What can the Galapagos finch beaks really tell us?

It is common wisdom that the shapes of bird beaks are closely linked to feeding ecologies.  Texts similar to the following are not difficult to find: “Changes in the size and form of the beak have enabled different species to utilize different food resources such us insects, seeds, nectar from cactus flowers as well as blood from iguanas, all driven by Darwinian selection.”  The historical context for such statements can be found in observations of finches made by Charles Darwin when he visited the Galapagos Islands.  Beak shape appeared to correlate with diet and feeding behaviour.

It is only in recent years that quantitative analyses of beak shapes have provided evidence to support Darwin’s theory of evolution by natural selection, although the number of such studies has been few.  Textbooks recount the finch beak analyses to support a general rule that feeding ecology drives the selection of bird beak shapes.  However, what has been called a “truism” has now been shown to be highly misleading.

A Press Release from the University of Bristol summarises the new findings as follows:

An international team of scientists from the United Kingdom, Spain and the US used computational and mathematical techniques to better understand the connection between beak shapes and functions in living birds. By measuring beak shape in a wide range of modern bird species from museum collections and looking at information about how the beak is used by different species to eat different foods, the team were able to assess the link between beak shape and feeding behaviour.

Professor Emily Rayfield, from the University of Bristol’s School of Earth Sciences, and senior author of the study, said: “This is, to our knowledge, the first approach to test a long-standing principle in biology: that the beak shape and function of birds is tightly linked to their feeding ecologies.”

Guillermo Navalón, lead author of the study and a final year PhD student at Bristol’s School of Earth Sciences, added: “The connection between beak shapes and feeding ecology in birds was much weaker and more complex than we expected and that while there is definitely a relationship there, many species with similarly shaped beaks forage in entirely different ways and on entirely different kinds of food. This is something that has been shown in other animal groups, but in birds this relationship was always assumed to be stronger.”

Darwinism has had a long tradition of extrapolating small changes in morphology to major changes affecting the complexity of organisms and their body plans.  So strong has been the Darwinian attachment to the concept of natural selection, that specific cases involving minor dimensional changes have become generalised to explain all bird beaks as the product of adaptive evolution.  They think it is self-evident that natural selection is the key to understanding the connection between beak shape and feeding behaviour.

A team of scientists have been testing the hypothesis that “beak morphology predicts feeding ecology in birds”.  The data set involved 175 species, from 94 families and 38 orders of birds.  They found that “diet accounts for less than 12% of beak shape variation”.  This means that the hypothesis has “little explanatory power”.  The scientists write: “Our results suggest that adaptation to dietary composition is not as fine-tuned as generally perceived, and there is not a close to one-to-one mapping of beak shape on feeding ecology.”

It is to be hoped that textbook authors will take these findings on board, and make the appropriate revisions.  These should include the following.

1. Present the Galapagos finch data in the context of microevolution, with reversible adaptations to changing environments;
2. In the absence of an evidence base, avoid the temptation to extrapolate microevolution to explain all the diversity in living organisms;
3. To recognise that scientists have only recently been able to lay aside the dogmatism of Darwinism, and realise that much research remains to be done. The lead author of the research is quoted as saying: “Really, we’re just starting to scratch the surface, and a lot more research is needed to fully understand the drivers behind beak shape evolution.”

David J. Tyler (8 February 2019)