Reproductive Isolation
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[edit] Definition
Reproductive isolation is the heritable genetic inability to interbreed with organisms outside one's own species.
[edit] Prezygotic isolation
Prezygotic isolation is so called because it takes place before the formation of the zygote. It takes four forms: behavioral isolation, temporal isolation, mechanical isolation, and gametic incompatability.
- Behavioral isolation: Two animal species may never interbreed even though they are capable of it. An interesting example is given by cichlid fish. Many species can interbreed; however, mate selection, which is done on a visual basis, ensures that they don't. When human activities make the water turbid and visibility poor, the barriers to reproduction are removed and the species merge. As another example, songbirds which are potentially interfertile can be separated by having and responding to different mating calls.
- Temporal isolation is often seen in plants, when we have two related species of plants which could in principle cross, but which bloom at different seasons, so that in fact no cross occurs.
![Male genitalia of different species of damselfly, after Eberhard [1]](/index.php/Image:Damselfly_genitalia.jpg)
- Mechanical isolation: On a less subtle biological level, two animals may be willing to try to breed, but the necessary parts may not fit together, as happens in some insects (see diagram, right).
- Gametic incompatibility is the inability for a sperm to penetrate and fertilise the egg. For example, in abalones, the sperm carries a lysin protein that dissolves a hole in the egg's envelope, but only in eggs from the same species of abalone (D.J. Futuyma, Evolutionary Biology, 1998)
[edit] Post-zygotic isolation
This also comes in various forms:
- Inviability: The hybrid may not be able to develop or survive at all, due to radical genetic incompatibility.
- Hybrid sterility: Hybrids may be viable, but unable themselves to reproduce, as is the case with mules (horse-donkey crosses).
- Hybrid breakdown: In some cases, the first generation of hybrids may be fertile, but subsequent generations are sterile.
These effects may affect hybrids of different sexes differently. In cases where just one sex is absent (due to inviabilty) or sterile, it is almost always the sex with heterozygous sex chromosomes (e.g. the males in mammals, since male mammals have XY sex chromosomes and female mammals have XX): this principle is known as "Haldane's rule".
[edit] Natural selection
We might mention one more case, which is not strictly speaking reproductive isolation, but which is worth mentioning. Suppose, as sometimes happens, that two groups of animals live in adjacent territories and are perfectly capable of interbreeding, but that the cross-breeds are invariably less fit than the uncrossed varieties. Then natural selection will ensure that the hybrids are contained in a narrow band along the border between the two varieties, so that although individuals may mix their genes, the two populations will remain separate and distinguishable.
