Genetic Drift

The Founder Effect means from the same original population, isolated smaller populations may evolve differently due to genetic drift

Through random chance, some alleles may not be represented in subsequent generations. This is a problem for small populations where advantageous or deleterious alleles can reach fixation.  For founder populations (e.g the first populations to inhibit an island), genetic drift may help drive speciation.

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Glossary of Galapagos