Evolution CH8.docx

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PCB 4674
Colette Marie St Mary

CHAPTER 8 Evolution in Finite Populations Manx cat short tail long legs. Manx phenotype: includes both the reduced or absent tail and longer hind legs than forelegs. Single autosomal locus M. Dominant. Usually Mm. MM is lethal. Usually homozygote lethality = strong natural selection against M. Found only on island, M become common. Founder Effect: Chance variation in the initial allele frequencies sin a founding population may lead to dramatically different allele frequencies on an isolated island compared to on a mainland. 8.1 Random Change and Genetic Drift In experiments with small sample sizes, realized frequencies are not always very close to the expected frequencies. Wright-Fisher Model: small-population version of Hardy-Weinberg. Assumes: 1. Natural selection is not operating on the trait or traits affected by the locus in question. 2. Mating in the population is random with respect to the locus in question. 3. No mutation is occurring. 4. There is no migration into the population from other populations. Modeling a Hermaphroditic or monoecious species: model where male and female are placed into the same gamete pool instead of having separate pools. Genetic Drift: the process of random fluctuation in allele frequencies due to sampling effects in finite populations. Three general consequences of genetic drift: 1. In a finite population, allele frequencies fluctuate over time, even in the absence of natural selection. 2. Some alleles are fixed, others are lost, and the fraction of heterozygotes in the population decreases over time. 3. Separate populations diverge in their allele frequencies and in terms of which alleles are present. Genetic Drift Causes Allele Frequencies to Fluctuate over Time The rate at which allele frequencies fluctuate because of drift depends on the size of the population. Selectively neutral: there is no fitness difference between the two alleles. Genetic drift is completely random, unsure which will be lost. If there is k number of copies of a given allele, each of these copies has a 1/2N chance of being fixed, for a total of k/2N. Genetic Drift Causes Heterozygosity to Decrease within a Population over Time Genetic drift reduces variation within populations. Observed heterozygosity: the fraction of individuals in the population that is heterozygous at the given locus. Equation: Ho= 1- (sum n,i=1) f[AiAi] Expected heterozygosity: the fraction of heteroqygotes expected under the Hardy-Weinberg model, given the allele frequencies in the population. Equation: H e 1 - (sum n, i=1) p i 1/2N: when N is large, there is little change. When small there is a large amount of change. Drift reduces heterozygosity. Effective population size: the population that is actually contributing. Drift and Divergence in the Galapagos Archipelago Slippage-induced mutation: copy number changes readily. 8.2 Coalescent Theory and the Genealogy of Genes Coalescent theory: how gene copies spread through a finite population over generations. From Species Trees to Gene Trees Coalescent point: separation point in gene tree. Red: separation point Green: separation for all genes in tree. The Coalescent Process and Genetic Variation Any allelic differences among a set of gene copies at the same locus must have arisen by mutation subsequent to the coalescent point for this set of gene copies. The deeper the coalescent point, the more variation we expect to see in the population. Process by which variation arises at the locus as the result of two separate processes: 1. The genealogical process by which a coalescent tree is formed 2. The mutation process by which variation arises along the coalescent tree Drift will act more strongly to reduce heterozygosity in a small population than in a large one. The pattern of variation that we see in a neutral locus is therefore the result of two sources of randomness superimposed on one another: 1. The randomness associated with which particular genealogical history happens to occur-that is, the coalescent tree of the present population 2. The randomness associated with where mutations arise along this coalescent tree. Two sources of randomness that cause variation around this average number of differences: 1. Genealogical history is a random process, so the two alleles may be separated by considerably more or less than 4N generations 2. The mutation process varies, so if the two alleles are separated by say 1000 generations, we may see more or less than 1000u mutations distinguishing them. 3. Selection drives alleles quickly to fixation, leading to a more recent coalescent time. Gene genealogy for new mutants: o Neutral o Positively selected o Subject to balancing selection 8.3 Demography, Biogeography, and Drift Population Bottlenecks Population bottleneck: brief period of small population size. Elephant seals, almost extinct, bottleneck, lost much genetic heterozygous diversity. Founder Effect Founder Effect: change in allele frequencies that result from the sampling effects that occur when a small number of individuals from a large population initially colonize a new area and found a new population. Leading edge subpopulations: near edge when
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