Divergence in call types results in reproductive isolation.
New species can arise despite ongoing gene flow. One possible route is ecological speciation where populations become reproductively isolation due to divergent natural selection on particular traits. A textbook example of such a scenario concerns Crossbill (genus Loxia). These birds have diversified in beak morphology because they specialized on eating the seeds from different conifer species. The different beak shapes lead to different call types which are used to form flocks. Because the birds pick their partner within a flock, there is reproductive isolation between the different call types. And it all started with a cone seed…
A recent study in Proceedings of the Royal Society B focused on the Cassia Crossbill (L. sinesciuris) which recently diverged from the Red Crossbill (L. curvirostra). About 5000 years ago, the ancestors of the Cassia Crossbill colonized the South Hills. Because the dominant seed predator – the American Red Squirrel (Tamiasciurus hudsonicus) – was absent, the birds adapted to the lodgepole pine and developed larger bills compared to the Red Crossbills. More recently, a certain type of Red Crossbill – the Ponderosa Pine Crossbill – entered the South Hills. The secondary contact between the Cassia Crossbill and the Ponderosa Pine Crossbill allowed the researchers to test ecological speciation scenario outlined above. They made two predictions:
- The calls of the Cassia Crossbill and the Ponderosa Pine Crossbill should become more different over time.
- The birds should flock according to call type.
Divergence in call type?
To test the first prediction, the researchers analyzed more than 3000 recordings of Cassia Crossbills over a period of 20 years (1998-2018). The results show that the calls of these birds steadily become more and more different from the Ponderosa Pine Crossbill (orange line in figure). Next, they compared the calls of Cassia Crossbills with Lodgepole Pine Crossbills, a population that does not coexist with the Cassia Crossbills. Because these species do not interact, there should be not divergent selection on the call types. And indeed, the difference between calls of Cassia Crossbill and Lodgepole Pine Crossbills remained stable over the study period (purple line in figure).
The first prediction is confirmed! What about the second one? Do crossbills flock according to call type? To assess the second prediction, the researchers performed playback experiments: they played recordings of Cassia Crossbills that were similar or dissimilar to Ponderosa Pine Crossbill calls. According to the ecological speciation scenario, more Cassia Crossbills should be attracted to the calls that sounded different from the Ponderosa Pine Crossbill. And this was indeed the case as shown in the barplots below.
This case nicely exemplifies how cultural evolution can contribute to speciation. The calls are learned from the parents and can thus be considered a cultural trait. The authors conclude that “these increasingly divergent vocalizations are imitated by offspring, leading to call divergence at the population level and reduced heterospecific flocking. […] Because Crossbills flock year-round and choose mates from within flocks, increased reproductive isolation is probably a byproduct of character displacement in call structure.”
Porter, C. K., & Benkman, C. W. (2019). Character displacement of a learned behaviour and its implications for ecological speciation. Proceedings of the Royal Society B, 286(1908), 20190761.