Why do some phenotypic traits change while others
remain the same during evolution?
Student engagement: Canalization and evolvability: Darwin’s finches beaks vs. Drosophila sex combs. To motivate students to join this research project, I used to asked them about evolution. As it is natural, we ended up talking about the morphological changes observed in Darwin’s finches. After few mintues of brainstorming ideas, we agreed that it was necessary to change the shape of the beak, while other aspects of this trait, such as coloration and hardness, needed to remain the same.. Then I explained to them that some evolutionary studies focus on how aspects of a phenotypic trait change (evolvability) while others concentrate on how they remain the same (canalization). However, it is unknown how these two opposing processes can take places simulateneously and whether developmental processes can play a role in solving this paradox. Fortunately, sex comb evolution is an ideal model system to study this interplay, as while sex comb orientation is constantly changing during evolution, its shape remains stable.
Findings: Our work shows that physical forces acting on tissues are key to understanding the mechanisms that prevent and promote morphological variation in sex comb evolution. I will only concentrate on explaining, the role of tissue mechanics in maintaining a similar shape during evolution. As a comb forms a column of bristles, sex combs follow the buckling principle which governs other columns in nature. Long columns require less forces to bend them than short columns. As a result, combs of small and intermediate size tend to be straight, while long combs display multiple developmental modifications to remain as straight as possible. Our results are supported by genetic pertubations in D. melanogaster, morphometric analysis in Drosophila species and computer simulations. This work demonstrated that tissue mechanics not only provide an important insight to understand how phenotypes originate, but also how phenotypes evolve.