I bought a fine copy of the Life and Letters of Thomas Henry Huxley in 3 volumes. On page 343f of volume I I find his thoughts on speciation, addressed to the Rev. Charles Kingsley (author of the Water Babies) on April 30, 1863.
My dear Kingsley
With respect to the sterility question [between species], I do not think there is much doubt as to the effect of breeding in and in [we would now use the neologism “inbreeding”] in destroying fertility. But the sterility which must be obtained by the selective breeder in order to convert his morphological species into physiological species – such as we have in nature – must be quite irrespective of breeding in and in.
There is no question of breeding in and in between a horse and an ass, and yet their produce is usually a sterile hybrid.
So if Carrier and Tumbler [varieties of pigeon], e.g., were physiological species equivalent to Horse and Ass, their progeny ought to be sterile or semi-sterile. So far as experience has gone, on the contrary, it is perfectly fertile – as fertile as the progeny of Carrier and Carrier or Tumbler and Tumbler.
From the first time that I wrote about Darwin’s book in the Times and the Westminster [Review] until now, it has been obvious to me that this is the weak point of Darwin’s doctrine. He has shown that selective breeding is a vera causa [Whewell’s term for a true cause, taken from Newton] for physiological species.
But I entertain little doubt that a carefully devised system of experimentation would produce physiological species by selection – only the feat has not been performed yet.
150 odd years later, it still has not. Most biologists would agree with Huxley that sterility is not the outcome of selection, but of drift and hitchhiking upon selection for ecological effects. [In animals, at any rate. Most of the time.]