What happens during speciation? Well, what happens is that, as [Theodosius] Dobzhansky called it, the so-called isolating mechanisms develop. That means, any kind of characteristics of a species and its individuals that separates them or makes them unwilling to mate with an individual of another species. Now, that in animals very often is a behavioral barrier, that the two individuals of two different species meeting with each other have different displays and therefore break it off because they realize they don't belong together. In plants quite often it is a sterility factor; the pollen is indeed carried to the other plant but no fertile seeds develop because there's the sterility factor. All these properties of the genotype that prevent successful cross-breeding are called isolating mechanisms. Now, Dobzhansky when he first proposed the term got a little confused and he included geographic barriers. Geographic barriers are not isolating mechanisms… isolating mechanisms are all properties of genotypes, properties of individuals.