Leslie Samuel
Issues in Origins and Speciation
Chapter 6
Mendelian Genetics in Populations II:
Migration, Genetic Drift,
And Nonrandom Mating
6.1 Migration
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In an evolutionary sense, is the movement of alleles between populations
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Mechanisms: range from long-distance dispersal of juvenile animals to the transport of pollen, seeds, or spores by wind, water, or animals.
Adding Migration to the Hardy Weinberg Analysis: Migration as an evolutionary force.
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The one-island model involves migration, and the accompanying gene flow, being effectively one way, from continent to the island
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This takes the allele and genotypic frequencies on the island away from Hardy-
Weinberg equilibrium.
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However, a single bout of random mating will put the population back into
Hardy-Weinberg equilibrium.
Empirical Research on Migration as a Mechanism of Evolution
Color pattern of water snakes of Lake Erie is determined by single locus with 2 alleles.
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Virtually all water snakes on mainland are banded. On the island – unbanded.
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On islands – limestone rocks. Unbanded individuals more cryptic.
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Banded snakes move from mainland to islands preventing the island population from being fixed
Migration as a Homogenizing Evolutionary Force Across Populations
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Migration tends to homogenize allele frequencies, opposing selection
6.2. Genetic Drift
Results from violation of the assumption of infinite population size
Model:
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If population is small, chance events produce outcomes that differ from theoretic expectations – sampling error.
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Selection is differential reproductive success that just happens
Genetic Drift and Population size
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Genetic drift is most important in small populations
Sampling error diminishes as sample size increases.
Empirical Research on Sampling Error as a Mechanism of Evolution: The founder Effect

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Founders effect is a result of a small population that was just founded by a group of individuals that have moved, or been moved, to a new location.
Typically represents the instantaneous evolution of differences between the new population and the old one.
Random Fixation of Alleles and loss of Heterozygosity
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Under genetic drift, every population follows a unique evolutionary path. Genetic drift is rapid in small populations and slow in large populations.
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Eventually alleles drift to fixation/loss and the frequency of heterozygote declines o Buri’s experiment with fruit flies – The frequency of heterozygotes declined with time o Natural experiment with collared lizard conformed that small isolated populations lose their genetic diversity as a result of drift.
The Rate of Evolution by Genetic Drift
Mutation is the creation of a new allele; substitution is the fixation of the new allele, with or without additional mutational change
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The neutral theory holds that advantageous mutations are exceedingly rare and that most alleles of most genes are selectively neutral
The selectionist theory holds that advantageous mutations are common enough that they cannot be ignored
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Summary – genetic drift is a nonadaptive mechanism of evolution – simply a result of chance
6.3 Genetic Drift and Molecular Evolution
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Early molecular evidence indicated that rates of change were high and constant through time
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This was in conflict with the theory of natural selection
The Neutral Theory of molecular evolution
Models the fate of new allele that were created by mutation and whose frequencies change by genetic drift
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Claims to explain most evolutionary change at the level of nucleotide sequence
Pseudogenes establish a canonical rate of Neutral evolution
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Silent Sites change faster than replacement sites in most coding loci
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Variation among Loci: Evidence for Functional Constraints
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The Neutral Theory as a null hypothesis: Detecting Natural Selection on DNA
Sequences
Current Status of the Neutral Theory
It acts as a null hypothesis for detecting positive selection in molecular evolution.
6.4 Nonrandom Mating
Inbreeding decreases the frequency of heterozygotes and increases the frequency of homozygotes compared to expectations under Hardy-Weinberg assumptions o Malaria parasite – Data revealed a deficit of heterozygotes and an excess of homozygotes may be evidence of inbreeding

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Inbreeding may lead to reduced mean fitness if it generates offspring homozygous for deleterious alleles.
6.5 Conservation Genetics of the Illinois Greater Prairie Chicken
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Early efforts failed because birds were suffering from inbreeding depression
Inbreeding depression was caused by a loss of allelic diversity under genetic drift
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Migration appears to be restoring genetic diversity to remnant populations and alleviating inbreeding depression