This idea would follow from the dobzhansky-mayr theory of speciation.

a.) geographic isolation happens before speciation.
b.) speciation is not related to changes in the environment.
c.) isolated populations evolve differences gradually as they adapt to the environment.
d.) the origin of species happens through adaptation by natural selection.

A.) Geographic isolation happens before speciation.

Explanation:

Dobzhansky-Mayr's theory of speciation states that when two populations become geographically isolated, whether because of a geological event, migrations or local extinctions, they evolve separately. After sufficient time for them to accumulate genetic differences, they will be reproductively isolated. If isolation is complete, speciation has occurred

With this we can conclude that the letter A represents an idea derived from Dobzhansky-Mayr's theory of speciation.

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your answer is "the 1950's"