Download Genetic drift is random

Genetic drift
Random factors in evolution
 Mutation is random
 Genetic drift is random
 Genetic drift is random fluctuations in frequencies of alleles or haplotypes
 Due to violation of HW assumption of large population size
 Can result in nonadaptive evolution
Sampling error
 Sample drawn from a population is likely to vary from the population by chance
 Genes that make it into the next generation can just be a random sample of
genes in the population (assume alleles are neutral with respect to fitness)
 Imagine:
 50 individuals, every allele unique
 100 alleles, qi = 0.01
 Random mating: Draw gametes from population at random, with
replacement
 By chance, 10 individuals don’t get sampled
 20 alleles are lost!
 Some other alleles were drawn more than once - their frequency is now >
0.01
Random walk
 Frequency of one gene will eventually reach 0 or 1 by chance if there is no
stabilizing force to return the frequency towards 0.5
Genetic drift
 Allele frequencies fluctuate at random within a population, until eventually one
or another allele becomes fixed
 Frequency of heterozygotes H = 2p(1-p) declines
 Rate of decline in heterozygosity is used as a measure of rate of genetic
drift
Genetic drift
 The probability that a given allele will become fixed is equal to the initial
frequency of that allele
 An allele is more likely to be fixed in a small population than a large population
 p = 1/2N
 Therefore as N gets larger, p gets smaller
Genetic drift
 Happens faster in small populations
 Average time is 4N generations
Demes
 Small independent populations are called demes
 A group of demes is called a metapopulation
 Allele frequencies may vary among the demes
 Variance among the demes increases from generation to generation
Coalescence
 As time goes on, more lineages become extinct
 Populations coalesce back to a single common ancestor
 One allele becomes fixed
Effective population size (Ne)
 Effective population size is smaller than censused population size
 Nonbreeding individuals are not included in effective population size
Factors that reduce Ne
 Variation in number of progeny
 Uneven sex ratio
 Natural selection
 Generations overlap
 Fluctuations in population size
Population bottlenecks
 Restrictions in size through which populations may pass
 Founder effect occurs when a new population is established by a small
number of colonists
Bottleneck
 Mauna Kea Silversword
 Down to 16 Individuals
 Monocarpic Perennials
 Self-incompatible
 Most s-alleles lost by drift
 Some are left with no possible mate!
Bottleneck
Founder Effect
 Amish Colonies
 Founded 1720-1770, N<200
 Married within religion, few converts after 1800
 8000 in Lancaster Co. PA, 100 founders
 Lancaster Amish: Ellis-van Creveld: p=0.13
 Pyruvate kinase deficiency

Ohio Amish: High frequency of hemophilia
Invasive species
Reduced genetic diversity
Neutral theory
 Asserts that the great majority of mutations that are fixed are neutral with
respect to fitness
 Fixed by genetic drift
 Creates molecular clock
 DNA sequencing supports neutral theory
 Synonomous mutations happen more often than replacements
 Rates of substitutions are higher in introns and pseudogenes
 Rate of evolution is higher in genes that are least likely to affect function
Gene flow
 Rate of fixation is inversely proportional to population size
 Drift is counteracted by gene flow (m) from other populations
Measuring gene flow
 Nm = ((1/FST)-1))/4
 Nm is number of immigrants per generation
 Assume alleles are neutral
 Assume that allele frequencies have reached an equilibrium between
genetic drift and gene flow
Migration counteracts divergence by drift
FST = 1 / (4Nm + 1)
Pocket gophers
Coalescence theory
 Coalescence takes longer in larger populations
 Can determine coalescence time to common ancestor based on DNA
sequences
 Can determine historical effective population size
Humans
 mtDNA
 Time to coalescence of common ancestral gene is 156,000-250,000 years ago
 Effective population size was 4600 – 11200 people