism must be versatile enough to do many things. • Not all evolution is adaptive (example genetic drift in small populations) • Selection can only EDIT variations that exist. New alleles/genes are not formed by “mutation on demand”