What is a species, anyway?
Hilary Rosner has an interesting piece at Wired Science on the campaign to keep the critically endangered Devils Hole pupfish from going extinct.
Background: during the Pluvial period there were a lot of occasionally interconnected lakes in the American southwest, and some of those lakes had little fish in them of what we’d later call genus Cyprinodon. Climate changed, the desert dried up, and desert Cyprinodons found themselves restricted to smaller and smaller bodies of water in the desert. Predictably, speciation occurred when populations were separated, and now five species live in the little bits of remaining fresh and not-so-fresh water in the desert.
Cyprinodon is a cosmopolitan genus, and many of its member species are doing okay, but desert Cyprinodons are having a tough time, as you might expect of freshwater fish in the desert. The Devils Hole pupfish, Cyprinodon diabolus, is facing the biggest threat of all: its only remaining natural habitat is Devils Hole with no apostrophe, a geothermal spring (92°F/33°C) in a limestone cave in Nevada, with the breeding habitat of the entire species limited to a small rock shelf just beneath the surface of the water. A century of groundwater pumping for agriculture has made that habitat even more tenuous. The fish live a year, and there’s a significant seasonal population crash each winter when less light means less algae. In September, according to Rosner, the global population of Devils Hole pupfish dropped from its usual level in the mid-100s to about 75 individuals.
There’s some question rising whether the Devils Hole pupfish is actually a “good” species, or simply a perpetually starved and overheated strain of the neighboring Cyprinodon nevadensis mionectes, the Ash Meadows pupfish. (Cyprinodon nevadensis is commonly split up into six subspecies, all of them but the extinct Cyprinodon nevadensis calidae still holding on slightly better than their cousin in Devils Hole. As Rosner describes, when an Ash Meadows pupfish somehow snuck into a holding tank with some Devils Hole pupfish in it, the pupfish suddenly started breeding more successfully, and within a few generations all the fish had Ash Meadows ancestry. Predictably, a pupfish biologist has suggested that a few Ash Meadows pupfish be slipped into Devils Hole to reinvigorate the population, and just as predictably others are horrified at the idea.
Rosner’s got a pretty good rundown of the politics involved, though she stumbles a bit in her handling of the definition of “species”:
Martin wants to cross two separate species. That’s supposed to be a no-no. In fact, by one definition of what constitutes a species, it shouldn’t even be possible. Scientists have long thought of species as reproductively isolated units. In the days before Darwin, if two animals couldn’t produce fertile offspring, it meant they were different species. Then things got complicated.… But it turns out that biology doesn’t even adhere to those categories. For example, the ability to reproduce can evolve far more slowly than other traits. So when one species branches off from another, it may still be able to breed with its relatives up the evolutionary tree. “It raises the question, what really is a species? It’s very hard to clearly articulate,” says M. Sanjayan, lead scientist for the Nature Conservancy. “There are lots of things that can breed together but look morphologically and genetically different.” That means scientists triaging endangered species might have more options than they thought.
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