In the apple maggot fly, the frequency of certain alleles increases steadily as one goes from North to South. This is an example of: Sympatric speciation.
Sympatric speciation is observed when there are no physical barriers which prevent any members of a species from mating with another, and all members are in close contact to one another. A new species, possibly established on a distinct food source or trait seems to develop instinctively.
The example of apple maggot fly supports a model for sympatric speciation in progress. Hawthorn is the native host for the apple maggot fly in North America. However, in the mid-1800s, a new population developed on introduced, domesticated apple.
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