March 7, 2017 by Bruno Martin
A study of the super-diverse bird groups, which include Darwin's finches, has found that modular skull parts helped them adapt to different roles.
Adaptive radiation is the rapid evolution of many diverse species from a single ancestor. Typically this occurs in the colonization of island habitats, as a group of birds diversifies to exploit different ecological niches – the lifestyles, or roles, available within an ecosystem.
Two classic examples of adaptive radiation are Darwin's finches - a group of species native to the Galapagos Islands which have different beak shapes depending on whether they feed on seeds, fruits or insects - and Hawaiian honeycreepers, some of which have long beaks adapted to feeding on the nectar of different flowers.
Darwin's finches were named after Charles Darwin, whose study of this group of birds played a central role in his research into evolution.
Previously, scientists have analysed the shape of beaks to understand how birds diversify and adapt to these specialist ecological functions. However, because the beak is integrated with the rest of the skull, examining beak evolution alone does not provide a complete picture of how these island birds have achieved such remarkable diversity.
In a new study, published in Philosophical Transactions of the Royal Society B, researchers compared the 3-D structure of the entire skull in Darwin's finches, Hawaiian honeycreepers, and in their common ancestral group, from which the two diverse groups are descended.
The authors performed X-ray microcomputed tomography scans (very fine resolution CAT scans) of hundreds of cleared and dried bird skeletons and fossils of extinct species to compare the external and internal structure of their skull.
They found that skull shapes are much more diverse in Darwin's finches, and especially in Hawaiian honeycreepers, than in their ancestral group or in non-related groups of other bird species. This whole-skull variation was greater than the variation observed from looking at just their beaks. The finding suggests that variation of the entire skull plays an important role in the adaptive radiation of these birds.