13.8 The gene pool of a nonevolving population remains constant over the generations Hardy-Weinberg equilibrium states that the shuffling of genes during sexual reproduction does not alter the proportions of different alleles in a gene pool p2 + 2pq + q2 or (homozygous dominant genotypes + heterozygous genotypes + homozyogous recessive genotypes) ** know for test (p + q = 1; p = dominant allele and q = recessive allele frequencies) To test this, let’s look at an imaginary, non- evolving population of blue-footed boobies Webbing No webbing Figure 13.8A

We can follow alleles in a population to observe if Hardy-Weinberg equilibrium exists Phenotypes Genotypes WW Ww ww Number of animals (total = 500) 320 160 20 Genotype frequencies 320/500 = 0.64 160/500 = 0.32 20/500 = 0.04 Number of alleles in gene pool (total = 1,000) 640 W 160 W + 160 w 40 w Allele frequencies 800/1,000 = 0.8 W 200/1,000 = 0.2 w Figure 13.8B

Recombination of alleles from parent generation W sperm p = 0.8 W egg p = 0.8 SPERM EGGS WW p2 = 0.64 w sperm q = 0.2 w egg q = 0.2 WW qp = 0.16 Ww pq = 0.16 ww q2 = 0.04 Next generation: Genotype frequencies 0.64 WW 0.32 Ww 0.04 ww Allele frequencies 0.8 W 0.2 w Figure 13.8C

Hardy-Weinberg practice

13.9 Connection: The Hardy-Weinberg equation is useful in public health science Public health scientists use the Hardy-Weinberg equation to estimate frequencies of disease-causing alleles in the human population Example: phenylketonuria (PKU) PKU occurs 1/10000 babies. Changes in the results demonstrate a potential issue if there is an increase in PKU occurrence.

13.10 Five conditions are required for Hardy-Weinberg equilibrium (H-W equilibrium rarely exists but . . . ) The population is very large The population is isolated (no migration of individuals, or gametes, into our out of the population Mutations do not alter the gene pool Mating is random All individuals are equal in reproductive success (no natural selection or fitness differences)

13.11 There are several potential causes of microevolution Genetic drift is a change in a gene pool due to chance The effect of a loss of individuals from a population is much greater when there are fewer individuals Genetic drift can cause the bottleneck effect (results from a disaster reducing population size, i.e. elephant seals) Original population Bottlenecking event Surviving population Figure 13.11A

or the founder effect (colonization of an area by a small number of individuals) Think Galapagos Islands or The Mutiny on the Bounty and Fletcher Christiansen with native islanders Figure 13.11B, C

Gene flow is gain or lose of alleles from population due to immigration or emigration Mutations are rare but occur constantly. Never usually directly responsible for evolutionary change but provide raw material for which other mechanisms of microevolution work Nonrandom mating is usually the case where choice of mater is important part of behavior but it does not typically change allele frequency or microevolution. Reproductive success leads to adaptive changes in gene pool

13.12 Adaptive change results when natural selection upsets genetic equilibrium Natural selection results in the accumulation of traits that adapt a population to its environment If the environment should change, natural selection would favor traits adapted to the new conditions Amount and kind of genetic variation in population can limit the degree of adaptation. Need to have the possible genes in the first place

13.13 Variation is extensive in most populations VARIATION AND NATURAL SELECTION 13.13 Variation is extensive in most populations Phenotypic variation may be environmental or genetic in origin But only genetic changes result in evolutionary adaptation Variation of phenotypes due to attributes of polygenic inheritance or combination of multiple genes for a given trait.

Many populations exhibit polymorphism and geographic variation Polymorphism = contrasting forms are present (often due to polygenic inheritance. Geographic variation = based on environment Stratification or clinal, smooth across population To measure Determine ave. percent of gene loci heterozygous Determine nucleotide diverstiy of DNA sequences Figure 13.13

13.14 Connection: Mutation and sexual recombination generate variation Parents A1 A1 A2 A3 Mutation is the ultimate source of all variation MEIOSIS A1 A2 A3 Gametes FERTILIZATION Offspring, with new combinations of alleles A1 A2 A1 A3 and Figure 13.14

Sexual recombination shuffles alleles in diploid organisms. Mutations normally harmful but they may help organism’s adaptation with a changing environment Sexual recombination shuffles alleles in diploid organisms. Independent assortment Crossing over Random fertilization Organisms reproducing sexually with longer life spans and sexual recombination necessary to increase the variation stemming from single mutations.

