Most of you have heard of convergent evolution. To some extent it’s often most clear and visible in morphological characteristics which are shaped by the basic physical parameters of the universe around us. Physics is nicely predictable. Bats and birds are subtlety different, but a rough congruence is body plan is evident. More striking are the parallels between dolphins, tuna, and the long extinct Ichthyosaurs. There are a finite manner of ways you can be optimally shaped as a vertebrate if you wish to be fast in the viscous waters of our planet. Living torpedoes need to emulate the sleek lineaments of the torpedo. This is probably where you are wondering what’s so interesting about this, as you read this in the illustrated evolution books you perused as a child. Well, Nature Genetics has a neat new comparative genomic paper out, Convergent evolution of the genomes of marine mammals, which explores on the genomic level what is visible to our naked eyes in terms of macroevolution made flesh in morphological similarities.
It’s an open access paper, and quite short and succinct, so I invite readers to check it out. The methods are straightforward, they sequenced the marine mammal lineages highlighted in red above, and compared them to their sister lineages which had not taken to the oceans, as well as with each other. After comparing regions of the genome they found five genes with evidence of selection across all the parallel lineages, which evolved from very distinct clades of mammals. Some of these genes made sense in terms of their functional relevance for marine organisms. In some cases the substitutions within the gene were distinct. This is to be somewhat expected, as genes are big, and there may be several ways to skin the cat. In contrast in other genes it was the exact same substitution, indicating strong constraint. All good. But then near the end they add this coda:
Our comparison of the genomes of marine mammals has highlighted parallel molecular changes in genes evolving under positive selection and putatively associated with independently evolved, adaptive phenotypic convergence. It has been hypothesized that adaptive evolution may favor a biased subset of the available substitutions, to maximize phenotypic change…and this hypothesis may explain some of our findings of convergent molecular evolution among the marine mammals. However, we also found widespread molecular convergence among the terrestrial sister taxa, suggesting that parallel substitutions might not commonly result in phenotypic convergence. The pleiotropic and often deleterious nature of most mutations may result in the long-term survival of substitutions at a limited number of sites, leaving a signature of molecular convergence within some coding genes. The parallel substitutions in 15 positively selected genes identified in this study likely represent a small proportion of the molecular changes underlying adaptive and convergent phenotypic evolution in marine mammals. Our data therefore indicate that, although convergent phenotypic evolution can result from convergent molecular evolution, these cases are rare, and evolution more frequently makes use of different molecular pathways to reach the same phenotypic outcome.
Basically the expectation here is that obvious convergent evolution is going to drive some similarities on the genomic level. This isn’t too strange an assumption when you note that groups of genes like the Hox show up over and over across many taxa. But the authors also found lots of genomic convergence between terrestrial lineages. This surprised them, because to our naked eye there isn’t any parallelism of form equivalent to what you see among the marine organisms. I think perhaps one aspect we need to consider is that some convergent evolution is in the eye of the beholder. The authors seem to be cryptically pointing to the genotype space being constrained by the genetic correlation matrix, the inability of substitutions to occur because of pleiotropic effects. But what about the possibility that there are similarities between lineages which are not salient in the form of gross morphology? For example, social structure and population density vary between lineages with no particular rhyme and reason (I exaggerate some, but you get the picture when you think of the eusocial mole rat and the eusocial hymenoptera).
Ultimately I think this paper is less important in and of itself than the fact that it sheds light on the possibility that in the near future we’ll get a good sense of the genomic shape of the tree of life, and we’ll have recourse to many high quality genomes all across the tips of the phylogenetic tree. The sorts of comparative methods utilized in this paper also have the good feature of being rather transparent and less abstruse that you often find in population genomic papers.