10
02/10
Reigning in Evolution
Life is diverse. There is no doubting that. In fact, the diversity has led many to believe that the possibilities offered by the seed of complex interactions, several thousand genes and their myriad products are so large, that we shouldn’t really be surprised at just about anything we see in the biosphere.
Adding a little bit of spicy strangeness along those lines, is the phenomenon of ‘convergent evolution’. This is one of those not-so-rare cases when species that aren’t very similar or closely related , turn out to have a feature that is strikingly, almost awe-inspiringly, similar. Wikipedia has a pithy list of examples of convergent evolution, my favorite one being the case of the human eye, and the Octopus eye. See how similar* they look, even though Cephalopods(them) and Vertebrates(us) diverged in evolution millions of years ago, before any eyes of similar nature were invented by the ubiquitous tinkering of evolution.
There are many explanations for why convergent evolution might occur, but they are circumstantial at best, speculative at worst and usually not very convincing. If you are a biologist, you will know that finding out the truth in such cases is intensive enough to just suffice ourselves with Orgel’s Second rule “Evolution is cleverer than you are”, and leave it at that.
But it’s starting to turn out, that while evolution may be a clever tinkerer, there aren't too many ways to skin a cat. A couple of papers published in the last few weeks show that while evolution may create similar characteristics in unrelated organisms, it tends to use the same mechanism at the molecular level to do so. Chan et al report in Science, that sticklebacks in different isolated populations tend to lose a feature of their anatomy: the pelvic girdle, by the same molecular event – the loss of a genetic regulatory element called the Pitx1 enhancer. Now you may say that these are the same species, and not nearly as spectacular as the Octopus-Human comparison, but remember that these populations do not interbreed, and not all populations have sticklebacks lose their pelvic girdle. It is pretty astounding that three populations who lose their pelvic girdle do so in the same manner – by eliminating Pitx1.
More exciting was a paper published in PloS Genetics by Counterman et al. , who examine the genes that produce the color in Heliconius butterflies. In contrast to the earlier paper, Counterman et al. study populations of a species of butterfly called the Small Postman (H.erato), that is a Muellerian mimic of its poisonous relative , the Postman butterfly (H.melpomene). The idea is that birds know that the Postman butterfly is poisonous , and will not eat it. H.erato will try to mimic the appearance of the its toxic cousin, so that birds leave it alone in a case of mistaken identity. Both H.erato and H.melpomene are poisonous, and have independently evolved similar appearances despite having diverged millions of years ago. The mimicry is a huge benefit to both species, since it ensures that predators have one thing firmly entrenched in their collective memory – if it looks like Postman butterfly, do not eat it .
[The author of the paper has educated me about the nature of interaction between these two species. See comments for details].
Heliconicus melponeme Heliconicus erato
Of course, the various populations of H.erato are not all equal, some seem to be better make-up artists and look much more like their miasmatic relative. Counterman et al. perform a population genetic analysis of these, and find, to their surprise that all of them show a dependence on only one site (locus) in the genome of these butterflies.
By doing more intensive analysis on this locus, they are able to determine that in fact, it is only one gene – kinesin, that seems to make most of the difference between a good mimic , and a bad one. Once more, the appearance and nature of a rather complex trait, is shown to be linked to a rather small region of DNA. If you want to look like the Postman butterfly it seems, you need to mess with the Kinesin gene.
In the light of these findings, a hint of a molecular explanation for convergent evolution begins to emerge. As the authors of either paper suggest, there may be evolutionary hotspots in our genome for a given trait. Whenever natural selection pressures the species into changing the trait, it is this genomic hotspot that feels the brunt. At the molecular level, evolution seems to be much more constrained than the medley of phenotypic outcomes lead us to believe.
I wouldn’t be surprised if one day we found that the developmental genes or regulatory programs that make an octopus eye and a human eye are more akin than their tentacles and our hairy heads seem to suggest.
*Note : It is interesting that despite the compelling similarity, the eyes are not completely identical. The retina of the Octopus eye actually has the reverse order of cell layers , when compared to the Human eye. This actually makes the Octopus eye somewhat better, since its optic nerve does not have to penetrate the retina – thus, an Octopus eye has no blind spot! Morever, because its photoreceptors have no obstrcutive cell layer (unlike us), it probably has a higher sensitivity to light as well.
References
Chan, Y., Marks, M., Jones, F., Villarreal, G., Shapiro, M., Brady, S., Southwick, A., Absher, D., Grimwood, J., Schmutz, J., Myers, R., Petrov, D., Jonsson, B., Schluter, D., Bell, M., & Kingsley, D. (2009). Adaptive Evolution of Pelvic Reduction in Sticklebacks by Recurrent Deletion of a Pitx1 Enhancer Science, 327 (5963), 302-305 DOI: 10.1126/science.1182213
Counterman, B., Araujo-Perez, F., Hines, H., Baxter, S., Morrison, C., Lindstrom, D., Papa, R., Ferguson, L., Joron, M., ffrench-Constant, R., Smith, C., Nielsen, D., Chen, R., Jiggins, C., Reed, R., Halder, G., Mallet, J., & McMillan, W. (2010). Genomic Hotspots for Adaptation: The Population Genetics of Müllerian Mimicry in Heliconius erato PLoS Genetics, 6 (2) DOI: 10.1371/journal.pgen.1000796