Studying the Beaks of Darwin’s Finches

Mere months after the 150th anniversary of the publication of Charles Darwin’s “On the Origin of Species”, the finches that inspired Darwin’s theory of divergent evolution are once again at the forefront of scientific research.

A team of Harvard evolutionary biologists and applied mathematicians led by Organismic and Evolutionary Biology Professor Arkhat Abzhanov published a study which reveals how variation in certain genes translates into observable phenotypic differences in the beaks of Darwin’s finches.

The team found that the link between genotype and phenotype—as it relates to beak shape—is much simpler than previously thought, with beak length, depth, and width each controlled by a single gene.

In addition, the order of the mathematical transformations necessary to create the different beak shapes and sizes corresponds to the taxonomical differences between the birds themselves.

Using a number of different analytical techniques, the team sorted 14 seemingly disparate beak shapes into three broader categories.

The team—which sought to apply quantitative methods to describe the beaks in a more mathematical way—conducted analysis that consisted of quantitative comparisons of the shapes of different species’ beaks, according to Otger Campàs-Rigau, a postdoctoral fellow in applied math specializing in morphogenesis, or the process by which an organism’s shape is determined.

“Scientists had wondered for a long time how different the beaks really are, and had analyzed their differences in a more qualitative way,” Campàs-Rigau said.

The team found that finches within the genus Geospiza all have beaks differentiated only by scaling transformations, with only a single gene controlling each of the three dimensions. But for Galápagos finches outside of Geospiza, higher order mathematical transformations were necessary to account for the physical differences in the beaks, Campàs-Rigau said.

According to team leader Abzhanov, the findings shed new light on the work of D’Arcy Wentworth Thompson, a Scottish biologist who used mathematics in the early 20th century to explore the morphological relationships between different species of crabs.

The Harvard team’s exposition of the scaling relationships present in the beaks of Darwin’s finches takes Thompson’s findings one step further.

Abzhanov said that the next challenge is for researchers to explain the genetic origins of the higher order transformations that account for the differences in beak shapes above the species level.

The team’s research was published in the Feb. 16 Early Edition of the Proceedings of the National Academy of Sciences.