1 Genes Within Populations Chapter 20

2 Darwin: Evolution is descent with modification. Evolution: changes through time. 1.Species accumulate difference; 2.Descendants differ from their ancestors; 3.New species arise from existing ones. Genetic Variation and Evolution

3 Natural selection: proposed by Darwin as the mechanism of evolution. individuals have specific inherited characteristics; they produce more surviving offspring; the population includes more individuals with these specific characteristics; the population evolves and is better adapted to its present environment. Natural selection: mechanism of evolutionary change

4 Darwin’s theory for how long necks evolved in giraffes

5 Natural selection: mechanism of evolutionary change Inheritance of acquired characteristics: Proposed by Jean- Baptiste Lamarck. Individuals passed on physical and behavioral changes to their offspring; Variation by experience…not genetic; Darwin’s natural selection: variation a result of preexisting genetic differences.

6 Lamarck’s theory of how giraffes’ long necks evolved.

7 Godfrey H. Hardy: English mathematician. Wilhelm Weinberg: German physician. Concluded that: The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met: Hardy-Weinberg Principle

8 Five assumptions : 1.No mutation takes place 2.No genes are transferred to or from other sources 3.Random mating is occurring 4.The population size is very large 5.No selection occurs Hardy-Weinberg Principle

9 Calculate genotype frequencies with a binomial expansion: (p+q) 2 = p 2 + 2pq + q 2 p = individuals homozygous for first allele; 2pq = individuals heterozygous for both alleles; q = individuals homozygous for second allele; because there are only two alleles: p plus q must always equal 1 (that is the total population) Hardy-Weinberg Principle

10 Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations.

11 Agents of Evolutionary Change Mutation: A change in a cell’s DNA. –Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. –Ultimate source of genetic variation. A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population.

12 Gene flow: A movement of alleles from one population to another. Powerful agent of change; Tends to homogenize allele frequencies.

13 Agents of Evolutionary Change Nonrandom Mating: mating with specific genotypes: –Shifts genotype frequencies –Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals –Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes

14 Genetic Drift Genetic drift: Random fluctuation in allele frequencies over time by chance. important in small populations –founder effect - few individuals found new population (small allelic pool) –bottleneck effect - drastic reduction in population, and gene pool size

15 Genetic Drift: A bottleneck effect

16 Selection Artificial selection: a breeder selects for desired characteristics.

17 Selection Natural selection: environmental conditions determine which individuals in a population produce the most offspring. 3 conditions for natural selection to occur: –Variation must exist among individuals in a population; –Variation among individuals must result in differences in the number of offspring surviving; –Variation must be genetically inherited.

18 Pocket mice from the Tularosa Basin Selection

19 Selection to match climatic conditions Selection for pesticide resistance

20 Fitness and Its Measurement Fitness is a combination of: –Survival: how long does an organism live –Mating success: how often it mates –Number of offspring per mating that survive Body size and egg-laying in water striders.

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

22 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

23 Maintenance of Variation Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population. –Negative frequency-dependent selection: rare phenotypes are favored by selection. –Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population. Strength of selection changes through time.

24 Negative frequency - dependent selection Maintenance of Variation Positive frequency- dependent selection.

25 Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time. Galápagos Islands ground finches –Wet conditions favor big bills (abundant seeds); –Dry conditions favor small bills; Maintenance of Variation Heterozygotes may exhibit greater fitness than homozygotes.

26 Homozygous dominant phenotype: no anemia; susceptible to malaria. Heterozygous phenotype: no anemia; less susceptible to malaria Maintenance of Variation Frequency of sickle cell allele.

27 Disruptive selection for large and small beaks in black- bellied seedcracker finch of west Africa. Maintenance of Variation Disruptive selection: acts to eliminate intermediate types.

28 Directional selection for negative phototropism in Drosophila. Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes

29 Stabilizing selection for birth weight in humans. Maintenance of Variation Stabilizing selection: acts to eliminate both extremes.

30 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

31 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

32 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

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

34 Phenotypic variation in insect ommatidia. Experimental Studies