Speciation is caused by

Adaptations of an animal are their:

The idea of natural selection as the fundamental process of evolutionary change was reached by whom and when?

Genetic variation produced in species due to crossing over is an important process for which of the following?

Given below are the two statements.
Statement $I$: Directional isolation favours the intermediate forms and eliminates both phenotypic extremes.
Statement $II$: In directional isolation,more individuals acquire a value other than the mean character value.
In the light of the above statements,choose the correct answer from the options given below:

Genetic drift occurs due to . . . . . . .