[7] Population Genetics - II

[7] Population Genetics - II - Migration and the H-W...

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Migration and the H-W principle
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Migration (p 225) Movement of alleles between populations Gene flow Caused by anything that moves alleles from one population to another Long-distance dispersal Transport of pollen etc. by various sources Migration varies among different species Island populations are excellent “laboratories” for examining migration Figure 7.4, pg. 225 Can migration from the continent to the island shift allele frequencies from H-W equilibrium?
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Can allele frequencies shift under migration? p226 Our mouse population has 2 Alleles: A 1 and A 2 Before migration the frequency of A 1 is 1.0 Will migration from the continent affect the allele frequencies? 1000 individuals –A 1 A 1 = 800/1000 = 0.8 –A 2 A 2 = 200/1000 = 0.2 After migration: Allele frequencies have changed Genotype frequencies are initially not consistent with H-W Expected H-W genotype frequencies: –A 1 A 1 = 0.64 (p 2 ) –A 1 A 2 = 0.32 (2pq) –A 2 A 2 = 0.04 (q 2 )
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One round of random mating • Allele frequencies are now: A 1 = 0.8 and A 2 = 0.2 Random now mating occurs: p 2 + 2pq + q 2 = 1 (0.8) 2 + 2(0.8)(0.2) + (0.2) 2 = 1 0.64 + 0.32 + 0.04 = 1 A 1 A 1 = 0.64 (p 2 ) A 1 A 2 = 0.32 (2pq) A 2 A 2 = 0.04 (q 2 ) Our population is now back in H-W equilibrium What would happen if migration occurred again? It takes one round of random mating to restore HW equilibrium
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An example: Lake Erie Water Snakes Example of migration from mainland to island Snakes vary in colour pattern which is determined by a single locus with two alleles Unbanded island snakes have a higher rate of survival than banded immigrants Selection favours unbanded snakes on the islands pg. 228
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An example: Lake Erie Water Snakes cont. Why don’t the islands consist of only the unbanded snakes? Migration! It Prevents the island population from becoming fixed for the unbanded allele Migration can prevent the divergence of populations Figure 7.7, pg. 228 A= unbanded D= strongly banded
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What is the effect of migration? In the case of the water snakes, migration maintains the banded snakes on the Islands, even though they have lower fitness (migration is working against selection in this example, but it could also do the opposite). Migration is a force that generally tends to homogenize populations
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Migration and Homogenization Population 1 p= 0.1, q= 0.9 Population 2 p= 0.6, q= 0.4 Population 3 p= 0.4, q= 0.6 p = 0.3667, q = 0.6333 average allele frequencies among the three populations Migration generally acts to homogenize populations, or make their allele frequencies the same. If these three populations all exchange migrants at an equal rate, eventually each population will converge on an equilibrium frequency of alleles, and this equilibrium will be equal to the average frequencies of the 3 populations in the absence of any other evolutionary forces migration between populations
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Genetic drift and the H-W principle
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[7] Population Genetics - II - Migration and the H-W...

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