All species are transitional

Though this is more properly reproductive failure than isolation (unless you consider failure to be total isolation), one unfortunate, novel cause is pollution: pesticides and endocrine disruptors are impacting birds and aquatic vertebrates alike. I saw some popular coverage of this a few weeks ago; I'll see if I can come up with a link later.Perhaps natural (botanical) endocrine disruptors or other naturally occuring substances could biochemically change reproductive behavior in one part of a species' habitat range but not another. If the species had complex reproductive behavior, the loss of a single behavioral component (say singing remains but tail display lost) might begin to isolate that population. Or maybe a hyperpredator on the margin of a range starts wolfing down every cock who crows too loudly--a few nerdish cocks might survive and pass on their lives of quiet desperation Failure to respond to reproductive stimuli, failure to respond appropriately to threat, gonadal development failure--the soup of complex compounds we have made of the earth is beginning to show up in unexpected ways.I have always felt uncomfortable about evaluations of toxicity performed on isolated compounds: sure, 1 part per brazillion demonstrates no toxicity, but what if we put that compound into a body/ecosystem that already contains a thousand other pollutants?Anyway, that's an off-topic pet peeve...

Another possibility is extrinsic causation: for example, the drug colchicine can create polyploidal plants.If an external stimulus (chemical, viral, etc.) creates polyploidy, and a significant fraction of a population (rather than just an individual) is subjected to the stimulus, then finding a polyploidal mate might be fairly likely.

I ran across a proffered example last night--I believe it was bivalves. I'll see if I can find it again.