Evolutionary analyses attempt to pinpoint the success of this adaptive radiation.
When I write Galapagos Islands, you might think about Darwin’s Finches. Indeed, this group of birds has become an iconic example of an adaptive radiation on these islands. However, several other bird species reached this archipelago, but never diversified in terms of species numbers or morphology. Think of the Yellow Warbler (Setophaga petechia) or the Little Vermillion Flycatcher (Pyrocephalus nanus). Or what about the Galapagos mockingbirds that are represented by just four species with little morphological differences. The contrast between these species and the more extensive radiation of the Darwin’s Finches raises an intriguing question: what is so special about these finches? A recent study in the journal Ecology and Evolution took the first steps in solving this mystery.
Ashley Reaney and his colleagues collected morphological data on 349 species from the Thraupidae family (to which the Darwin’s Finches belong). Next, they ran the Bayesian Analysis of Macroevolutionary Mixtures (BAMM) program to detect changes in evolutionary rates along the phylogeny of this bird group. These analyses revealed that the majority of Thraupidae experienced an early burst in diversification, followed by decreasing rates until the present. There were, however, two exceptions: the Darwin’s Finches and the seedeaters (genus Sporophila). These sections of the evolutionary tree experienced a recent increase in diversification rate – 6 million years ago for the Darwin’s Finches and 21 million years ago for the seedeaters. This dramatic contrast is nicely illustrated in the figure below where the rapid diversification (in red) stands out against the decreasing rates in the overall phylogeny (in blue).
These findings highlight the unique radiation of the Darwin’s Finches, but still leave our original question unanswered: what is so special about these birds? Additional analyses revealed that these finches occupy a far larger area of the beak morphospace compared to the other species (including the seedeaters). In other words, the Darwin’s Finches show a greater variety of beak shapes, allowing them to enter more ecological niches and diversify into several species. And although ecological opportunities and biogeographic factors certainly played a role in the radiation of Darwin’s Finches, the researchers suspect that the unique developmental and genetic features of these birds were equally (or perhaps even more) important.
It is possible that the ancestor of the Darwin’s Finches that arrived on the islands was “already endowed with the genetic propensity to produce the high levels of beak variation needed to explore new dietary niches.” This propensity for diversification – also known as evolvability – concerns several intrinsic factors that allow certain species to rapidly adapt to new environments. These factors might be related to genetics (e.g., certain mutations or gene flow from other populations) or particular developmental programs. Previous research demonstrated that the cranium of Darwin’s Finches is highly modular, allowing different beak traits to evolve independently from one another. Another possibility – which might seem contradictory – involves the integration of the entire cranium through developmental and genetic connections between the different beak traits. The interplay between modular change and integration might explain the impressive evolvability of the Darwin’s Finches.
The researchers conclude that these hypotheses will need to be tested in other adaptive radiations, such as the Hawaiian honeycreepers or the Malagasy vangas. Moreover, future research should focus on the evolution of developmental genetic programs, including those underlying beak morphology (which I covered in this blog post). If we want to understand the diversification of life on our planet, we will have to combine evolutionary analyses with detailed developmental studies. Time for some evo-devo.
Reaney, A. M., Bouchenak‐Khelladi, Y., Tobias, J. A., & Abzhanov, A. (2020). Ecological and morphological determinants of evolutionary diversification in Darwin’s finches and their relatives. Ecology and Evolution, 10(24), 14020-14032.
Featured image: a collage of different Darwin’s Finches (Geospiza magnirostris, Geospiza fortis, Certhidea fusca, Camarhynchus parvulus) © Kiwi Rex | Wikimedia Commons