an early birthday surprise

 Say ‘Charles Darwin’ and (after ‘evolution’!) many people would probably say ‘Galapagos’. The tortoises, mockingbirds, finches & iguanas that he observed and collected on the Galapagos Islands contributed to his development of the theory of evolution by means of natural selection.

Darwin noted that there were two species of iguana on the islands – the land-living species, and the marine iguanas, which swim down to graze on algae that grow on the rocky seabed. Now, just a few weeks before we celebrate the 200th anniversary of Darwin’s birth, comes the announcement of a third Galapagos iguana: a pink, or ‘rosada’, iguana (Gentile et al 2009).

I first read about it in the morning paper, which described this as ‘the pink iguana that Charles Darwin missed’. Hardly fair to Darwin, given that it seems that rosada is found only on the volcano Volcan Wolf on the island of Isabela – & Charles didn’t explore that area. In fact, rosada wasn’t noticed until 1986, despite the fact that it’s phenotypically distinct from the other Galapagos iguanas, with its lack of prominent dorsal spines & its pink body marked by spots and vertical stripes of black. There are also differences in between the species in the head-bobbing behaviour used in courtship and territory defence. (Its anonymity might reflect its rarity – so far researchers have found only 36 rosada individuals.)

Previous genetic comparisons of the land and marine iguanas suggested that they might have diverged around 10.5 million years ago (mya). This was before the current islands, which are all volcanic, had emerged from the ocean. This earlier research also indicated that the land iguanas (there are 2 species) diverged from each other much more recently, during the Pleistocene (<2.5mya). In the latest study, Gentile et al. used mtDNA sequencing, & microsatellite DNA genotyping, to see where rosada fitted in.

Their phylogenetic analysis of mtDNA from all four species showed that the marine iguana (Amblyrhynchus cristatus) is quite distinct from all the land species, and that rosada diverged from the other land iguanas (Conolophus) around 5.70 mya. Yet the volcano where rosada is found, Volcan Wolf, is less than 0.5 million years. This suggests that the the pink iguana is a relict population that must surely have once been more widespread.  

The microsatellite analyses showed that mutation and genetic drift had contributed to the marked genetic differences between pink rosada and the yellow Conolophus species. While the two forms share 26% of their alleles, there was no evidence of recent interbreeding despite the fact that pink & yellow iguanas are found in the same habitat. The authors comment that while reproductive isolation between these two forms may still be incomplete, this isn’t surprising, as scientists have recorded hybridisation between marine (Amblyrhynchus) and land (Conolophus) iguanas. Consequently, they feel rosada should have the status of a new species.

And one which may well be highly endangered, given its apparent low numbers and single location. Let’s hope that the year in which we celebrate Darwin’s immense contribution to biology isn’t also the year in which rosada is lost forever.

G. Gentile, A. Fabiani, C. Marquez, H.L. Snell, H.M. Snell, W. Tapia & V. Sbordoni (2009) An overlooked pink species of land iguana in the Galapagos. Proceedings of the National Academy of Sciences 106(2):507-511 doi 10.1073/pnas.0806339106