1 Genes Within Populations Chapter 20

2 Darwin’s Theory

22 years old! Darwin’s Theory

(a) Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp. (c) Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground. (b) Insect eater. The green warbler finch (Certhidea olivacea) uses its narrow, pointed beak to grasp insects. Darwin’s Theory

Warbler finch Woodpecker finch Small insectivorous tree finch Large insectivorous tree finch Vegetarian tree finch Cactus finch Sharp-beaked finch Small ground finch Medium ground finch Large ground finch Insect eaters Bud eater Seed eaters Cactus eater Warbler finch Tree finches Ground finches Ancestral species Descendant species Darwin’s Theory

Correlation of species to food source Adaptive radiation Rapid speciation: new species filling niches because they inherited successful adaptations Seed eaters Flower eaters Insect eaters

7 Darwin (1859) descent with modification (current sp. descended from the same ancestral sp.) Genetic Variation and Evolution

8 Evolution (1930s) all changes that have transformed life through time  sp. accumulate difference  descendants differ from their ancestors  new sp. arise from existing ones Genetic Variation and Evolution

9 Natural selection mechanism of evolution  ind. have sp. inherited characteristics  some prod. more surviving offspring  pop. begins to include more ind. with these sp. characteristics  pop. evolves and is better adapted to its env. Natural Selection

10

11

12 Godfrey H. Hardy: English mathematician Hardy-Weinberg Principle Wilhelm Weinberg: German physician HOW DO WE KNOW IF EV. IS OCCURING?

13 Hardy-Weinberg Principle Hypothetical situation: nonevolving pop. Serves as null hypothesis Orig. proportions of the genotypes in a pop will remain constant from gen. to gen.

14 1.No mutation takes place 2.No genes are transferred to or from other sources (gene flow) 3.Random mating is occurring 4.The population size is very large (prevents genetic drift) 5.No selection occurs Hardy-Weinberg Principle

15 Calculate genotype frequencies with a binomial expansion (p+q) 2 = p 2 + 2pq + q 2 p 2 = individuals homozygous for 1 st allele 2pq = individuals heterozygous for both alleles q 2 = individuals homozygous for 2 nd allele because there are only 2 alleles: p + q must always = 1 Hardy-Weinberg Principle

16 Hardy-Weinberg Principle

17 Hardy-Weinberg eq. is used to predict allelic freq. in subsequent gen. Hardy-Weinberg Principle

18 A pop. not in Hardy-Weinberg equilibrium indicates that 1 or more of the 5 ev. agents are operating in a pop.

19 Agents of Evolutionary Change 1.Mutation change in a cell’s DNA –ultimate source of genetic variation –rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles

20

21 Agents of Evolutionary Change 2.Gene flow movement of alleles from one population to another –powerful agent of change –can homogenize alleles btwn. pop.

22

23 Agents of Evolutionary Change 3.Nonrandom Mating mating with sp. genotypes –shifts genotype freq. –assortative mating doesn’t change freq. of ind. alleles; increases proportion of homozygous ind. –disassortative mating phenotypically diff. ind. mate; produces excess of heterozygotes

24 Assortative Disassortative

25 Genetic Drift 4.Genetic drift random fluctuation in allele freq. over time by chance –important in small populations founder effect - few ind. found new pop. (small allelic pool)

26 Plolydactyly in Amish Pop.

27 Genetic Drift bottleneck effect - drastic reduction in pop. and gene pool size (ex: endangered sp.)

28

29 Selection 5.Selection –Two types: 1)Artificial selection: a breeder selects for desired characteristics

30 Selection 2)Natural selection env. cond. determine which ind. in a pop. prod. most offspring only agent that produces adaptive change –3 conditions: 1.variation must exist among ind. in a pop. 2.variation among ind. must result in diff. # of offspring surviving 3.variation must be genetically inherited

31 Pocket mice from the Tularosa Basin Selection to Avoid Predators

32 Selection to Match Climatic Conditions

33 Selection for Antibiotic Resistance

34 Selection for Pesticide Resistance

35 Fitness and Its Measurement Fitness phenotype with greater fitness usually increases in freq. –combination of: survival: how long does an organism live mating success: how often it mates # of offspring/mating that survive

36 Body size and egg-laying in water striders Fitness and its Measurement

37 Interactions Among Evolutionary Forces Mutation and genetic drift may counter selection The magnitude of drift is inversely related to population size

38 Gene flow may promote or constrain evolutionary change –Spread a beneficial mutation –Impede adaptation by continual flow of inferior alleles from other populations Extent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow –High in birds & wind-pollinated plants –Low in sedentary species Interactions Among Evolutionary Forces

39 Degree of copper tolerance Interactions Among Evolutionary Forces

40 Maintenance of Variation Frequency-dependent selection depends on how freq. or infreq. a phenotype occurs in a pop. –Neg. freq.-dependent selection rare phenotypes are favored

41 Maintenance of Variation –Positive freq.-dependent selection: common phenotypes are favored

42 Oscillating selection selection favors 1 phenotype at 1 time, and a diff. phenotype another time –env. changes cause –Ex: Galápagos Islands finches wet conditions favor big bills dry conditions favor small bills Maintenance of Variation

43 Maintenance of Variation

44 Heterozygote advantage –heterozygotes may exhibit greater fitness than homozygotes –keep deleterious alleles in a population –Example: Sickle cell anemia homo. rec. : severe anemics, no malaria homo. dom.: no anemia, malaria hetero.: no anemia, no malaria Maintenance of Variation

45 Maintenance of Variation Frequency of sickle cell allele

46 Disruptive selection: acts to eliminate intermediate types Maintenance of Variation

47 ex: large and small beaks in black-bellied seedcracker finch of west Africa Maintenance of Variation

48 Directional selection: acts to eliminate 1 extreme from an array of phenotypes Maintenance of Variation

49 ex: directional selection for negative phototropism in Drosophila Maintenance of Variation

50 Stabilizing selection: acts to eliminate both extremes Maintenance of Variation

51 ex: stabilizing selection for birth weight in humans Maintenance of Variation

52 Experimental Studies of Natural Selection In some cases, evolutionary change can occur rapidly Evolutionary studies can be devised to test evolutionary hypotheses Guppy studies (Poecilia reticulata) in the lab and field –Populations above the waterfalls: low predation –Populations below the waterfalls: high predation

53 High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age. Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories. Experimental Studies

54 The evolution of protective coloration in guppies Experimental Studies

55 The laboratory experiment –10 large pools –2000 guppies –4 pools with pike cichlids (predator) –4 pools with killifish (nonpredator) –2 pools as control (no other fish added) –10 generations Experimental Studies

56 The field experiment –Removed guppies from below the waterfalls (high predation) –Placed guppies in pools above the falls –10 generations later, transplanted populations evolved the traits characteristic of low-predation guppies Experimental Studies

57 Evolutionary change in spot number Experimental Studies

58 The Limits of Selection Genes have multiple effects –Pleiotropy: sets limits on how much a phenotype can be altered Evolution requires genetic variation –Thoroughbred horse speed –Compound eyes of insects: same genes affect both eyes –Control of ommatidia number in left and right eye

59 Selection for increased speed in racehorses is no longer effective Experimental Studies

60 Phenotypic variation in insect ommatidia Experimental Studies