Zoology 10th Edition by Stephen Miller,John Harley

Exercise 2

The biological definition of a species is a group of populations in which genes are actually or potentially exchanged through interbreeding. This definition is difficult to apply in all instances because some species reproduce only asexually, some species are represented only by fossils, and in others reproductive criteria are difficult to test. Speciation requires that subpopulations are prevented from interbreeding. This reproductive isolation commonly occurs as a result of geographic barriers isolating subpopulations (allopatric speciation). Speciation can also occur within small local populations (parapatric speciation), or within a single population (sympatric speciation).
How does reproductive isolation occur in each of the three forms of speciation described in this section?