Changing Allele Frequencies

Allelic Frequencies Change When There Is: Non-random mating Gene flow/Migration Genetic drift Mutation Natural selection

Non-random Mating We marry people similar to ourselves 80% of the time 1/3 of all marriages occur between people who were born <10 miles apart Certain individuals contribute more to the next generation than others –Prize bull semen –Chinese immigrant to South African with rare dominant mutation that causes teeth to fall out by age 20 had 7 wives –Albinism in Hopi Indians –Genocide by rape in Darfur –Consanguinuity (a.k.a. inbreeding) –Endogamy =marriage within a community

Non-random Mating

Gene Flow/Migration Individuals may join the migration Individuals may mate within other populations along the way Immigrants introduce alleles and emigrants remove them A cline exists when neighboring populations have differing allelic frequencies –Geographical barriers (e.g. mountains) –Language barriers

Gene Flow/Migration

Genetic Drift Chance sampling of alleles from the whole population Due to: –Founder effect –Population bottleneck

Founder Effect Small group leaves the population to start a new settlement New colony may have different allele frequencies than the original population Ex: Small religious sect community in Utah/Arizona –Founded by 2 individuals in the 1930’s –50% of all fumarate deficiency Mental retardation, seizures, coma

Population Bottleneck Many members of a population die and only a few are left to re-populate Much more restricted gene pool than original population Ex: Pingalapese people of the East Caroline Islands in Micronesia –Typhoon wiped out all but 9 males and 10 females –Autosomal recessive achromatopsia very prevalent Color-blindnesss, nearsightedness, and cataracts

Genetic Drift

Mutation Major and continual source of genetic variation One allele changes into another Must occur in gamete in order to affect future generations and allele frequencies

Genetic Load Mutations that lead to lethal traits are often eliminated from the gene pool, however, some mutant alleles can persist in heterozygotes Genetic load refers to the collection of these deleterious alleles in the population –Each of us has 5-10 recessive lethal alleles

Mutation

Natural Selection Guided by changes in the environment Individuals with certain phenotypes are more likely to survive and have reproductive success Ex: Antibiotic resistance –Antibiotics select for those bacteria which are antibiotic-resistant

Natural Selection

Forces that Alter Allele Forces that Alter Allele Frequencies