Allele Frequencies

Introduction Darwin proposed natural selection Certain traits get passed on Genetics = change in allele frequencies to cause evolution Genetic Factors + Natural selection = Mutations Gene flow Genetic drift Founder effect Bottle neck Nonrandom mating

Natural selection Increase or decrease in allele frequencies due to impact of environment

Mutations Introduction of new alleles that may provide a selective advantage Most mutations are deleterious “causing harm”

Gene Flow Describes movement of individuals between populations Removal of alleles from population (emigration) Introduction of alleles to population (immigration)

Genetic Drift Random increase or decrease of alleles 2 types: Due to chance Usually when populations are small Associated with flipping a coin 2 types: Founder effect Bottleneck

Founder Effect Allele frequencies in a group of migrating individuals differ from their population origin Ex. Germans Amish community possessed allele for polydactylism 200 years of reproductive isolation = over 8,000 in population had polydactylism (exceeding number of cases around the world) Amish Community in Pennsylvania Polydactylism = more than 5 fingers and toes

Bottleneck Population’s dramatic decrease in size Catastrophe, predation, disease, etc. Small population becomes severely vulnerable

Nonrandom Mating Individuals choose mates based on particular traits Can occur when mates choose only nearby individuals Certain, selected traits passed to next generation 2 main examples: Inbreeding Sexual selection

Inbreeding Individuals mate with relatives

Sexual Selection Females choose males based on attractive appearance or behavior

Sources of Variation in Populations Must be variation for natural selection Types: Mutations Sexual reproduction Diploidy Outbreeding Polymorphism

Mutations Provide raw material for new variation Rearranging existing alleles in new combinations Invent alleles that have never existed in gene pool Ex. antibiotic and pesticide resistance

Sexual Reproduction Creates individuals with new combinations of alleles Genetic recombination Crossing over Independent assortment Random joining of gametes Crossing over – exchange DNA between non-sister chromatids Independent assortment – metaphase I making random combinations of maternal and paternal chromosomes Random joining of gametes – fertilization contributes to diversity of gene combinations in zygote

Diploidy Presence of 2 copies of each chromosome in a cell Recessive allele Stored for future generations

Outbreeding Mating with unrelated partners Increase possibility of mixing different alleles and creating new allele combinations

Balanced Polymorphism Maintenance of different phenotypes in population 1 phenotype may provide best adaptation Alleles for advantageous traits increase Alleles for disadvantageous traits decrease Heterozygote advantage = resistance to malaria Hybrid vigor = hybrid of corn = resistance to disease and produce larger corn ears Minority advantage = least common phenotypes have advantage

Neutral Variation Not all variation has selective value Fingerprint patterns are neutral variation = everyone has different fingerprints Most cases, environment determines whether variation is neutral or selective Monocultures Overuse of antibiotics

Monocultures No genetic variation due to agriculture Potato famine

Overuse of Antibiotics Reduces variation in bacterial populations by eliminating individuals susceptible to disease Non-susceptible bacteria increase Domination of the population New outbreaks of diseases (harder to destroy)