My latest column for Seed is now available online. It’s an abbreviated summary of how vertebrates make segments (so it’s illustrated with a fly…), with special emphasis on the global and clocklike mechanism we use.

I’ve written a long introduction to the work I’m about to describe, but here’s the short summary: the parts of organisms are interlinked by what has historically been called laws of correlation, which are basically sets of rules that define the relationship between different characters. An individual attribute is not independent of all others: vary one feature, and as Darwin said, “other modifications, often of the most unexpected nature, will ensue”.

Now here’s a beautiful example: the regulation of the growth of mammalian molars. Teeth have long been a useful tool in systematics—especially in mammals, they are diverse, they have important functional roles, and they preserve well. They also show distinct morphological patterns, with incisors, canines, premolars, and molars arranged along the jaw, and species-specific variations within each of those tooth types. Here, for example, is the lower jaw of a fox. Look at the different kinds of teeth, and in particular, look at the differences within just the molars.

This example — the lower teeth of a grey fox — shows the three-molar dental phenotype typical of placentals.

Note that in this animal, there are three molars (the usual number for most mammals, although there are exceptions), and that the frontmost molar, M1, is the largest, M2 is the second largest, and M3, the backmost molar, is the smallest. This won’t always be the case! Some mammals have a larger M3, and others may have three molars of roughly equal size. What rules regulate the relative size of the various molars, and are there any consistent rules that operate across different species?

To answer those questions, we need to look at how the molars develop, which is exactly what Kavanagh et al. have done.

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