Testing different evolutionary models in the Eastern Double-collared Sunbird species complex.
Evolution is often depicted as the slow accumulation of genetic differences, potentially giving rise to new species when two lineages diverge. However, evolution can also proceed quite rapidly. Long periods of stability – or stasis – are punctuated by bursts of evolutionary change (I covered a possible genetic mechanism for this model of punctuated equilibria in this blog post). We now know that both models of evolution – gradualism and punctuated equilibria – occur, but the relatively contribution of these models remains a matter of debate. Moreover, they might just represent the extremes on a continuum of evolutionary speed. For each and every trait, we can investigate whether their evolution was gradual or punctuated. A recent study in the journal Proceedings of the Royal Society B took a closer look at the evolution of certain song characteristics.
Before we dive into the study, let’s see if we can deduce the most likely evolutionary model for bird song. In most species, song is learned by imitating conspecifics. Small mistakes during this learning process can result in the development of new song variants or dialects. These mistakes might accumulate gradually, giving rise to almost continuous variation. This scenario has been documented in the Greenish Warbler (Phylloscopus trochiloides, see also this blog post). Alternatively, female birds might not be impressed by the new songs and select for males with a more conservative repertoire. This stabilizing selection can lead to long periods of stasis. When a new song variant pops up that is favored by females, it will quickly spread through the population. A punctuated change. This might have happened in some populations of White-throated Sparrows (Zonotrichia albicollis). Clearly, both gradual and punctuated changes are possible when it comes to bird song.
In the current study, Jay McEntee and his colleagues focused on members of the Eastern Double-collared Sunbird (Cinnyris mediocris) species complex. These colorful birds can be found in the mountainous areas of East Africa where different populations have evolved distinct songs. The researchers quantified the variation in 14 song traits across the distribution of these sunbirds. After checking that each genetic lineage produces its own distinct song (which was indeed the case), different evolutionary models were tested on the dataset. These analyses revealed that seven song traits followed a pattern of punctuated equilibria, while the remaining traits could not be assigned to a specific evolutionary model. Interestingly, the bursts of evolutionary change for four traits were all located on the branch leading to one set of populations belonging to the Forest Double-collared Sunbird (Cinnyris fuelleborni).
Speculating about Mechanisms
These findings suggest that some song traits in the Double-collared Sunbird species complex show punctuated patterns. However, it remains to be determined which mechanisms underlie periods of stasis and bursts of rapid evolutionary change. When it comes to the stasis of songs, the researchers offer two possibilities: stabilizing selection (as discussed above) or occasional gene flow between the populations. The latter explanation seems unlikely, because most populations are isolated on distant mountains. And although some populations are interbreeding – such as C. fuelleborni and C. moreaui – the levels of genetic and cultural exchange are probably to low to have a significant stabilizing effect on the evolution of song.
And what about the punctuated changes? One hypothesis states that these changes are a by-product of morphological evolution. Indeed, previous work has shown that the evolution of bird song is tightly connected to changes on morphology (see for example this blog post). However, the researchers deem this explanation unlikely, because “there is limited morphological evolution” in this species complex. Moreover, the largest species – the Loveridge’s Sunbird (C. loveridgei) – produces the highest frequency songs, while you would expect lower frequency songs based on its body size. However, I find this reasoning unconvincing. Besides body size, there are numerous other morphological changes that can have an impact on song frequency. Detailed morphological analyses are needed here.
An alternative possibility concerns rapid range expansion in which a series of founder effects introduces new song variants throughout the range of another population. Selection for certain variants can then result in the rapid rise of a new song. Reconstructing the evolutionary history of the Double-collared Sunbird species complex can reveal whether this scenario is plausible or not.
McEntee, J. P., Zhelezov, G., Werema, C., Najar, N., Peñalba, J. V., Mulungu, E., Mbilinyi, M., Karimi, S., Chumakova, L., Burleigh, G. J. & Bowie, R. C. (2021). Punctuated evolution in the learned songs of African sunbirds. Proceedings of the Royal Society B, 288(1963), 20212062.
Featured image: Eastern Double-collared Sunbird (Cinnyris mediocris) © Nigal Voaden | Wikimedia Commons