Its not that the subspecies no longer produces fertile offspring, it’s just that one subspecies evolves into another, with neither population going extinct. Both groups continue to produce viable offspring, but one group simply contains slightly different organisms from its parent.
I'm from Fort Pierce, Florida, where a type of lizard called the Cuban anole (
Anolis sagrei sagrei) is extremely common. They are an introduced species which seems to be forcing out the native species of lizards, green anoles (
Anolis carolinensis) for example. Cuban anoles have been prevalent there for a few decades, according to my mother, who has lived in Fort Pierce for 50 years.
Now I go college at UF, which is in Gainesville, about 200 miles north of Fort Pierce. I haven't seen many lizards around here, but all the ones I have seen are Cuban anoles with slightly different markings. They are a bit darker, and appear smaller as well. The few males I've seen lack crests, perhaps because they are not as healthy, or maybe for some other reason. There aren't nearly as many of them. Also, one morning I noticed one outside my window, and it looked much less jittery than the ones from Fort Pierce. They move much more slowly and deliberately.
I think the reason for this is that the lizards here have begun to adapt to the slightly colder winters, and fewer of them can survive each year. They are not cold tolerant, and during the winter at night it sometimes gets into the 20s, while in Fort Pierce it rarely gets below 30, and I would imagine their native home of Cuba is even warmer, though I haven't looked it up. Gainesville is in a less tropical region than Fort Pierce, and it’s also farther from the coast, so the different environment appears to be having an effect on this species. I think that over time, they may be able to invade areas even farther north, where there may be more open niches for them. They eat insects, so if they can adapt to the cold, they will probably do well.
My point is that they are evolving by a mechanism similar to that of ring species. Over just a few decades, Cuban anoles have spread hundreds of miles into a less favorable climate, and have begun to adapt to it through natural selection. Though they can probably still interbreed successfully with the anoles from Fort Pierce or even Cuba, they are already showing noticeable changes in their coloration and behavior, and I believe that if they are allowed to continue farther northward, they will be less able to interbreed with the parent population in Cuba.
If anyone would like to see pictures of these lizards, I don't have any that would show the differences between those here and in Fort Pierce. This website has a few pictures of what the ones in Corkscrew Swamp Sanctuary look like, with information on each species. Corkscrew Swamp is far south of Fort Pierce, closer to the end of the peninsula.
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