13.15 Overview: How natural selection affects variation Ancestral population is varied with characteristics suited for many environments Over generations, individuals with best characteristics for environment leave more offspring Natural selection tends to reduce variability in populations or less suited organisms produce less offspring The diploid condition preserves variation by “hiding” recessive alleles. Not seen because with a dominant allele. May take many generations to get rid of disadvantageous recessive alleles

Balanced polymorphism may result from the heterozygote advantage Heterozygote is favored over either homozgyote, so variation is maintained in the population. i.e. sickle cell, heterozygote is immune to malaria but does not have trait. Homozygote dominant, no sickle cell and no immunity from malaria.

13.16 Not all genetic variation may be subject to natural selection Some variations show neutral variation, providing no apparent advantage or disadvantage Example: human fingerprints Frequency changes due to genetic drift, not natural selection May be some unknown benefit Figure 13.16

13.17 Connection: Endangered species often have reduced variation Low genetic variability may reduce the capacity of endangered species to survive as humans continue to alter the environment Studies have shown that cheetah populations exhibit extreme genetic uniformity Thus they may have a reduced capacity to adapt to environmental challenges Rosenberg connection SSP Figure 13.17

Rosenberg connection SSP

Although inbreeding can reduce individual fitness and contribute to population extinction, gene flow between inbred but unrelated populations may overcome these effects. Among extant Mexican wolves (Canis lupus baileyi), inbreeding had reduced genetic diversity and potentially lowered fitness, and as a result, three unrelated captive wolf lineages were merged beginning in 1995. We examined the effect of inbreeding and the merging of the founding lineages on three fitness traits in the captive population and on litter size in the reintroduced population. We found little evidence of inbreeding depression among captive wolves of the founding lineages, but large fitness increases, genetic rescue, for all traits examined among F1 offspring of the founding lineages. In addition, we observed strong inbreeding depression among wolves descended from F1 wolves. These results suggest a high load of deleterious alleles in the McBride lineage, the largest of the founding lineages. In the wild, reintroduced population, there were large fitness differences between McBride wolves and wolves with ancestry from two or more lineages, again indicating a genetic rescue. The low litter and pack sizes observed in the wild population are consistent with this genetic load, but it appears that there is still potential to establish vigorous wild populations. http://rspb.royalsocietypublishing.org/content/274/1623/2365.full

13.18 The perpetuation of genes defines evolutionary fitness An individual’s Darwinian fitness is the contribution it makes to the gene pool of the next generation relative to the contribution made by other individuals Production of fertile offspring is the only score that counts in natural selection

13.19 There are three general outcomes of natural selection Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Stabilizing selection Directional selection Diversifying selection Figure 13.19

Explaining outcomes Stabilizing selection – narrows the range of population variability towards intermediate form. Usually occurs in stable environment Directional selection – moves from toward one of the extremes. Usually occurs during environmental change Diversifying selection – environmental conditions vary where both extremes favored over intermediate

13.20 Sexual selection may produce sexual dimorphism Sexual selection leads to the evolution of secondary sexual characteristics These may give individuals an advantage, look bigger, stronger in mating (and disadvantage, easier for predator to see) Figure 13.20A, B

13.21 Natural selection cannot fashion perfect organisms This is due to: historical constraints – can’t restart just because need for change adaptive compromises, one adaption might be advantage or later a disadvantage chance events, organism depends on nature, not usually the other way around. availability of variations, can’t change to something better if no genetic foundation to draw from – may result in extinction

Movie: Desire Questions?

13.22 Connection: The evolution of antibiotic resistance in bacteria is a serious public health concern The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria Example: Mycobacterium tuberculosis Figure 13.22