In Mexico's Cueva del Azufre (the Sulphur Caves), a small fish called the shortfin molly (Poecilia mexicana)is on the way to becoming two separate species. Those that live in the dark interior of the caves are very different to their relatives that swim in the bright, surface waters. They have lighter colours and live more solitary lives. Their eyes are smaller, less sensitive and have lower levels of light-sensitive pigment. Instead, they rely instead on a hypersensitive pressure detector - the lateral line - to sense disturbances in the water.
Their differences aren't just skin deep either. Michael Tobler from the Texas A&M University has been studying the mollies for years, and has shown that the surface and cave populations have started to become genetically distinct. The question is why? The caves are an open habitat with no physical barriers separating the two populations. What's stopping them, and their genes, from mingling?
Tobler has found that one of these barriers is a living one. The mollies are hunted by an insect, the giant water-bug (Belostoma spp). It's about the same size as the fish and lurks close to the water's surface, waiting to stab passing prey with stiletto-like mouthparts. In the gloom of the cave, the surface fish are more vulnerable to the bug, but in the light, it's the cavefish that are at a disadvantage. The same predator, hunting throughout the Sulphur Caves, is keeping two populations of the same species apart.
Tobler collected the bugs and both types of mollies and housed them in large plastic bottles, which mimicked the cramped, shallow spaces of their natural habitat. He placed the bottles either within the cave or in a shaded spot at its mouth, and waited.
After a day, he found that in the light, the cave-adapted fish suffered most at the mouths of the bugs, and experienced four out of every five stab-wounds. In the dark, the situation was reversed, and around two-thirds of the bug attacks were inflicted on the surface fish. Even if Tobler allowed the captured mollies to acclimatise to their environments for a day before adding the predator to the mix, they still suffered the same proportion of attacks in the two environments.
There are many reasons why species can start to split into two, even though their members occupy the same range. The two populations may become separated in time rather than space. They could grow to find each other unattractive, be unable to produce a fit or healthy hybrid, or simply become unable to mate.
But Tobler's study illustrates one of the simplest blockades - immigrants between two populations do poorly. In this case, the mollies' senses can detect a predator's threat in their home environment, but not in the alternative one. Moving from one to the other greatly increases their odds of a sticky end. Tobler plans on assessing the bug's true impact in future experiments, but for now, he believes that it's a good example of the influence that predators wield over the rise of new species.
Reference:Tobler, M. (2009). Does a predatory insect contribute to the divergence between cave- and surface-adapted fish populations? Biology Letters DOI: 10.1098/rsbl.2009.0272
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