Evolution & Natural Selection

1. Evolution & Natural Selection Natural selection Why do gene/trait/allele frequencies change? Types of change (aka natural selection)

2. Some things are not just left to chance . . .

3. Evolution /Natural Selection • The word “evolution” refers to how an entity changes through time • Darwin initially used the phrase “descent with modification” to explain the concept of evolution • ..all species arise from other, pre-existing species • But, they accumulate differences

4. Natural Selection • The process occurs in steps: • Gene variation must already exist (individuals can’t choose to adapt) • Variation is passed on to offspring • All populations ‘overproduce’ and only a few offspring ‘survive’ to reproduce • Its not random… who survives & reproduces • Inherited traits aiding survival increase chance of leaving offspring • Over time, such traits become prevalent

5. Natural Selection is Microevolution • Microevolution: • Evolutionary changes within a species, (small scale) • Changes in gene frequency within a species over time Some individuals having certain traits… produce more surviving offspring

6. Natural Selection . . . The basics of how it works. Long necked more successful… . . . Gradually more long necked giraffes

7. Evolution of insecticide resistance in insect populations

8. Evolution of drug resistance in HIV

9. Artificial selection: diverse vegetables derived from wild mustard

10. Evidence for Evolution • Evidence comes from: Fossil record Molecular record Anatomical record

11. Fossil Record • Provides… most direct evidence for macroevolution • Fossils are preserved remains, tracks, or traces of once-living organisms • Created when organisms become buried in sediment • Calcium in bone mineralizes • Arraying fossils according to age often provides evidence of successive evolutionary change

12. Evolution in the titanotheres Large blunt horns Small bony protuberance

13. Whale “missing links” • Fossils link all major groups The forms linking mammals to reptiles are particularly well known

16. Molecular Record • New alleles arise by mutations and • they come to predominance through favorable selection Evolutionary changes = continual accumulation of successful genetic changes • Distantly-related species accumulate greater # of differences compared to closely-related species Example: 146 aa hemoglobin b chain

17. DNA Hybridization:uses DNA similarity between species

18. An example of molecular homology. Human beta chain 0 Gorilla 1 Gibbon 2 Rhesus monkey 8 Dog 15 Horse, cow 25 Mouse 27 Gray kangaroo 38 Chicken 45 Frog 67 Lamprey 125 Sea slug (a mollusk) 127 Soybean 124 Hemoglobins • The numbers represent the number of amino acid differences between the beta chain of humans and the hemoglobins of the other species. In general, the number is inversely proportional to the closeness of kinship. • All the values listed are for the beta chain except for the last three, in which the distinction between alpha and beta chains does not occur. • The human beta chain contains 146 amino acid residues, as do most of the others.

19. DNA mutations reflect evolutionary divergence The greater the evolutionary distance The greater the number of amino acid differences

20. Cytochrome C • Same pattern seen for DNA sequences Changes occur at constant rate Called: Molecular Clock

21. Anatomical Record & Development • All vertebrates share basic set of developmental instructions

23. Anatomical record • Homologous structures • Have different structure and function • But all derived from same part of common ancestor The same basic bones are present in each forelimb Homology among vertebrate limbs

24. Convergent evolution Different species adapt in similar ways when challenged by similar opportunities

25. Anatomical record • Vestigial organs • Structures no longer in use! Example: The human appendix Apes… have a much larger appendix Involved in digestion, attached to gut tube contains bacteria to digest cellulose food Chimpanzee Pan troglodytes Bonobos Pan paniscus

26. Microevolution is the change in gene frequency of a species. Why do gene frequencies change? • Due to 5 evolutionary forces: Mutation Migration Genetic Drift Nonrandom mating Selection

27. Mutation • Errors in DNA replication • Ultimate source of new variation • Mutation rates • too low to significantly alter allele frequencies on their own

28. Migration…A very potent agent • Movement of individuals… from one population to another Immigration… in to Emigration… out of

29. Genetic Drift • Random loss of alleles… Occurs in small pop’s Founder… leaves Bottleneck… decrease Small group to new location Sudden decrease in pop due to natural forces

30. Founder Effect • Small population can cause extreme genetic drift • Small gene pools allow for rapid change • Example: polydactyly in Pennsylvania Amish

31. Bottleneck Effect

32. Nonrandom mating More or less… Frequently than expected by chance • Mating occurring… Inbreeding… With relatives = homozygosity Outbreeding… With non-relatives = heterozygosity

33. Types of Selection (natural) • Its statistical! Cant predict fate of single individual… But, can predict which kind will become more prevalent in a population Three types of selection: 1. Stabilizing… eliminate extremes 2. Disruptive… eliminate intermediates 3. Directional… eliminate single extreme

34. Three Kinds of Natural Selection 1. 2. 3.

35. Stabilizing Selection Increase in the frequency of the intermediate phenotype Intermediate weight = • Human infants… Survival rate Chicken eggs… Intermediate weight = Hatching success

36. Disruptive Selection Can open tough shells of large seeds • African seedcracker finch Large & small beaked Intermediate-beaked birds… at a disadvantage Unable to open large Too clumsy to open small More adept at handling small seeds

37. Male Lazuli Buntings showing variation in the brightness of their breeding plumage Low plumage Brightness score High plumage Brightness score

38. Drab vs Bright: Lazuli Bunting Survival Strategy Selection Plumage brightness From Greene et al 2000, Nature 407:1000-1003

39. Directional Selection Fly toward light… = Eliminated! • Drosophila flies… These flies were an extreme phenotype of the population Experiment…. Mate 20 times… Phototropism reduced Phototropic flies are far less frequent in the population Called: Phototropism

40. Sickle Cell Anemia: Stabilizing Selection Homozygosity = reduced life span ss = sickled cells SS = normal cells, but also more susceptible to malaria But if heterozygous (Ss) = less susceptible to malaria and not so many sickled cells . . .