In regard to the original question:
As Mike has explained, you won't find proof in the true sense of the word; all you will find is evidence.
One line of evidence that is compelling to me is the fact that virtually every species is surrounded by a spectrum of other species that are related to it in varying degrees.
At the "near" end of the spectrum, you will find sub-species so closely related that minor changes in size, color, features and/or behavior are the only real differences between them. Examples would include the Asian and African Lion, and the Summatran and Bengal Tigers.
As we start to move away from a given species' most closest kin, we find other, more distinct species that often can still actively interbreed and produce fertile offspring. (Sometimes even in the wild) Lions and Tigers, for example can be bred to produce "Ligers" or "Tygons." (Lion/Tiger and Tiger/Lion respectively) Other examples would include Leopard/Lion, Lynx/Bobcat, Puma/Leopard, Polar Bear/Brown Bear, Polar Bear/Kodiak Bear, Forest/Savannah African Elephants, Blue/Black Wildebeast, Eland/Kudu, Masai/Rothschild's Giraffe, Harp/Hooded Seal --the list goes on and on.
Every one of these pairings represent two species whose ancestery crosses or actually merges at some point in the past. It is intuitively apparent then that new species do diverge from common stock. This in and of itself would not be compelling if there was a clear "line" of gametic isolation conforming to the notion of Genesis "kinds" If that existed, we could write this all off as simply variartion within a "Kind" (e.g. The Lion and Tiger are both members of the "Cat Family" and hence the same basic "Kind.")
However this doesn't seem to be the case. Some species which don't appear that closely related actually are. Camels and Llamas both have 74 chromosomes. A Llama/Camel cross (By artificial insemination) produces the "Cama" --a creature with the short ears and long tail of the camel, no hump and the Llama's cloven hoves.
Other species obviously are related, but not nearly as close as we might think. Donkeys have 62 chromosomes, but horses have 64. Crossing a Donkey stallion with a Horse mare will give you either a Mule or a Molly. Because of this mismatch in karyotype, Mules are always sterile and Mollies almost always are. Similarly, crossing a Horse Stallion with a Donkey mare will sometimes produce the smaller boned Hinny, but often, it produces nothing at all. Mules, Mollies and Hinnies have 63 chromosomes.
Some species are even more distantly related. Sheep have 54 chromosomes and Goats have 60. Crosses between sheep and goats will develop to the point of birth, but they are usually stillborn, although there have been documented exceptions. The resultant creature, which has long, goat-like legs and a heavy sheep-like body has 57 chromosomes
Although a cross between the Bison and Domestic Cattle will produce healthy, fertile offspring, crossing the Water Buffalo with Domestic Cattle will produce a living Zygote, (Which means the Sperm and Egg did manage to successfully combine) but the Zygote dies when it splits beyond eight cells.
Are sheep, cattle and goats one kind or three kinds? To me, neither answer is really attractive from the standpoint of creationism. If the answer is "One kind", why do they each have distinctly different genetic karyotypes respectively? If the answer is "Three kinds", then why can some combinations still interbreed?
This to me is strong evidence that the idea of "species purity" is a human invention. Nature defies these attempts to compartmentalize creatures into static species. Life is a constantly changing genetic continuum rather than a rigid set of self-contained "Kinds."
