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Evolutionary Theory

Author(s): Tadzia GrandPré, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Migration

Migration, or the geographical movement of organisms, can cause deviations from Hardy-Weinberg expectations (and therefore can provide raw material for evolution) by changing the gene pools of different populations. Through migration, new alleles can be introduced or taken away from a population, or the frequencies of alleles and genotypes in a population can be altered.

Suppose that population 1 has only alleles "a" and "b," while population 2 has only alleles "x" and "y." If individuals from population 2 move into the same area as population 1 and breed with individuals from population 1, they will introduce the "x" and "y" alleles.

Now suppose that all individuals from population 1 carrying the "a" allele migrate into the area containing population 2 and interbreed with that population. Population 1 (individuals that did not migrate into the area of population 2) will undergo a decrease in genetic variation, because it is left only with individuals carrying the "b" allele. Meanwhile, population 2 will experience an increase in variation, because it has gained the "a" allele.

A more subtle way for migration to alter Hardy-Weinberg expectations is by shifting the relative frequencies of alleles, even when the number of alleles remains unchanged. Suppose population 3 also has only alleles "a" and "b." And suppose the "a" allele is common in population 1, but rare in population 3. If population 1 migrates and interbreeds with population 3, allelic frequencies will change and the differences between the two populations will be reduced.