Context and overall objectives
Today on our planet we see an enormous variety of living creatures, including ourselves. But how did this remarkable plethora of diverse forms come into existance? According to the integrative field of evolutionary developmental biology (evo-devo), the morphological changes seen in adult organisms first appear during their embryonic development, as a result of molecular modifications. Therefore, revealing which particular developmental program is responsible for a specific evolutionary change is pivotal to our understanding of the origin of diversity. Of all land vertebrates, birds are the most species-rich and, arguably, most diverse group with about 10,000 species. Much of this success can be attributed to their toothless beaks, a novelty of the bird lineage. The astonishing variation in shape and size of bird beaks reflects a wide range of dietary specializations, and hence of ecological niches they occupy. This makes birds an interesting model to study general evolutionary principles by focusing on mechanisms underlying their craniofacial development.
The iconic Darwin’s finches inhabit Galápagos and Cocos Islands. They are a group of about 15 songbird species all of whom arose from a single colonizing ancestor and now occupy ecological niches normally filled by representatives of multiple bird families. While very similar in overall appearance, Darwin’s finches are strikingly distinct in the size and shape of their beaks and in feeding behaviours (Figure 1). Hence, they represent a classical example of, and an inviting model for studying factors underlying rapid morphological evolution and adaptive radiation. Darwin’s finches have been extensively studied but knowledge on the molecular mechanisms controlling their embryonic development has been lacking and was provided by Dr. Abzhanov, the supervisor of this project. His team has used a combination of morphometrics, comparative developmental genetic and functional tests to reveal basic principles underlying beak shape morphogenesis. The team discovered several signaling pathways that during embryonic development control independently the three axes of beak morphology (length, depth and width). The three axes represent separate developmental modules which allow for independent variability among them, thus increasing overall variation in beak shape in the course of evolution. The team showed that the enormous beak diversity in Darwin’s finches could be reduced to three “group shapes” (A, B, and C) where group members are related by scaling, and revealed specific molecular mechanisms controlling the variation within group “A”. Within-group variation is due to scaling but the difference between groups is determined by beak curvature. The main objective of this project was to identify the developmental programs underlying the leaps of beak shape diversification during Darwin’s finches’ adaptive radiation – the variation between “group shapes” (Figure 2).
