Intro to evolution week 8 notes
Intro to evolution week 8 notes BZ 220
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This 4 page Class Notes was uploaded by Courtney Parks on Monday March 28, 2016. The Class Notes belongs to BZ 220 at Colorado State University taught by Angeloni, Lisa Maria in Fall 2016. Since its upload, it has received 162 views. For similar materials see Introduction to Evolution in Biology at Colorado State University.
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Date Created: 03/28/16
Week 8 Nonrandom mating Nonrandom mating- when individuals do not pair up randomly with respect the genotype of phenotype o Violates H-W equilibrium o Changes genotype frequencies o Not a mechanism of evolution Kinds of nonrandom mating o Assortative mating: similar individuals are more likely to mate than expected Inbreeding-mating among genetic relatives Phenotypic assertive mating- “likes attract” More homozygotes o Disassortative mating: dis-similar individuals are more likely to make than expected by chance Outbreeding: avoid mating with genetic relatives Phenotypic disassortative mating: “opposite attract” More heterozygotes o Selfing: an extreme case of inbreeding More homozygotes Genotype frequencies changed but allele frequencies didn’t Identical by descent- when two alleles are identical because both were inherited from a single copy of that allele in a common ancestor Inbreeding coefficient: a measure of inbreeding o F (always between 0 and 1) o The probability that an individual taken at random from a population will have alleles that are identical by descent o In extreme inbreeding everyone is identical by descent, no heterozygosity Inbreeding and genotype expectations o f(AA)= p (1-F)+pF o f(Aa)= 2pq(1-F) o f(aa)= q (1-F)+qF F=2 F=1 f(AA) p P f(Aa) 2pq 0 f(aa) q 2 q o Continued inbreeding reduces heterozygosity and F increases o If F is high, heterozygosity is low suggesting there has been lots of inbreeding Inbreeding plus selection o Inbreeding affects the outcome of selection An excess # of homozygotes are produced, which might have different fitness from heterozygotes Recessive deleterious alleles (normally hidden in heterozygotes) can come together in homozygous inbred offspring and be expressed o Sometimes inbred offspring have reduced fitness o Inbreeding depression Reduction in fitness of inbred individual relative to outbred individual, caused by expression of deleterious recessive alleles in homozygous genotypes Can depend on 1. Environmental stress- more severe under stressful conditions 2. Stage in life cycle- effects might not show up until later in life 3. Degree of inbreeding 4. How long a population has been inbreeding- long-term may lessen inbreeding depression if selection has already “purged” deleterious alleles o Purging- the removal of deleterious alleles by selection Overtime reduces the magnitude of inbreeding depression by removing deleterious alleles A population may no longer experience any negative effects of inbreeding o Avoidance of inbreeding Mate choice Genetically controlled self-incompatibility Dispersal o Phenotypic assortative mating “likes attract” If based on underlying genotypic similarity this can also lead to increase in homozygotes for that trait Conclusions o Changes genotype frequencies Violates H-W equilibrium Assortative mating-less heterozygotes Inbreeding common in small populations- loss of heterozygosity in small populations o Acting alone does not change allele frequencies Not a mechanism of evolution o Inbreeding plus selection Reduced fitness Decrease in frequency of deleterious alleles Conservation genetics Genetic concerns in conservation o Genetic drift: long-term loss of genetic variation 1. Decreased potential to evolve in response to environmental change 2. Susceptibility to disease o Inbreeding depression: short term fitness reduction 1. Decreased reproduction 2. Mutational meltdown Genetic drift o Rate of loss genetic variation depends on smallest population size and length of time at smallest population size o Genetic variation declines in small populations o Can reduce ability to respond to environment o Can increase disease susceptibility o Strategy 1: Do nothing If different alleles lost in different populations so genetic variation is maintains overall and can be reintroduced later Problem: only works if multiple populations persist o Strategy 2: Introduce alleles from another population Problem: may destroy local adaptations Inbreeding depression o Increases homozygosity, exposing deleterious alleles Short term: decrease in fitness Long term: Selection may purge o Mutational meltdown Snowball effect o Conservation Inbreeding depression seems to be commons and is worse under stress Inbreeding depression + genetic drift can cause population decline= mutational meltdown o Strategy 1: do nothing Problem: may not recover o Strategy 2: Introduce unrelated individuals Problem: if populations is already purging, this may add extra deleterious alleles which would create more inbreeding depression Minimum population sizes to avoid negative genetic effects o 50- prevent inbreeding depression o 500- maintains genetic variation (as drift decreases it) o 5000- maintain genetic variation (as drift and selection decrease it)
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