Whatever they are called, even one instance of fertile offspring proves that horses and donkeys are NOT different species (unless, of course, you believe in the sort of speciation that gives you nearly 7 million different species of salmon, to say nothing of tuna).
"Whatever they are called, even one instance of fertile offspring proves that horses and donkeys are NOT different species."
Sure they are. The fertility rate of the offspring is almost zero (60 documented cases in 500 years of a fertile mules), and far far below replacement levels.
Certainly the notion of "species" is a slippery one (and, to a large degree, arbitrary) but I think it is generally agreed that horses and donkeys are different species because they have different numbers of chromosomes, mating isn't natural and hybrid offspring are nearly always sterile.
Imagine the following situation. One billion individuals in species A. One billion individuals in species B. Both species are genetically diverse, but there is a high probability that any two individuals of A (of the opposite sex) could successfully breed; likewise B. It just happens that there is one male in species A which would produce fertile offspring with 1 female in species B if they happened to meet and "get the urge". This is an extreme example of ring speciation. In such a situation would you seriously claim that A and B are still the same species? If one of the potential co-breeders died would you still claim that they were the same species, or would you admit to a speciation event at that point? This could lead to a bizarre situation where the two groups "flickered" in and out of being the same species as the rare potential co-breeders were born and died.
The whole concept of species is a difficult one to pin down, as examples like that demonstrate.
(unless, of course, you believe in the sort of speciation that gives you nearly 7 million different species of salmon, to say nothing of tuna).
The substance of your point about salmon is unclear to me. Are you asserting that there are seven million distinct groups of salmon that have difficulty interbreeding?
So what do you think of this?:
http://www.freerepublic.com/focus/f-chat/1351793/posts