30 July 2008

Saving Darwin's muse

31 July 2008: Taxonomy updates! See footnotes 3 and 4.

Today at work I sat for several minutes gazing in gobsmacked awe at a newly arrived, bubble-wrapped package containing small, dried-out footpads taken1 from two specimens of Nesomimus Mimus2 trifasciatus (Galápagos mockingbirds from the island of Floreana, formerly Charles Island). Interesting, yeah? Well don't click away just yet, because it gets way better...

These two specimens were collected from the island before the species went extinct there (it's still hanging on by a thread in the form of approximately 100 individuals inhabiting two nearby lumps of rock in the sea -- Gardner-by-Floreana and Champion islands -- and is thus one of the rarest birds in the world), so we3 hope these two footpads contain enough intact DNA to tell us more about the diversity of the original population on Floreana -- information that can help us determine the best strategy for selecting individual birds from the remaining populations (rapidly diverging already, Darwin would be pleased to learn) to reintroduce back to Floreana in the hopes of saving the species from extinction.

N. M.2 trifasciatus went extinct on Floreana before the end of the 19th Century, so it's not particularly easy to find specimens that were collected there before then. In fact, you can probably count on one hand the number of specimens of N. M.2 trifasciatus collected from Floreana proper (we may never know for sure how many there are out there). In other words, these specimens are really rare ...and we've got two of them here at the Natural History Museum. Have I got your attention yet? I hope so, because I've saved the best part for last...

Now let's think really hard about who might have collected such specimens from the Galápagos islands before 1900. If you don't see where I'm going with this now then you've quite literally got blinders on because it's splashed all over this blog ...yes, there was a little ship called HMS Beagle that called into Floreana in 1835, and both her captain Robert FitzRoy and her naturalist Charles Darwin disembarked, and each one of them collected a specimen of the mockingbird they found there. When Darwin saw the bird, he noticed right away what he did not notice about the finches until much later when Professor Gould set him straight: that each island in the Galápagos appeared to have its own unique species of mockingbird. In the words of ornithologist Frank Steinheimer (references removed for clarity, see full text here):
Sulloway has already shown that the mockingbirds from the Galápagos Islands rather than the finches were used in Darwin's On the Origin of Species, though some later authors have still succumbed to confusion on this matter.
In a chapter entitled "Geographical distribution", Darwin noted: "But we often take, I think, an erroneous view of the probability of closely-allied species invading each other's territory, when put into free intercommunication. Undoubtedly, if one species has any advantage over another, it will in a very brief time wholly or in part supplant it; but if both are equally well fitted for their own places, both will probably hold their separate places for almost any length of time."
In short, Darwin recognized the importance of building different ecological niches ("own places") or the establishment of two very similar species in the same geographical region. As an example he used the Galápagos mockingbird Nesomimus parvulus, the Charles mockingbird Nesomimus trifasciatus, and the San Cristòbal mockingbird Nesomimus melanotis, of which he collected one and, respectively, two specimens of each (Darwin's numbers 2206, 3307, 3349, 3350)4. In his Transmutation Notebooks (Notebook B from 1837–1838) he goes on to say that: "It may be argued representative species [such as the South American and Galápagos mockingbirds] [are] chiefly found where barriers […] interrupt[ed…] communication," stressing the importance of geographical isolation for speciation processes.
The birds of the Galápagos Islands are also cited, albeit in general terms, as an example of how vague the distinction is between what one sees as a species and what scientists refer to as subspecies. The mockingbirds of the Galápagos Islands collected by Darwin have been classified both as subspecies of a single species (Mayr and Greenway 1960, pp. 447–448), and as three different (allo-)species (Dickinson 2003, p. 649).
Darwin turned again to the Galápagos birds when explaining endemism. Darwin showed that of the 26 land birds known at the time, 21, or perhaps even 23, were believed to be endemic to the Galápagos Islands, whereas of 11 marine birds only 2 were considered endemic (Darwin 1872, pp. 405–406).
In other words, these very birds -- the ones collected by Fitzroy and Darwin during the voyage of the HMS Beagle 1831-1836 -- the ones which now reside in a quiet specimen drawer at the Natural History Museum's bird collection in Tring -- the ones whose footpads are newly gracing my desktop today in two rather anticlimactic Eppendorf tubes -- were a seed if not the seed of Darwin's later questioning of the immutability of species, and ultimately led to the publication On the Origin of Species by Means of Natural Selection, which, arguably, has changed the world (for the better in my humble opinion).

I am honoured to be given a chance to work with these rare specimens (Darwin's is the invaluable holotype - the founding specimen of the described species), and, more importantly, to be given a chance to do my small part3 in helping rescue from extinction -- by means of a genetically well-informed reintroduction strategy -- the Floreana mockingbird, Darwin's real muse in the Galápagos.

