Convergence, adaptation, and constraint

Jonathan B. Losos. Evolution 65(7): 1827-1840. DOI:10.1111/j.1558-5646.2011.01289.x. Convergence, adaptation, and constraint

Are these Anolis dewdaps constrained? Maybe more than you’d think… (PLoS One; click for source)


Will Pearse

Will Pearse

We’ve covered too many data papers recently (that’s not a joke, but it does read like one), and so I picked this paper to help us step back a little and think. I’m pleased with the result: this is an excellent essay, that really made me think about what convergent evolution actually is. I’m particularly keen to hear what you all think of my comments about history!

Losos argues convergence is scale-dependent: there are many ways to evolve a long beak, and while there may be divergent evolution of the actual genes involved, the resulting phenotype (a long beak) is convergent. We’ve covered convergent evolution in bacteria, where the same genes (but different regions of those genes) mutated in parallel in separate lineages. I like this scale-dependency – it allows us to define convergence so that it’s amenable to study at all levels from phenotype to genetic mechanism.

I think we can push this framework further, and compare very different systems in meaningful ways. For instance, maybe examining constraints to evolution in responses to predation in Daphnia is easier when you consider what constrains their tolerance of the abiotic environment. Maybe seeing particular stressors and evolved responses as analogous to one another allows us to better compare evolution among clades, and view constraints to evolution in a more holistic way..

Apparently, there are some who take the view that evolutionary changes are incomparable historical events, and so the whole idea of convergence is a nonsense. I think this is rather peculiar; while there is a debate in history as to whether the field is a science (I think it is, but I’m not a historian!), every historian I know compares periods and events in history, with the precise aim of drawing parallels among periods. Thus I think the argument that evolution is the study of history, and therefore will not allow us to compare events, is not one even a historian would agree with!


Lynsey McInnes

Lynsey McInnes

Commenting on a Losos paper is always going to be tricky, as this is a man who knows his evolutionary biology! You can tell this in two ways, first simply by the breadth of examples he draws on and second by his daring to question the be all and end all of phylogenetically-informed analyses, another recent examplesof his critique of such analyses can be found here.

Like Will, I appreciated having a week off from data bashing and am currently juggling all the different issues that Losos brings up on what is and is not convergence, parallelism, adaptation, exaptation, etc. The biggest take home message I got from the essay was that, as always, scale matters. Birds and bats both have wings that let them fly, are these convergent traits? Depends on your scale of comparison. It seems like identifying instances of convergent evolution would be simplified immeasurably if the researcher concerned just set out the scale across which he is looking and perhaps also mentions whether he is worried about the trait being the ‘same’ at the genetic, phenotypic, morphological and/or morphological level. Hey presto, confusion and agro could be gotten rid of.

I can’t help comparing the issues brought up here to the ones Losos, and plenty of others, have attempted to deal with concerning identifying instances of niche conservatism. Again, it all depends on scale. Cooper et al. provide an excellent roadmap for conducting analyses on nice conservatism, I’d like to see a companion piece to this essay detailing the practical approaches to sensible analyses of putative instances of convergent evolution.

I’ve recently shifted the scale of my own analyses to incorporate (currently to deal exclusively with) intraspecific variation. In practice, this has meant starting to think about different models of mutation (infinite site, infinite allele, shitty recombination raising its ugly head begging to be dealt with) so I find my scale of analysis shifting to the genetic level, wanting to see mutations in same genes, indeed at the same sites to qualify as parallel evolution. For this reason, I really appreciated this essay as it forced me to address my newfound genetics-only bias and realise that interesting, valid and evolutionarily important convergent changes at the functional (or even just phenotypic) level need not be produced from identical genetic changes.

The recent bacteria study that we discussed here at PEGE was a brilliant example of a standalone set-up for studying evolution across these different levels (genetic, phenotypic, etc.), the next step, as always, is to devise a set-up that facilitates similar inference in systems where access to all these levels might be patchy. Losos’ essay will undoubtedly be helpful in this regard.

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About will.pearse
Ecology / evolutionary biologist

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