Understanding Evolution

Research Profiles : The genes that lie beneath :

Discovering disparity

baboon tooth cross-section
In this baboon tooth cross-section, the enamel is the thick white layer along the top of the tooth.
When Leslea and her team examined the outcome of the baboon tooth analysis, she was particularly struck by their enamel — the hard, thin outer coating of the tooth that helps form a sharp edge for slicing flesh or dull surface for grinding grain. First, she found that baboons with thick tooth enamel tended to be related to other baboons with thick tooth enamel — indicating that the thickness of tooth enamel is genetically-influenced. No surprise there. But Leslea also discovered an unexpectedly large amount of variation in enamel thickness; within a single, tight-knit population, some baboons had just a thin veneer of enamel and some had a thick coating.

What's so interesting about this high level of variation? For one thing, it means that enamel thickness may evolve easily. If a trait does not vary within a population, natural selection is powerless to act on that trait: the population is stuck with whatever trait it's got — even when faced with a changing environment (e.g., being stuck with a low heat tolerance in an environment with rising temperatures). However, when a trait varies a lot within a population, it can evolve quickly, since natural selection can easily weed out the variants that are less successful in the new circumstances (e.g., if a population had a lot of variation in temperature sensitivity and heat tolerance, it would be in a much better position to survive and evolve when faced with rising temperatures).

The speed with which enamel thickness can evolve in primates helps validate a method that paleontologists have been relying on for hundreds of years: using tooth shape and enamel thickness as indicators of diet and lifestyle. For example, we generally assume that a fossil mammal with thick enamel and rounded cusps used its teeth to grind hard objects, like seeds. But one could also imagine a situation in which a species' diet changed rapidly (e.g., a key food source went extinct and the species wound up relying on a new food source) but the evolution required to shape the species' teeth for the new food lagged behind — in that case, there would be a mismatch between the fossil remains of the animal and its actual lifestyle. However, Leslea's findings downplay that possibility: natural selection can quickly mold enamel thickness to reflect diet and lifestyle changes, so there should be a tight match between the two.

Enamel thickness is a good indicator of lifestyle.

This means that enamel thickness is a better indicator of current lifestyle than of evolutionary history. Traits that evolve slowly record evolutionary history and can help us figure out how different organisms are related to one another — but traits that evolve rapidly often overwrite evidence of a species' evolutionary history and instead reflect local ecological pressures.


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Baboon tooth cross-section scan provided by Leslea Hlusko

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