Isolating Mechanisms

Isolation is the separation of members of a single population into sub-populations, which preserves the genetic integrity of each resulting group. Closely related species living in the same geographic area typically do not interbreed because they are kept apart by isolating barriers — any evolved trait of the two species that prevents interbreeding. The most fundamental distinction among these barriers is between prezygotic mechanisms, which prevent the two species from even coming into reproductive contact in the first place, and postzygotic mechanisms, which act only after fertilisation has already occurred. Prezygotic barriers include: (i) Ecological/habitat isolation — related species occupy different microhabitats within the same broad region, e.g. Rana areolata breeds in shallow grassy ponds while Rana grylio breeds in deep water. (ii) Seasonal isolation — species breed at different times, e.g. Bufo americanus breeds early in spring while Bufo fowleri breeds much later in the season. (iii) Sexual/ethological (behavioural) isolation — differing mating calls or courtship behaviour prevent mating even without any difference in time or place, e.g. the distinct mating calls of the grey tree frog (Hyla versicolor) and pine woods tree frog (Hyla femoralis). (iv) Morphological/mechanical isolation — physical differences in external genitalia prevent successful mating, e.g. the size difference between Bufo quercicus and Bufo valliceps. (v) Physiological isolation — mating occurs but fertilisation fails due to biochemical incompatibility of the gametes, e.g. sperm of Drosophila virilis surviving only about a day inside the reproductive tract of Drosophila americana, versus the much longer survival of D. americana's own sperm there. (vi) Cytological isolation — differing chromosome numbers between species prevent fertilisation, e.g. between the bullfrog Rana catesbeiana and the gopher frog Rana areolata. Postzygotic mechanisms include: (vii) Hybrid inviability — fertilisation succeeds and the embryo develops, but the hybrid dies before reaching reproductive maturity, seen in certain fish, frog and beetle crosses due to genetic incompatibility. (viii) **Hybrid steril …