Evolution Study Guide Genetic Drift Genetic drift Random changes in allele frequency within a population caused by chance events Differential reproductive success that just happens Sampling error in the production of zygotes from a gene pool Typically results in non adaptive evolution in which traits spread in a population that do not increase fitness Because it s a collective cumulative effect of RANDOM events it cannot produce adaptation Genetic drift is a random evolutionary force Genetic drift is sensitive to population size drift is strongest in small populations Drift results in random fixation of alleles and loss of heterozygosity As alleles become fixed there is an overall decline in heterozygosity as each population become homozygous for one or the other of the alleles o Fixation of alleles when every individual in a certain population has the same allele at a particular locus Once the frequency of the allele is at 100 every individual of this population has this fixed gene at a particular locus Fixation occurs rapidly in small populations more slowly in larger populations but it eventually occurs no matter the population size o Time to fixation Fixation time time it takes for a new allele to become fixed in a certain population Note Selection can modify these outcomes by either facilitating or preventing fixation extinction of alleles Unlike selection mutation or migration drift never maintains allelic diversity It always decreases it Violation of the Hardy Weinberg equilibrium principle the population must be large and mating is random Genetic drift violates this by acting on small populations in which random events radically alter allele frequencies Gamete sampling error a chance difference between the frequency of a trait in a subset of individuals from a population vs the frequency of the trait in the entire population will result in random changes in allele frequency from generation to generation Sampling error is larger for small samples than for big ones Founder effect when a small number of individuals start a new population the small size of a founder population almost guarantees that its allele frequencies will not be identical to the parent population Allele frequencies are likely to be different by chance from the source population Population bottleneck a population crash occurs and the surviving individuals are founders of the new population Type of founder event effect Allele frequencies after the crash will probably differ from those before the crash Heterozygosity The most common measure of genetic variation measure of the amount of heterozygosity across loci can be used as a general indicator of the amount of genetic variability Useful for diploid species every diploid cell has two alleles one inherited from each parent If an individual has two different alleles at a specific locus the individual is heterozygous at that locus Range is 0 1 Not sensitive to additional variation since the upper limit is 1 for any number of alleles How does genetic drift affect heterozygosity Expected H W heterozygosity H 1 p q As an allele reaches fixation or loss heterozygosity should decrease Process Increase or Decrease genetic diversity in a population Genetic Drift Decrease Natural Selection Migration Mutation Increase however stabilizing selection results in a decrease Increases Increase Non random Mating and Inbreeding Depression Assortative mating similar individuals are more likely to mate than expected by chance If assortative mating is based on genotypic similarity this can also lead to an increase in homozygotes loss of heterozygotes Phenotypic assortative mating likes attract People who had high self perceptions were more discriminating in mate choice People typically ranked an attribute as important in a mate if they perceived themselves as ranking highly in that attribute Population subdivision Certain genotypes are physically isolated Isolated populations may be small Inbreeding mating between genetic relatives A population experiencing inbreeding cannot be in Hardy Weinberg b c inbreeding change in Genotype frequencies Inbreeding selection inbreeding depression o Ratio of fitness between inbred and outbred individuals caused by expression of recessive deleterious alleles o 1 Wi Wo o Wi relative fitness of inbred individuals o Wo relative fitness of outbred individuals o is always between 0 and 1 o High means high inbreeding depression Common in small populations But small populations may not be able to avoid inbreeding depression w out migration so all individuals will be related This is a conservation concern Changes genotype frequencies Populations are not in H W equilibrium Genotype frequencies p 2pq q Coefficient of inbreeding F o The probability that an individual taken at random from a population will have alleles that are identical by descent Allele frequencies do not change Loss of heterozygosity Effects of inbreeding depend on extent of genetic relatedness Selfing sibling mating cousin mating Selfing an extreme case of inbreeding Common in plants Relatively rare in animals How does inbreeding affect genotype frequencies heterozygosity and homozygosity Reduction of heterozygotes increase in homozygotes over time Change in genotype frequency not in allele frequency Inbreeding vs Selection Inbreeding alone does not affect allele frequencies But it can affect the outcome of SELECTION which does change allele frequencies An excess of homozygotes are produced if homozygotes have different fitness from heterozygotes selection can alter allele frequencies Recessive deleterious alleles normally hidden in heterozygous state can come together in homozygous inbred offspring Selection acts on changed genotype frequencies Selection is the force causing evolution Inbreeding simply creates the conditions under which selection can act Disassortative mating dissimilar individuals are more likely to mate than expected by chance If disassortative mating is based on genotypic similarity this can also lead to an increase in heterozygotes Outbreeding avoid mating with genetic relatives Avoided inbreeding by Mate choice Genetically controlled self incompatibility Dispersal Phenotypic disassortative mating opposites attract Identity by descent IBD Two alleles are identical because both were inherited from a single copy of that allele in a common ancestor Mutational meltdown refers to the process by which a small population accumulates harmful mutations which leads to