How did the diversity of life come to be? Darwin proposed a simple yet elegant answer to this longstanding question: adaptation to alternative ecological conditions ultimately results in the formation of new species. There is now a large body of evidence that strongly supports Darwin's hypothesis. However, beyond a well-established association between adaptive divergence and speciation, we have little understanding of how these two key evolutionary processes become coupled. My work attempts to help fill this gap. Outlined below are a few specific areas of personal interest on the origin of species.

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Why is sympatric speciation rare in birds?
Allopatric speciation is the primary driver of bird diversity. But closely related bird species often show the sort of niche divergence required for sympatric speciation. Among birds, few groups conform to theoretical conditions for sympatric speciation better than red crossbills, yet only a few lineages (e.g., the Cassia crossbill) have completed this process. My work suggests that, when the feeding tradeoffs driving adaptive divergence coincide with breeding, the conditions for sympatric speciation are met in most crossbill ecotypes. However, sympatric ecotypes sometimes breed during conditions of weak feeding tradeoffs and high resource availability, which leads to increased levels of gene flow that may constrain divergence. Because many animals time reproduction to coincide with abundant, easily-accessible food resources, this may explain the rarity of sympatric speciation except in groups like phytophagous insects.



A recording of my 2021 Evolution talk on this topic is to the right ->

Grouping behavior's role in speciation
Diverging lineages can become reproductively isolated if phenotypically similar individuals form aggregations wherein mating occurs. Such "grouping" mechanisms automatically generate assortative mating with respect to the traits that mediate group formation. Classic examples of grouping mechanisms playing a key role in speciation include host plant choice in some herbivorous insects (e.g., Rhagoletis flies, Enchenopa treehoppers). Red crossbills show a different type of grouping mechanism that has received little study in the speciation world. They feed in large flocks, where they use public information on flock mates' feeding rates to more efficiently assess resource quality. This favors flocking among phenotypically similar individuals that have similar feeding abilities. Because crossbills choose mates from within flocks, this "assortative flocking" leads to reproductive isolation between ecotypes and may have been key to crossbill diversification. I am also exploring social grouping behavior's role in speciation in other taxa.

Can cultural evolution promote speciation?
Biologists have long suggested that cultural evolution might have contributed to the extraordinary diversity of birds with learned songs (songbirds, hummingbirds, and parrots). However, direct evidence for cultural evolution's role in speciation has been challenging to come by, and some evidence indicates that learned traits may impede speciation. Moreover, most evidence suggests that learned traits diverge through random processes in allopatry. Using a long-term dataset of banded Cassia crossbill audio recordings, we found evidence for rapid divergence in a learned trait (contact calls) between sympatric crossbill lineages. Comparative analyses of call structure, playback experiments in the field, and previous genomic and paleobotanical studies all suggest that rapid evolution of contact calls may have been key to the rapid diversification of crossbills in sympatry. Importantly, call evolution appears to be linked to adaptive divergence in this and perhaps other systems.