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migration
- movement of alleles between populations
- results in gene flow: transfer of alleles from the gene pool of one population to another
- can increase genetic variation within populations
- tends to decrease variation between populations
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models describing migration
-continent island
-island
-stepping stone
-isolation by distance
- -continent island: moving from mainland to bare islands
- -island: all islands exchange equally
- -stepping stone: populations next to eachother are exchanging equally
- -isolation by distance: more migration to closer populations
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genetic drift
-2 types
- chance or random event that influences evolution
- effects are immediate and cummulate over time
- larger effect on smaller populations
- with times, drift changes allele frequencies no matter what the population size is
- founder event: moving from big population to smaller, allele frequencies are not the same, lose diversity
- bottle neck: a few individuals contribute genes to the entire future population of the species, one type survives better due to luck- not because they are better
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rate of fixation
- depends on population size
- eventually all populations would fixate, but migration (gene flow) doesnt let that happen
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genetic diversity
- effected by population size
- needed to adapt to changing environments
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patterns of natural selection
- directional (purifying)- homozygote advantage, over time favored allele frequency will equal 1 (fixed), reduces diversity, disadvantageous allele lost over time
- stabolizing- favores intermediate variants and acts against extreme phenotypes, heterzygote advantage, balencing selection (keeping both in population)
- diversifying- selects against phenotypes near the average value, favoring extreme phenotypes, can lead to speciation (new species), difficult to get populations to diverge
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varying selection
- temporal fluctuation: different phenotypes favored in different generations
- multiple niche polymorphism: if you have the allele you survive, intermediates tend not to survive to adulthood
- frequency dependent selection: kind of balancing selection, results in maintenance of polymorphism
- ---inverse (rare is good): side mouth sucking fish, the rare one can find more food
- ---positive (common is good): whatever alleles present are determined by evolutionary standpoint (mimcry)
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