How are species created?

Genes in a given population are co-adapted; that is, the genes reinforce each other to produce individuals that will leave behind a maximum number of offspring. When two populations first start to diverge from each other, crosses between the two can produce fertile offspring. But as divergence continues, the mixing of genes from two different populations produces offspring with genes that are not co-adapted. Consequently, these offspring are not as fertile as their ancestors were. The crosses that produce the most offspring are those between two individuals in the same population. Furthermore, there is selection in each population for individuals that do not interbreed with individuals in a different population. This selection gives rise to isolating mechanisms of different types--behavioral, temporal, chemical, and mechanical.