When, why, and how does speciation take place? Travis Ingram, professor at the University of Otago in New Zealand, tackled this question in his talk at Evolution 2017 (and in this paper) by examining Anolis speciation in the context of anoles’ most enigmatic trait–the dewlap.
Ingram posited that we can think of relationships between speciation rates and the value of particular traits in two ways. One possibility is that the value of a particular trait in a lineage influences the probability that that lineage speciates, trait evolution facilitating speciation. Conversely, particular traits may be especially likely to diversify at speciation events, in response to speciation. Ingram tested these two hypotheses in Anolis, crowd-sourcing photographs of outstretched anole dewlaps to quantify dewlap size and ending up with analyze-able dewlap size information for 184 species from across the whole clade.
Ingram detected no relationship between speciation rates and dewlap size, indicating no evidence for dewlap-size-dependent speciation in anoles (possibility 1 above). However, probing a bit further, Ingram considered why bigger dewlaps may be related to speciation rates–what if a bigger dewlap allows for greater pattern complexity, allowing more species to coexist by accessing more axes along which their dewlaps can diverge? Quantifying dewlap complexity as the number of colours on a dewlap, Ingram did find a relationship between size and complexity, but curiously, more complex dewlaps were linked to lower, and not higher, speciation rates. Why remains a mystery. Suggesting evidence for speciational evolution (possibility 2 above), 34% of dewlap size evolution was associated with speciation events. Intriguingly, this pattern was driven almost entirely by mainland and not island anoles.
In sum, though the precise processes linking speciation and dewlap evolution remain rather enigmatic, it seems to me that Ingram’s macroevolutionary approach has given us a number of directions in which to take microevolutionary and behavioral ecological studies to understand why dewlaps vary in the ways that they do!