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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 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 – 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) 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 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 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 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)