* * *

For more information about the Floreana mockingbird please visit Robert Curry's most excellent webpage: Darwin's Mockingbirds: The endemic mockingbirds of the Galápagos, and the Charles Darwin Foundation's press release, Action Plan to Save the Floreana Mockingbird of Galápagos.


1. Carefully dissected by the steady hand of Mark Adams, a curator in the Bird Group at the Tring outpost of London's Natural History Museum.

2. Many thanks to Professor Robert. L. Curry of the Department of Biology at Villanova University who kindly made me aware of updated taxonomic placement, writing, "The South American Classification Committee of The American Ornithologists’ Union decided recently that the genus Nesomimus should be merged into Mimus, based largely on the analyses in our 2006 paper in Evolution (Arbogast, B. S., S. V. Drovetski, R. L. Curry, P. T. Boag, G. Seutin, P. R. Grant, B. R. Grant, and D. J. Anderson. 2006. The origin and diversification of Galapagos mockingbirds. Evolution 60:370-382). The evidence now available strongly supports the conclusion that the Galapagos populations (regardless of how many species might be recognizable there) are an offshoot of mockingbirds from the mainland (although probably from Middle America rather than South America). This means that a mockingbird in Galapagos is more closely related to some continental and Caribbean mockingbirds than those are to other mockingbirds from, say, Argentina. So, when writing about “Darwin’s mockingbirds,” it would be best to refer to them as Mimus trifasciatus (and M. parvulus, M. macdonaldi, M. melanotis) rather than Nesomimus. (The old name Nesomimus could still be used to refer to the clade of species in Galapagos, but only as a sub-genus, in which case the name wouldn’t be part of the formal Latin binomial.)"

3. I am but a tiny player in this large, multi-layered project involving the Galápagos National Park and the Charles Darwin Foundation. Particular mention must be made of Paquita Hoeck, a PhD student in Lukas Keller's research group at the Zoological Museum of the University of Zurich, who is including these specimens (she'll receive a matching pair of Eppendorf tubes this week) in her much larger microsatellite analysis of Galápagos Nesomimus which includes hundreds of specimens of N. trifasciatus from Gardner-by-Floreana and Champion islands -- both historic specimens and birds they captured live and sampled on site over the last two years. I am doing an extraction and microsatellite amplification reaction in parallel in our Botany Department lab here at the NHM, to ensure that the data generated by Paquita in Zurich is indeed from these two specimens and not the result of contamination by fresher and therefore much more abundant DNA from more recent specimens that have passed through their lab. In other words, I'm just doing a control experiment, but I don't care, I still feel honoured to be participating at all.

4. Professor Curry also notes: "the section in which you quote Steinheimer includes an error, relative to modern analysis of the various populations in Galapagos, that dates back to John Gould and Darwin. Specifically, the statement that Darwin 'used the Galápagos mockingbird Nesomimus parvulus, the Charles mockingbird Nesomimus trifasciatus, and the San Cristóbal mockingbird Nesomimus melanotis, of which he collected one and, respectively, two specimens of each (Darwin's numbers 2206, 3307, 3349, 3350)' (besides using the older genus name) assumes that specimen 3350 was an individual of Mimus melanotis. However, Darwin collected that specimen on Santiago (James) Island, which is inhabited by one of the subspecies of M. parvulus. John Gould thought that 3307 and 3350 represented the same species (M. melanotis), but that interpretation does not seem to be correct. The bird illustrated in the Zoology of the Beagle voyage is clearly specimen 3307, as pointed out by Swarth (1930). An interesting historical tidbit here is that while Darwin collected his specimen of M. melanotis (on San Cristóbal) before his specimen of M. trifasciatus, the type specimen for the 'genus' (or subgenus) Nesomimus is the trifasciatus specimen (and it would be the type for all of Galapagos if the populations were treated as a single species, M. trifasciatus), because Gould happened to describe the Floreana specimen first."


Kevin Zelnio said...

You are my hero! Cool stuff. I can't wait to hear about the results of this awesome project!

Karen James said...

First "a Darwinian angel" and now your "hero" - I am having a very good week! I will keep updates coming on the project as they arrive...

Paquita said...

Congrats on your blog, Karen, it's fantastic. You are a very good writer!

Lacy said...

I am august, blown over, beaming... bravo.

Anonymous said...

A bird in the foot is worth two in the hand, or something like that.

Excellent post, thanks. More please!

Karl said...

The EXACT birds that Darwin collected? That's astounding.

It sure would be great to be able to buy prints of any Nesomimus (I know, whatever.) It would be especially great to be able to buy a print of a Gould's lithograph of a Nesomimus. If one were available. Anyone?

spudinski01 said...

Its definitely awesome stuff, cant wait to hear more about the beagle!