Saturday, April 24, 2010

Nature: April 22

Cultural Evolution: Those Damn Finches Again!
                Every student of biology has heard ad nauseum about Darwin’s finches and their beaks.  Although the myth generally goes that Darwin, being the magnificent observer that he was, noticed immediately that many species of finch coexisted on the Galapagos, the major difference being differences in beak sizes adapted to eat different foods, the truth is that it was only after returning home that this observation occurred, and only at the prompting of an ornithologist (John Gould) to whom Darwin had sent the samples.  It seems, in fact, that while on the Galapagos Darwin hadn’t really done anything but shoot (yes, shoot.  Darwin personally shot many of the specimens he brought home) as many species of bird as he could find, though the story as he reconstructs it in The Voyage of the Beagle would lead one to believe otherwise!  Keep in mind, however, that Voyage was published 3 years after his return from the trip, and 4 years after the Galapagos, and that Darwin was writing for a popular audience.  In fact, though at the time of its publishing Darwin was already entertaining thoughts of Evolution he was still 20 years from revealing this to the general public.  The words in Voyage are actually quite cryptic, and in typical Darwin fashion, very, very careful.  Note the last sentence, especially, which was actually only added in the second edition of the book:
The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza, from one as large as that of a hawfinch to that of a chaffinch, and (if Mr. Gould is right in including his sub-group, Certhidea, in the main group) even to that of a warbler. The largest beak in the genus Geospiza is shown in Fig. 1, and the smallest in Fig. 3; but instead of there being only one intermediate species, with a beak of the size shown in Fig. 2, there are no less than six species with insensibly graduated beaks. The beak of the sub-group Certhidea, is shown in Fig. 4. The beak of Cactornis is somewhat like that of a starling, and that of the fourth subgroup, Camarhynchus, is slightly parrot-shaped. Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.
One might really fancy that, Mr. Darwin!  Oh, what a marvelous fancy that is!  Tell us another one, Mr. Darwin!  But surely you don’t believe in such nonsense, do you?  You can actually read everything Darwin ever so much as scribbled onto a napkin online in search-able function here at the Darwin Online project
                Anyway, the research presented in the Nature article is not really about this history, nor even about the evolution of beak sizes in the finches, but is about their cultural evolution.  Yes, indeed, birds have something that you would call culture, and one of the most studied aspects of this culture has to do with their songs.  With modern software, bird songs can be recorded and analyzed to an incredibly precise degree based on many parameters, so that the songs of two different birds can be seen as distinctive as any human fingerprint, and can be passed down through the generations.  Now here’s the question: how is the song passed down?  Here questions of nature vs. nurture come to the fore.  Clearly, the song is not encoded in its audible manifestation somewhere deep in the genetic code of the birds, so that just by virtue of having this “song gene” a bird will grow up to sing a particular song.  We can tell this because by depriving a young bird of the song of its parents, it will learn no song at all.  However, just as clearly as the song is not encoded entirely in its genetic make-up, it is also clearly not “just” cultural – birds (save a wonderful smattering who can learn any noise in the world, including rain forest chainsaw!) clearly learn the songs of their parents over other noises and bird songs out there.  So, as per usual, when the question nature vs. nurture comes up in cultural evolution, the answer is “both”. 
In this particular study, Eben Goodale and Jeffery Podos of the University of Massachusetts compared recordings of finch songs from 1961 and 1999, and many of the songs remained remarkably similar over the four decades, pointing to an incredibly high fidelity of transmission from one generation to the next.  And if something is remarkably complex and effective in nature, it would have to have some sort of strong selective pressure to keep it tuned up – the authors suggest that the selective mechanism working here might be sexual selection.  Since songs are passed down from father to son, and they play a major role in mating rituals, it is proposed that by picking males that faithfully reproduce the song culture over ones that get it wrong female choice selects for males that are good learners.  Think about a party: two guys are playing guitar.  They both play The Beatles, only one is out of tune and keeps getting the words hilariously wrong while the other succeeds in faithfully reproducing Helter Skelter to a t.  Who gets the girls?  Now, it’s interesting that in human society (and in some of those outrageous creative birds), a little creativity is actually quite valued, and if you compared that same faithful Beatles reproducer to another guy who, upon being asked “who wrote that song? It was really good,” replied, “umm…actually I did,” we all know who the winner is. 
Hmmm…..I’ll be at my guitar.

Research Highlights. Nature 464, 1106 (22 April 2010)

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