There’s much excitement about a new paper in Science that shows how ants have hidden potential. In short, there are a few species of ants that can produce “supersoldiers”. Other ant species, however, can also make supersoliders when they are experimentally give the right dose of hormone.
Crudely, it looks like the ants’ ancestors had the ability by changing the hormone levels, but the pathway that was sensitive to the hormone remained. When species started to evolve differences in hormone levels again, the supersoldier body type “re-emerged” after a long period of suppression.
I wanted to talk not about the paper so much as a concept it illustrates that I don’t think has gotten enough attention in the evolutionary biology literature.
Homology is a critical concept in evolutionary biology. A generally used definition is a feature that two different species share because they inherited it from a common ancestor. But homology can be tricky, because a “feature” or “trait” isn’t a single thing. Features have different levels of organization: genes, cells, tissues, organs, and so on. And each level can evolve long a different path than the others.
Plus, the concept of homology was first proposed first by Richard Owen, who rejected the idea of evolution by natural selection. And it was widely used before we had learned a lot about the subtleties of genetics.
For instance, we’ve learned that just because you have a gene doesn’t mean it’s expressed. Much like the supersoldier ants, you can imagine a scenario where a gene (or gene network, etc.) is present in an ancestral species, but not expressed. The species diversifies, and diversified, leaving many daughter species with the unexpressed genes. Then, independently, several of the daughters of the original ancestor start expressing the genes, and the trait pops up in distantly related species.
From the point of view of the phylogeny, that feature doesn’t look homologous, even though there is evolutionary continuity at the genetic level from a common ancestor.
Butler and Saidel recognized this scenario some time ago. They coined the term “syngeny” to describe a feature arising from genes that are present, but rarely expressed. I heard Butler discuss this at a JB Johnston Club meeting, citing a particular fish brain structure that appears only in a few not closely related species. The idea was great.
What the ant story brings that they didn’t have at the time was the ability to bring their feature back through an experimental manipulation. It’s likely that many cases of syngeny are not going to be as easy to show in the lab as in the ants.
Rajakumar and colleagues, the authors on the ant paper, call what they’re seeing parallel evolution, but syngeny might be a better description.
Butler A & Saidel W. 2000. Defining sameness: historical, biological, and generative homology BioEssays 22(9): 846-853. DOI: 10.1002/1521-1878(200009)22:93.0.CO;2-R
Rajakumar R, San Mauro D, Dijkstra MB, Huang MH, Wheeler DE, Hiou-Tim F, Khila A, Cournoyea M, Abouheif E. 2012. Ancestral developmental potential facilitates parallel evolution in ants. Science 335(6064): 79-82. http://dx.doi.org/10.1126/science.1211451