POPULATIONS GENETICS

Population genetics A theory of evolution that incorporates genetics into Darwin’s model. Genetic changes within a population: microevolution Focuses on the gene pool, total collection of alleles in a population Five factors can disrupt (change) a gene pool; genetic drift, gene flow, mutation, nonrandom mating, natural selection

Gene Pool Sexual reproduction alone will not change frequency If there are no outside agents affecting the population, frequency will be constant Hardy-Weinberg equilibrium

Hardy-Weinberg 1.Very large pop. 2.Isolated pop. 3.Mutations do not alter gene pool 4.Random mating 5.Equal reproductive success, no natural selection Five factors can disrupt HW; genetic drift, gene flow, mutation, nonrandom mating, natural selection

Genetic Drift Change in a gene pool due to random chance, selection not involved Bottleneck effect: dramatic reduction in population size, random, natural disasters –Remaining population will have a different gene pool than the original Founder effect: colonization of a new habitat by a small number of individuals-Galapagos Islands

Other factors Gene flow: immigration and emigration, gain or loss of alleles Mutation: raw material for evolution –Can generate new alleles, good or bad Nonrandom mating: many organisms choose their mates, either by similar traits, desired traits, or proximity Natural selection

What is a species Biological species concept: sexual reproduction –Problems: extinct organisms, self- fertilization, asexual reproduction, overlapping populations with limited gene flow Evolutionary species concept: represents a genealogy or lineage of descent, unique genotypic and phenotypic traits

Geographic isolation Populations—and there gene pools— become separated by geographic structures. Block to gene flow leads to different gene pools in the populations –Mountain ranges, land bridges, continental drift, canyon, rivers, valleys, islands, etc. –Changes can lead to reproductive isolation and new species

Geographic Isolation Adaptive radiation: numerous species from one common ancestor introduced to new and diverse environments; island chains

Reproductive Isolation Prezygotic barriers prevent mating or fertilization –Temporal isolation (mating season), habitat isolation, behavioral isolation (mating rituals), mechanical isolation Postzygotic barriers operate after fertilization –Hybrid inviability (death), hybrid sterility (mule) –Often due to chromosome or gene mismatches

New species can arise in a single generation without geographic isolation. Failure of meiosis can create diploid gametes –Fertilization leads to triploid or tetraploid organisms

Speciation timeframe Gradualism: traditional view, slow gradual change over long periods of time Punctuated equilibrium: long periods of stability with short bursts of change and speciation