Well, we don’t really need any more evidence for evolution, do we? But it keeps pouring in, the latest in a paper from the Journal of Evolutionary Biology (link is to the abstract, and if you have journal access you’ll find the paper in the online “early edition”).
In WEIT I talk about olfactory receptor (“OR”) genes as evidence for evolution: these are genes that encode receptor proteins involved in smell, with each OR gene encoding a different protein. These genes provide evidence for evolution because, as I describe on pp. 69-71 of my book, in terrestrial mammals most OR genes are intact and functional, but in their aquatic mammalian relatives, like dolphins, whales, and sea lions, many of the OR genes are inactivated by mutations (80% of them in dolphins!). (These inactive genes are called pseudogenes.) Nevertheless, the DNA sequences of those dead genes clearly show their affinity to the active genes of terrestrial relatives. OR genes are probably useless underwater because they detect airborne and not waterborne odors. Moreover, marine mammals that spend some time on land, like sea lions, have fewer OR pseudogenes than do more-aquatic species like whales and dolphins, presumably because sea lions need to smell things when they’re on land.
The new paper, by T. Kishida and T. Hikida from Kyoto University, offers a remarkable parallel to the mammal results, but from snakes. To make a longish story short, the authors sequenced OR genes in two groups of sea snakes, those that give birth in the water, and hence never leave it, and those that are “oviparous”, and must leave the sea to lay eggs on land. They also looked at OR genes from the snakes’ fully terrestrial relatives. (We know from other data that sea snakes evolved from land-dwelling ancestors.)
The results: just like in mammals. In viviparous (live-bearing) sea snakes, about 30% of their OR genes had become nonfunctional pseudogenes, compared to only about 7% in their terrestrial relatives (elapids). That in itself is evidence for evolution, for what else could explain the presence of nonfunctional genes that are similar in DNA sequence — but broken by mutations — to active genes in relatives? And the inactivation of OR genes in aquatic snakes makes real evolutionary sense; genes that are unnecessary — indeed, that are a metabolic burden since unneeded — get zapped by mutation. The other cute result is that oviparous snakes that occasionally come ashore showed a lower proportion of OR pseudogenes genes (about 12%) than their viviparous cousins, precisely as expected if they, like sea lions, must retain some sniffing ability on land.
I love pseudogenes, for they constitute some of the most irrefutable evidence for evolution–and not just microevolution, either. The transition from terrestrial to aquatic mammals, or terrestrial to aquatic snakes, surely represents macroevolution. It’s hard to explain patterns like those of the OR genes by any form of creationism, unless you think the creator designed species to fool us into thinking they evolved!
Fig. 1. An oviparous sea snake, the highly venomous banded sea krait, Laticauda colubrina.
h/t: Matthew Cobb
Kishida, T., and T. Hikida. 2010. Degeneration patterns of the olfactory receptor genes in sea snakes. J. Evol. Biol. early view