Causes of speciationGeographic isolation
In the fruit fly example, some fruit fly larvae were washed up on an island, and speciation started because populations were prevented from interbreeding by geographic isolation. Scientists think that geographic isolation is a common way for the process of speciation to begin: rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations.
It doesn't even need to be a physical barrier like a river that separates two or more groups of organisms it might just be unfavorable habitat between the two populations that keeps them from mating with one another.
Reduction of gene flow
Even in the absence of a geographic barrier, reduced gene flow across a species' range can encourage speciation.
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