Gene Flow

The second requirement for genetic equilibrium is that the size of the population remains constant. If individuals move, genes move with them. Immigration is the movement of individuals into a population, and emigration is the movement of individuals out of a population.

The behavioral ecology of some animal species encourages immigration and emigration. Common baboons live on the savannas of eastern Africa in social and breeding groups called troops. A troop is dominated by a few adult males, and it may have from 10 to 200 members. Females tend to remain with the troop they are born into; however, younger or less dominant males leave their birth troop, eventually joining another troop. This constant movement of male animals ensures gene flow. Gene flow is the process of genes moving from one population to another. Gene flow can occur through various mechanisms, such as the migration of individuals or the dispersal of seeds or spores.

Word Roots and Origins immigration from the Latin immigrare, meaning "to go into"

figure 16-5

Genetic drift is significant only in small and medium-sized populations. In a small population, a particular allele may disappear completely over a few generations. In a larger population, a particular allele may vary widely in frequency due to chance but still be present in enough individuals to be maintained in the population. In a much larger population, the frequency of a particular allele may vary slightly by chance but remain relatively stable over generations.

Effect of Population Size on Allele Frequency


60 90


60 90


Population = 25

Population = 200

Population = 2,000

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