speciation in darwin’s finches

This afternoon the daughter sat Level 3 Biology (she seemed to quite like the paper). She said that one of the questions was about Darwin’s finches, based on some of the work of Peter & Rosemary Grant, who’ve been studying finches on the Galapagos Islands since 1973. During that time they’ve trapped, measured, banded & re-trapped every finch on the island of Daphne Major, collecting longitudinal data on the birds in perhaps the longest-running study of its kind. Among other things, the Grants were able to document how the finch populations changed in selection to the selection pressures imposed by a severe drought.: evolution in action. Nonetheless, a stock creationist remark concerning the Galapagos finches claims that this is not an example of evolution because the birds ‘are still finches and the changes cycle with changing environmental conditions.’ 

This really is a ridiculous statement: given the timespan involved, you can’t really expect more than gradual, incremental changes in population characteristics – changes that are not directional, or purposeful, & which may be reversed if selection pressures alter. Which makes a new, just-published paper by the Grants all the more exciting – it seems that they may be witnessing an example of allopatric speciation

When the Grants arrived, Daphne Major was home to two species of finch: the medium ground finch Geospiza fortis and the cactus finch Geospiza scandens. (A third species, G.magnirostris, began breeding there in 1983.) By 1981 the Grants had banded more than 90% of the birds on the island, which meant that when a new individual arrived from one of the other Galapagos islands, they were able to recognise it. The immigrant was an unusually large male ground finch with a distinctive song; genetic analyses suggested that it probably originated from the G.fortis population on the island of Santa Cruz – and also that it was a hybrid, with alleles from both fortis and scandens genomes. This bird went on to mate with another, rare, hybrid individual. The Grants note that they ‘have followed the survival and reproduction of this individual and all of its known descendants [the immigrant lineage]… for seven generations spanning 28 years.’ (The ‘immigrants’ were characterised by their larger size – including larger beaks, different song, & the presence of a unique genetic marker.)

The Galapagos Islands are periodically affected by severe droughts, which have a significant effect on the various finch populations. (The L3 exam focused on the impact of a drought in the mid-1970s.) One such drought, in 2004, saw the ‘immigrant lineage’ reduced to just two birds, a brother & sister. They bred with each other, & the Grants’ detailed genetic data show that from that point on the ‘immigrant lineage’ was reproductively isolated from the resident G.fortis population. They suggest that this isolation is due to two main factors: morphology, & song. The song is learned, & so an element of imprinting is involved. This is significant: the Grants observed only 13 instances of hybridisation between fortisscandens over 21 years, & comment that this ‘generally results from the learning of the song of another species during the early sensitive peirod of song learning’ (as is also the case for indigobirds). Morphological differences centre on the beak, which is noticeably larger in the ‘immigrant’ lineage.

This morphological difference already existed: the original immigrant probably came from Santa Cruz, where fortis individuals are bigger than those on Daphne Major. In other words, this difference arose allopatrically. However, once on Daphne Major the difference between the different groups was exaggerated by a drought in 2004, which saw a decline in the size of resident fortis birds (including beak size). And it was maintained by breeding within the ‘immigrant’ & resident lineages, mediated by those learned differences in song & by mate selection on the basis of beak size.

Are the differences enough to say that we are looking at a new species? To the Grants, the answer is, qyuite possibly –  they view the inbreeding, ‘immigrant’ group ‘as an incipient species because it has been reproductively isolated from sympatric G.fortis for three generations and possibly longer.’ Of course, whether this reproductive isolation will be maintained remains to be seen. Watch this space 🙂

See also Allan MacNeill’s very thorough commentary at The Evolution List .

P.R.Grant & B.R.Grant (2009) The secondary contact phase of allopatric speciation in Darwin’s finches. Proceedings of the National Academy of Sciences early edition, p 1-8. www.pnas.org/cgi/doi/10.1073/pnas.0911761106

Leave a Reply

Your email address will not be published. Required fields are marked *