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Points to remember about Natural Selection • Populations evolve, but individuals do not. I.E., Natural selection acts on the level of the individual, but populations are the smallest unit that can evolve. • Natural selection only works on heritable variations, not acquired traits. • Natural selection can only work with what it’s given. Variations are produced by different genetic mechanisms. • Natural selection is situational to a given environment in a given time and place. – No ideal organism. – Fitness of a given trait changes. Population Genetics Microevolution The historical context of Darwin’s life and ideas Evolution is…. • For Darwin (1859): Evolution is gradual change of heritable traits in a population across generations, eventually generating species over time. • For the Modern Evolutionary Synthesis (early 20th c.): Evolution is a change in allele frequency over time Population Genetics - Terms • Population: Localized group of individuals of the same species • Species: Group of populations whose individuals can potentially interbreed • Gene Pool: Total aggregate of genes in a population at one time • Allele: an alternative form of a gene • Homozygous: have identical alleles for a given trait (dominant or recessive) (e.g. AA or aa in a diploid) • Heterozygous: have >1 different alleles for a given trait (e.g. Aa or aA in a diploid) Why aren’t all individuals of one species identical? Genetic Diversity 1 1 Genetic Diversity is the product of… 1. Sexual reproduction 2. Genetic recombination 3. Mutation- key form of genetic variation in asexual reproduction, substrate for ‘new’ genetic material • Provides alternative alleles that may or may not be useful with changes in environment = raw material for evolution • R.A. Fisher's Fundamental Theorem of Natural Selection: the rate of natural selection is proportional to the degree of variation within a population The Modern Evolutionary Synthesis in action… Hardy-Weinburg Equilibrium p2 + 2pq + q2 = 1 Figure 23.3a The Hardy-Weinberg theorem Figure 23.3b The Hardy-Weinberg theorem The Modern Evolutionary Synthesis in action… Hardy-Weinburg Equilibrium p2 + 2pq + q2 = 1 Assumptions 1.Large population size 2.No gene flow 3.No mutation 4.Random mating 5.No selection The Modern Evolutionary Synthesis in action… • Microevolution (=shift in allele frequencies) happens when the H-W assumptions fail ! • Small population size –> Genetic Drift • Migration –> Gene Flow • Mutation • Natural Selection • Nonrandom mating Genetic Drift: random changes in allele (and genotype) frequencies from generation to generation due to sampling error Genetic Drift, part A: The bottleneck effect “Alleles” in original population “Alleles” remaining after bottleneck Bottleneck effect Cheetah www.petsdoc.com/pics/funpages/ wildlifephotos/cheetah.jpg Northern Elephant Seal Bottleneck Effect •http://www.icsi.berkeley.edu/~dbailey/gallery/image/elephant.jpg • Reduced to 20 individuals in 1896 • Now 30,000 individuals, with no detectable genetic diversity Genetic Drift, Part B: The founder effect • Change in allele frequencies when a new population arises from only a few individuals e.g., only a few fish are introduced into a lake e.g., only a few birds make it to an island www.fishbase.org Scorpaenidae Lionfish Pterois volitans Founder Effect •New Atlantic population, maybe from only 10 individuals Gene Flow • addition or removal of alleles due to individuals entering or leaving a population from another population • On average, two humans differ by 0.1% (1 in 1000 bp). • Is race a genetic reality? •NO: ~90% of genetic variation occurs within any single "race". LOTS OF OVERLAP •YES: only 10% of the variation can be explained by Asian, African, or European origin. Gene Flow Bluefin Tuna http://www.chambers-associates.org/Big-Marine-Fish/map_bluefin_spawning_areas.html Mutation • random, heritable changes in DNA that introduces new alleles into a gene pool Mutation = random, heritable changes in DNA sequences that introduce new alleles into a gene pool •members.tripod.com/~Alphacentaur/ X-men.JPG Modes of natural selection Modes of selection Directional selection Beak size selection in a Galápagos population of the medium ground finch Diversifying selection Diversifying selection Small bill size for soft seeds, large bills for hard seeds; selection against intermediate sized bills which crack both types of seeds inefficiently Stabilizing selection Stabilizing selection http://www.hummingbirds.net/gallery0.html What maintains genetic variation? • • • • • Diploidy Balancing selection Heterozygote advantage Frequency-dependent selection Neutral variation Sicle cell anemia is a good example of heterozygote advantage Sex The two-fold disadvantage of sex BUT, genetic diversity allows adaptation to changing environments Sexual Selection • non-random mating where individuals have sexually dimorphic traits that make them more attractive to mates Broad-billed hummingbirds •http://www.hummingbirds.net/ Sexual Selection - how can this be favored? Sexual selection and the evolution of male appearance