Evolution of Populations AP Biology Crosby High School

Modern Evolutionary Synthesis Modern Synthesis Emphasize importance of population as evolutionary unit Natural selection most important mechanism of evolution Gradualism explains large changes from accumulated small changes Population Genetics Emphasizes extensive genetic variability Recognized quantitative characteristics

Pop. Gen. Terminology Population: local group of individuals belonging to the same species Species: group of populations whose individuals have potential to interbreed and produce fertile offspring Gene Pool: total aggregate of genes in population at any given time Fixed gene pool: all individuals homozygous for an allele

Hardy-Weinberg Assumptions Equations Very large pop. Size No migration No net mutations Random mating No natural selection Equations p + q = 1 p2 + 2pq + q2 = 1

Causes of Microevolution Genetic Drift Bottleneck Effect Results from disasters and leads to little variability Founder Effect Huevos a doce Natural Selection Moths Gene Flow Migration Mutation

Bottleneck Effect

Genetic Variation w/in Populations Polymorphism More than one form of a characteristic Not quantitative traits Measuring Genetic Variation Gene Diversity % Heterozygous Humans: 14% Nucleotide Diversity % Difference in nucleotides of two individuals Humans: 0.1%

Genetic Variations Btw. Populations Geographic Variations: Result from different environments Cline: Graded change in a trait along geographic axis

Genetic Variation Mutation Sexual Recombination Diploidy Balanced Polymorphism Heterozygote advantage Sickle Cell carriers are advantageous Frequency-dependent Selection Two species affect each others evolution

Fitness Evolutionary: relative contribution an individual makes to the gene pool Darwinian: Contribution an individual makes to the gene pool of the next generation relative to the contributions of another individual Relative: Contribution of genotype to the next generation compared to other genotypes

Relative Fitness Most favored genotype set to a value of 1 Less favored genotypes are given values based on how much offspring they produce If they produce 80% as many offspring as the most favored genotype then Relative fitness = 0.8 Sterile individuals have relative fitness of 0.0

Modes of Selection Directional Selection: common during periods of environmental change or migration Diversifying Selection: favors both extremes over the intermediate genotype Stabilizing Selection: Acts against the extremes keeping the status quo Birth weight (3-4 kg)

Modes of Selection

Sexual Selection Benefits of sex Sexual Dimorphism: Difference in appearance between the sexes Intrasexual Selection: Direct competition between males of the same sex Rams Intersexual Selection: Individual of one sex are choosy in selecting a mate Peacocks

Nobody’s Perfect Evolution is limited by historical contraints Adaptations are often compromises Not all evolution is adaptive Selection can only edit existing variations