Check out the section called ‘Background Essay’….. brilliant for your revision, here is a sneak peak.
‘When explaining a breakup, couples will often say, “We grew apart,” or “We both changed in different ways.” That’s a good metaphor for how species are formed: members of a population somehow begin to diverge, usually as a result of being geographically separated from each other. Eventually, they can no longer interbreed, and at that point a new species has formed.
Yet if the two groups continued to live near each other, it’s likely that mating attempts between naturally varying members of the two populations would tend to allow the species to merge again. This is called “gene flow” between the two groups. What keeps this from happening, and what allows new species to arise and endure, are what are known as “isolating mechanisms.” These are either behavioral or structural differences between species that make mating impossible……’ see website for remainder of article.