ANIMALS and plants that go largely unnoticed because they occupy a small area are the ones that will provide the planet鈥檚 next batch of species.
At first glance it would seem that hugely successful species that range widely, such as the rat and housefly, are more likely to beget new species. The reasoning goes like this: a small group of individuals has a greater chance than most of getting cut off from their kin by a physical barrier, such as a mountain range or the changing course of a river. After this, they will either drift down a different evolutionary path or be forced to specialise. Also, animals and plants with large ranges are more likely to face, and have to adapt to, a wider variety of environmental conditions.
But David Jablonski at the University of Chicago says that being able to disperse widely and tolerate a range of conditions are precisely the attributes that prevent genetic divergence. 鈥淭he biological characteristics that allow a broad range also make species resistant to speciation,鈥� he says. And now he has empirical evidence to back his ideas.
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Jablonski and his colleague Kaustuv Roy at the University of California, San Diego, compared the geographical range and speciation rate of 420 species of marine snails that lived 65 to 85 million years ago, during the Cretaceous period. The fossils occur in a rock outcrop that extends 4000 kilometres, from New Jersey to Mexico. Species with small geographical ranges gave rise to more new species, the researchers will report in a future issue of Proceedings of the Royal Society B. An analysis of 135 living snail species revealed a similar, although smaller, trend.
Ironically, the very features that make species more likely to give rise to new species, such as limited dispersal and extreme specialisation to a particular niche, also make for a fragile existence. 鈥淎ttributes making you prone to extinction also make you prone to speciation,鈥� says Jablonski. But this might mean the effect is even stronger than the researchers were able to measure, because very short-lived species with tiny ranges are unlikely to turn up in the fossil record.