Without question, "species" is THE most important category in bio-classification. Now, there have been really big problems on how a species is defined. The layman's definition is "a group of creatures that can interbreed". Well, I think that's a poor definition, because it's not objective enough. Horses and donkeys can only have sterile kids. Lions and tigers sometimes have sterile kids. The way I see it, a species should be based on two simple criteria: 1. Creatures of the same species must RECOGNIZE each other, through some means, that they are of the same kind, and therefore WOULD mate with one another under natural settings. 2. Creatures of the same species must be able to have FULLY fertile kids. In other words, there must not be chromosomal mis-arrangements within a species, which would either always or sometimes cause sterility in the kids should mating occur. Is my definition good enough?
Heh, the species concept is slowly dying. But your definition completely omits asexually reproducing organisms. The species concept that you use is essentially that of Erns Mayr, a group of interbreeding natural population that is reproductively isolated from other groups. The recognition part is not really necessary, as it is already implicit in the definition. The second I can agree on, but the same goes for almost all species definitions, the population must be stable. That would not be the case if the kids were infertile. But as I said before, most (if not all) species concepts are lacking in one or the other aspect (especially when it comes down to prokaryotes). It is less universal as it was believed to be.
You've got it wrong. You're thinking of a genus. When I was a kid, before DNA analysis could be done, the definition of a genus was a group of species that could interbreed. A layman's definition of a species is more along the lines of: "A population of creatures that does not interbreed with other populations except in extreme circumstances." And that only works with animals; it's more difficult to define for plants. But there are plenty of examples of interspecies hybridization that produce fertile offspring. Wolves began to interbreed with coyotes in eastern Canada when their population declined, and now multi-generation wolf-coyote hybrids are spreading throughout the eastern USA. The black-headed grosbeak and the rose-breasted grosbeak began to crossbreed when the forest along the Mississippi River that separated their ranges was cut down and replaced with farmland; today multi-generation hybrid grosbeaks have crossed the Rocky Mountans and show up at our feeders in California. The establishment of bloodlines of fertile hybrids is common in the pet trade. Hybrid cockatoos, Amazons, conures and macaws are readily available, and many have been bred for a number of generations. Domestic cats have been crossbred with ocelots and the offspring are fertile. The same is true in farming; the American bison has been hybridized with domestic cattle. Cross-pollination of plants is unavoidable and hybrid seeds may germinate, but since they are not as well adapted to the environment as one or the other parent species, they seldom survive. But where man has disrupted the environment, such as by building highways, thriving stands of hybrid plants that are best-suited to the new environment are sometimes spotted.
However, if creatures who see one another as the same kind AND can have fully fertile kids, but cannot (at least temporarily) mate under natural settings due to a geographic barrier should STILL be the same species. Under the current definition, they would be different species, which I think is wrong. Has it been determined that wolf-coyote hybrids are FULLY fertile? How long have wolves and coyotes diverged? Are there any chromosomal inversions between them? Large chromosomal inversions would lead to occasional sterility, I think. Anyway, my definition of a species would call for FULL interfertility, and this can be objectively checked to see if there are any chromosomal inversions between two populations.
