Talk:Geographical isolation

Re: interbreeding being a sign of close relation between species

Just because two species can interbreed doesn't mean they're that closely related. Corn Snakes (Elaphe guttata) will breed with milk and king snakes (Lampropeltis), gopher (and probably bull and pine too) snakes (Pituophis melanoleucus), and most likely other species too, if given the chance. Claiming that Corns are close relatives of Milks would take some pretty convincing evidence.

And from the other side, Blue-Spotted (Ambystoma laterale) and Jefferson's (Ambystoma jeffersoni) Salamanders, both descendants of the (extinct) Blue Salamander and pretty close relatives, breed to produce offspring of several different species (ID'able only by genetic analysis), including species for which there are only females, who produce genetically identical offspring and mate with Jeferrson and Blue-Spotted males only to stimulate egg development, NOT for genetic material.

I don't know squat about mammals, but horses and donkeys produce mules. Are the two species that closely related? They have different numbers of chromosomes, which is why the offspring are sterile, right?

Actually, interbreeding is irrelevant here as it does not relate to the subject. Geographical isolation leads to organisms that can be destinguished from the original population over time.

As to the salamanders, they may be subspecies or varieties or whatever of the same species. This is for the specialists to decide. The thing is nomenclature is a battle field so many specialist so many views..

I'm afraid the above is partly wrong.

Geographical isolation is a key process in species formation, so the latter certainly needs to be mentioned in the article. Remember that the species is the only taxonomic level that is more than conventional; it is defined as the range of gene exchange, and it has to be determined empirically.

Obviously I wasn't clear that what I was talking about was fertile interbreeding. It's the capacity to do that that is the basic definition of a species. When two populations can interbreed and produce fertile young of both sexes, they are by definition a single species, because there is no barrier to gene exchange between them. Horses and donkeys are distinct species, because though they can interbreed and produce mules, the mules are infertiles, so there is no way that horse genes can get into the donkey gene pool or vice versa.

The barriers to interbreeding do not have to be physiological. For example, hamadryas and savannah baboons can interbreed - but the male offspring lack the required social behaviours to secure mates, so there is a behavioural barrier to gene exchange and they remain good species.

Evolutionary theorists have, however, got themselves into some fine logical tangles when the only barrier to fertile interbreeding is geographical isolation. If one population is in Europe and the other is in Australasia, they are effectively prevented from interbreeding; it seems reasonable to call them distinct species. But when people from Europe find their way to Australasia, and start taking members of either population with them, and they start interbreeding, do they then cease to be distinct species? And were they ever distinct species?

seglea 16:16, 9 Dec 2003 (UTC)