Download Allele Frequencies

Allele Frequencies
Introduction
• Darwin proposed natural selection
– Certain traits get passed on
• Genetics = change in allele frequencies to cause
evolution
• Genetic Factors + Natural selection =
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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)