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population shift

n. a change in the relative numbers of the different groups of individuals making up a population

Usage examples of "population shift".

Of more lasting significance was a large-scale population shift within Africa itself, the Bantu expansion.

Why doesn't the average tail length in the population shift to 4 inches under the influence of female sexual selection?

In summary, the standard version of the story describes a species of moth found in the British Midlands that were predominantly light-colored in earlier times but underwent a population shift in which a dark strain became dominant when the industrial revolution arrived and tree trunks in the moths' habitat were darkened by smoke and air pollution.