To add to David's admirable post, I thought I'd simply point out that there is only one species of tiger extant, and the Bengal, Sumatran, Siberian etc are merely breeds of the same. In fact there are only 5 extant species of big cat (allegedly a technical term referring to their ability to roar): tiger, lion, jaguar, leopard and snow leopard. The tiger and the lion have been known, on occasion, to cross-breed, giving rise to (depending upon which was the female parent and which the male) tigons (very large animals) and ligers (rather diminutive compared to the parent species). There have even been reports of these hybrids (the term for the offspring of two different species) being fertile and having produced offspring of their own.

It can be quite surprising when you find out the range of animals that can be considered a single species. For instance, my understanding is that ALL brown bears are members of the same species, from the giant Kodiak breed or sub-species to the much ickler brown bear that roams central and southern America, and Asia.

Also, though the coyote and grey wolf are considered separate species they interbreed very readily, and the product, according to some reports, is the red wolf. This has caused, I understand, some sort of a scandal in environmental circles because of the massive conservation effort in Florida and elsewhere to preserve the red wolf - except it turned out not to be a separate species but a hybrid, after all. (I could be wrong on this one - some years since I read the report in New Scientist?)

To a certain extent, in the past, the human species too could have been considered to be divided into breeds - except that we, unregenerate as we were, called them 'races'. The facts, as I understand them (vide Jared Diamond's Guns, germs and steel), are that while most human populations these days have exchanged genes with their surrounding populations to a great extent, there are still genetic markers that can be identified in groups that have remained isolated, or interbred, for many generations. The original five 'races' of humanity, on the other hand, all originated in Africa, and three of them are still confined to that continent (to a certain extent): the pygmies, the khoi-san and the 'blacks' (typified by the Bantu peoples, and being the stock from which the slavers kidnapped whole populations and shipped them off to the Americas and the Caribbean). In addition, the other two population strands were those that led to 1. the Northern African and Semitic people, and 2. everybody else. The latter have an interesting (at least to us) history, since the original population seems to have spread from Africa to east Asia, forming the basis of what we consider the 'oriental' populations, one strand of which, of course, travelled yet further east, via the Bering Land Bridge and became the 'native' American population. Another strand from east Asia travelled west, acquiring genetic mutations and markers as it went, until it settled down in Europe, to form the basis of what we call the 'white' or 'caucasian' population. So, if you like a neat reversal on racist thinking, being white represents being just one offshoot of an Asian population which itself was an offshoot of the original 5th human group! Another good reference on human population migration and genetic change is Luigi Luca Cavalli-Sforza's Genes, peoples and languages.

Perhaps an overlong response to your query, but I hope it helps.

cheer

the sunshine warrior