How did the SWS2 Gene Evolve?

Date Accepted: November 19,2014

Source: Wiley Online Library

Institutions Involved: Department of Ecology & Evolution, University of Chicago, Departments of Ecology & Evolutionary Biology, Cell & Systems Biology, University of Toronto, Centre for the Analysis of Genome Evolution & Function University of Toronto,

Summary: Researchers try and find out if color perception in birds had an influence on the evolution of a gene that controls feather color.

Did you ever wonder why some birds have bright feathers while others do not? The researchers from the University of Chicago and the University of Toronto had a similar question. Natasha Bloch, James Morrow, Belinda Change, and Trevor Price wondered, "Why did the Old World warblers have fewer pigmented feathers in comparison to the New World warblers?" So, they decided to conduct an experiment to find out. This particular experiment happens to be interesting because the researchers attempt to find out how the SWS2 gene (which codes for pigment) evolved.

This happens to be the cladogram from the paper. Here, you can see what is referred to as the Old World Warblers and what is referred to as the New World Warblers.

 For this experiment the researchers had three hypotheses. Their null hypothesis stated that the SWS2 gene remained unchanged during the course of evolution. The other two hypotheses each describe different ideas of how the SWS2 gene could have evolved. The first hypothesis believes that the gene is similar in both the New World and Old World Warblers, but was different in their respective anscestors; it also suggests that this difference in color is due to the environment of each of the birds. The second hypothesis states that the gene shifted as the New World Warblers diverged from Old World warblers, but they passed through another ancestor that occupied a different environment, which then lead to the different color perception and color difference in the feathers of each set of birds. This last hypothesis happened to be the researchers' most favored idea.

The researchers tested their ideas by sequencing the SWS2 gene from both of the warbler clades and also from some outgroups. For the New World warblers, the researchers collected birds that died from building collisions during migration in Chicago. For the Old World warblers, they used RNA samples. Once the researchers had  sequenced all of the SWS2 genes from various birs, they then reconstructed the SWS2 sequence evolution. They also identified all the variants with at least one substitution as candidates for shifts in color perception.They found that despite having 6 differences in the amino acid sequences, the ancestors of both

warbler clades had similar color perception.

In conclusion, they found that the color perception of the ancestors of both the New World warblers and the Old World warblers were the same, so this could not have contributed to the evolution (color variation) in the New World warbler's feathers. The researchers say that when they do figure out what the reason for behind the adaptiation is, they will then be easily able to determine if this evolved trait is due to historical  contingency or not.

Journal Reference: Bloch, N. I., Morrow, J. M., Chang, B. S. W. and Price, T. D. (2015), SWS2 visual pigment evolution as a test of historically contingent patterns of plumage color evolution in warblers. Evolution, 69: 341–356. doi: 10.1111/evo.12572