The evolution of mammalian teeth is characterized by the frequent and convergent evolution of new cusps. The evolution of new cusps can be linked to tooth development via population-level variation. This allows testing whether development increases the capacity to evolve, or evolvability, by facilitating and even directing morphological change. In a population sample of living seals, variation in cusp number of individual teeth is from three to five cusps, the variably present cusps being the shortest ones that also develop last. By factoring in recent evidence on development, I show that the variation in cusp number can be explained by a patterning cascade mode of cusp development that cumulatively increases and directs height variation in short cusps. The biased variation in seal tooth cusps supports the recognition of teeth as highly evolvable because only small developmental changes are needed to produce large changes in size and number of small cusps. This evolvability of tooth cusps may have facilitated the fast and independent acquisition of new cusps in mammalian evolution. In phylogenetic studies, small cusps may be unreliable as phylogenetic signals. Population level variation can be a powerful tool in testing and generating hypotheses in developmental evolution studies
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