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Chapter 7

Chapter 7-Population Genetics II.docx

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University of Waterloo
BIOL 359
Jonathan Witt

BIOL 359 Lecture 7 Janice Wong Evolution Population Genetics part II Chapter 7: Mendelian Genetics in Populations II- Migration, Genetic Drift, and Nonrandom Mating 7.1| Migration • Migration is the movement of alleles between populations • Migration means gene flow, transfer of alleles from the gene pool of one population to the gene pool of another population Adding migration to the HW analysis: Migration as a Mechanism of Evolution • Fig.7.4 • One-island model • Because the island population is so small relative to the continental population, any migration from the island to continent will be inconsequential for allele and genotype frequencies on the continent • So migration, and the accompanying gene flow, is effectively one way, from the continent to the island • Alleles arriving on the island from the continent represent a relatively large fraction of the island gene pool • Migration can alter allele and genotype frequencies • Violates HW conclusion 1 • Migration is a potent mechanism of evolution. In practice, migration is most important in preventing populations from diverging • One round of matign will put the population back into HW equilibrium for genotype frequencies Empirical research on Migration as a Mechanism of Evolution • Migration of individuals from the mainland to islands appears to be preventing the divergence of island versus mainland populations of Lake Eric water snakes • Mainland all snack are banded, whereas island snakes are unbanded • On island, snakes bask on limestone rocks and unbanded snakes are more hidden • Some island snakes move to the mainland area and bring back copies of alleles for banded coloration • Migration is acting as an evolutionary mechanism in opposition to natural selection, preventing the island population from becoming fixed for the unbanded allele Migration as a Homogenizing Evolutionary Process • Migration tends to homogenize allele frequencies across populations • If migration was allowed to proceed unopposed by any other mechanism of evolution, migration will eventually homogenize allele frequencies across populations completely Summary of the effects of Migration BIOL 359 Lecture 7 Janice Wong Evolution Population Genetics part II 1. Within a participating population, migration can cause allele frequencies to change from one generation to the next 2. For small populations receiving immigrants from large source populations, migration can be a potent mechanisms of evolution 3. Across groups of populations, gene flow tends to homogenize allele frequencies 4. Migration tends to prevent divergence of populations 7.2| Genetic Drift • Genetic drift doesn't result in adaptation but it does lead to changes in allele frequencies • In the HW model, genetic drift results from violation of the assumption of infinite population size A Model of Genetic Drift • In populations of finite size, change evens- in the form of sampling error in drawing gametes from the gene pool- can cause evolution • Both conclusions of the HW principle have been violated • Is a mechanism of evolution that simply happens by chance • Selection is differential reproductive success that happens for a reason • Genetic drift is differential reproductive success that just happens by chance • If the population is small, it cannot conform to the HW principle • Sampling error can cause evolution Genetic Drift and Population Size • Genetic drift is most important in small populations • Sampling error diminishes as sample size increases Sampling Error as a Mechanism of Evolution: The Founder Effect • Founder Effect: the allele frequencies in the new population are likely, simply by chance, to be different from what they were in the source population • It is the direct result of sampling error • The founding of a new population by a small group of individuals typically represents not only the establishment of a new population but also the instantaneous evolution of differences between the new population and the old one • When a new population is founded by a small number of individuals, it is likely that chance alone will cause the allele frequencies in the new population to be different from those in the source population= founder effect • Allelic diversity declines • Evolution occurs through sampling error and not natural selection Random Fixation of Alleles and Loss of Heterozygosity • Genetic drift can change allele frequencies in a single generation • Genetic drift is even more powerful as a mechanism of evolution when its effects are compounded over many generations • Fig. 7.15 • 3 patterns BIOL 359 Lecture 7 Janice Wong Evolution Population Genetics part II 1. Because fluctuations in allele frequency from one generation to the next are caused by random sampling error, every population follows a unique evolutionary path 2. Genetic drift has a more rapid and dramatic effect on allele frequencies in small populations 3. Given sufficient time, genetic drift can produce substantial changes in allele frequencies even in populations that are fairly large • If genetic drift is the only evolutionary mechanism at work in a population, no selection, no mutation, no migration, then sampling error causes allele frequencies to wander between 0 and 1. This can be seen in figure 7.15b • Wander of allele frequencies produces two important and related effects 1. Eventually alleles drift to fixation or loss 2. The frequency of heterozygotes declines Random Fixation of Alleles • As any allele drifts between frequencies of 0-1.0, sooner or later the allele will meet an inevitable fate • Its frequency will hit one boundary or the other • If the alleles frequency hits 0, then the allele is lost forever (assuming that mutation or migration do not reintroduce it) • If the allele's frequency hits 1, then the allele is said to be fixed • In figure 7.15b, there were 8 populations being tracked, the allele A d1ifted to fixation in one, and loss in 3 • It is a matter of time before A1will become fixed or lost in the other populations as well • As some alleles drift to fixation and others drift to loss, the allele d
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