The Evolution of Populations

Overview the smallest unit of evolution True or False: – Organisms evolve, in the Darwinian sense, during their lifetimes – Individuals evolve – Populations evolve – Genetic variations in populations contribute to evolution

Gene Pools and Allele Frequencies Population- a localized group of individuals that are capable of interbreeding and producing fertile offspring. Gene pool- the aggregate of genes in a population at any one time Thus a species may have two populations… ex. Caribou populations in the Yukon Fixed allele- all individuals are homozygous, only one allele exists in the population for that gene

Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution- change in the genetic makeup of a population from generation to generation Different from Darwin’s definition of evolution Darwin explanation of evolution considered unsatisfactory because did not consider how the heritable variations required for natural selection appear in populations or how organisms transmit these variations to their offspring

The Modern Synthesis Population genetics- the study of how populations change genetically over time Modern synthesis- a comprehensive theory of evolution that integrated ideas from many other fields. Mendelian genetics Darwinian theory of evolution through natural selection

Hardy- Weinberg Theorem This theorem states that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation , provided that only Mendelian segregation and recombination of alleles are at work. Involves gene pools that are not evolving Mendelian inheritance preserves genetic variation from one generation to the next Hardy-Weinberg equilibrium describes a population in which random mating occurs It describes a population where allele frequencies do not change

(p+q) X (p+q) = (p2+2pq+q2) Allele frequencies allele genotype Of male gametes frequencies frequencies of female in next generation gametes

Proportionally at a locus with with two alleles, the three genotypes will equal one: p2+2pq+q2=1 Meaning a population which continued to mate randomly generation after generation, would have allele and genotype frequencies which remained constant

For a Population to be in Hardy- Weinberg Equilibrium it must follow the five following standards

The population is large There is no gene flow Meaning there’s no migration of individuals or gametes into or out of the population Mutations do not alter the gene pool Mating is random All individuals are equal in reproductive success Meaning natural selection is not occurring within the population

When population deviate from Hardy- Weinberg equilibrium, the gene pool changes, and evolution occurs 4 main causes of evolutionary change are: 1) Genetic drift 2) Gene flow 3)Natural selection (only contributes to adaptive evolution) 4) Mutation (only over time)