Archive for the ‘exaptation’ Category
stuff on exaptation and Archaeopteryx
my photo of archaeopteryx, from London’s Natural History Museum
Through my various readings and researches, if they can be credited as such, I’ve from time to time come across the famous image of Archaeopteryx, a fossil (now the type specimen) of some sort of dinosaur-bird discovered in the early 1860s, and with some fanfare, I think, presented as proof of Darwin’s recently published thesis, On the Origin of Species (1859). Now, being an impoverished type, with an elementary scientific education, I’ve travelled little, and much of my reading has been from borrowed or scrounged titles, including the aforementioned work. However, a few years ago, due to a lucky and undeserved inheritance, I got to travel to London from my home in Australia, whereupon I paid a visit to the British museum. Wandering lonely as a cloud along the hallowed halls, I suddenly found myself face to face with the Archaeopteryx specimen so very occasionally but memorably depicted in the science texts previously perused, and I must say my heart skipped a beat. Not that I quite knew what all the fuss was about. It had something to do with missing links and flying dinosaurs and, as mentioned, confirmation of the controversial thesis.
Anyway a recent reading of the essay ‘Not necessarily a wing’, in Stephen Jay Gould’s Bully for Brontosaurus collection, has taken me back to Archaeopteryx, the first bird, as Gould calls it, though surely we can’t be certain about that. However the important point is that birds evolved from creatures that weren’t birds, and it likely wasn’t a sudden thing. As Gould describes it, arguments against Darwin’s theory, sometimes by evolutionists of a different stripe, questioned the development of features such as wings, and their usefulness before they were ‘developed’ enough to enable flight. Darwin responded with the concept of ‘functional shift’, more recently labelled ‘exaptation’, thanks to academic contributions from Gould himself, among others.
Wikipedia has it that ‘older potential avialans have [recently] been identified’. The term ‘avialan’, from Avialae (“bird wings”) ‘is a clade containing the only living dinosaurs, the birds, and their closest relatives’. In any case what’s more important is how flight developed in early dinosaurs. Archaeopteryx lived about 150 million years ago (the late Jurassic) and was about magpie-sized, but probably nothing like a modern bird in appearance. It had teeth, unlike modern birds, and a long bony tail, and various other features characteristic of dinosaurs. So it has been described as a transitional species. But what does this mean? Are we a transitional species? Isn’t every species transitioning in some way? (Add plug for transitioning to a bonoboesque humanity here).
What we can say is that Archaeopteryx was likely one of the first flying dinosaurs as well as the first bird or proto-bird, and we now have 12 separate fossils of the species. But of course discoveries continue. From Wikipedia,
… in recent years, the discovery of several small, feathered dinosaurs has created a mystery for palaeontologists, raising questions about which animals are the ancestors of modern birds and which are their relatives.
But let me return to this issue of exaptation – in this case, as regards wings and flight. Gould quotes from the 19th century dissenter vis-a-vis Darwin’s theory, St George Mivart, on the general problem:
Natural selection utterly fails to account for the conservation and development of the minute and rudimentary beginnings, the slight and infinitesimal commencements of structures, however useful those structures may afterwards become.
Mivart opted for a ‘comparatively sudden modification of a marked and important kind’, rather than incremental but still adaptive changes – that’s to say, changes naturally selected for, though not of course by any kind of design. But Darwin countered that these ‘sudden modifications’ smacked of the miraculous – ‘to admit all this is, as it seems to me, to enter into the realms of miracle, and to leave those of Science’.
Mivart worked hard to find large-scale modifications that might fit within an evolutionary framework, while at the same time Darwin had to work hard to show the adaptiveness of small-scale changes that might, over a long period, lead, for example, to flight. The organism would be gradually changed in terms of its hunting and feeding practices, for example, which would be incrementally selected for until the structural-anatomical changes led to an increasingly sustained ability to leave the ground for nutrition further afield and/or higher above ground.
Okay, this isn’t exactly what Darwin presents, which is change of function (functional shift – exaptation). I’m suggesting that the change itself would be so gradual as to be barely noticeable, functionally, except through the ‘compression’ of time afforded by our analyses of fossil records. However, unsurprisingly, analysts have been here before, as Gould’s essay tells us. In the case of insects, wing size compared to body size has been analysed carefully in recent decades, with respect to flight versus insulation or thermoregulation, and the results have favoured an exaptation model – first the advantages of thermoregulation, then as wings grew in proportion to body size, the advantages – and such advantages! – of flight.
The details here are complex almost beyond description – certainly beyond my ability to describe them – and we may perhaps never know whether Archaeopteryx is the direct descendant of modern birds or a cousin of that descendant. The Wikipedia article on the species is virtually book-length, with well over 100 references. The arguments run along similar lines regarding various human antecedents, with lumpers and splitters debating the position of Homo ergaster, Homo luzonensis et al. Gould points out, I think rightly, that evolutionary history is very bushy and human researchers are overly obsessed with finding direct, vertical lines of descent – ladders among the bushes, or transforming bushes into sets of ladders. The bushes themselves have their intrinsic interest and beauty.
References
Stephen Jay Gould, ‘Not necessarily a wing’, in Bully for Brontosaurus, 1991