09 December 2006

Contingency in History and Evolutionary Biology Pt 1

Wyrdr bio ful araed (Fate is inexorable.)
-Danish proverb
History doesn't repeat itself but it does rhyme
-Mark Twain

The paleontologist Stephen Jay Gould made a lot of heavy weather about the concept of contingency in evolutionary history. Its unclear to me if he was deliberately non-committal with his definitions or if he never worked out what exactly he meant by the phrase.
Contingency was Gould's answer to any lingering Linean tendencies to see progress in nature. The notion of progress he railed against was that one could rank species in terms of higher forms and lower forms along a Scala Natura.
Evolution wasn't a ladder of progress, just an unpredictable branching pattern whose form was based on accidents of history, contingencies. Replay the tape of life and you wouldn't likely get the same suite of species. That is to say you and I, not to mention the rest of our species, Homo sapiens, would likely not be here. Nor would mammals, or birds or reptiles among others. Gould thought this was one of the most important things to be said about evolutionary biology. In fact he may even have thought it revolutionary. He built his case for the pre-eminence of contingency as a significant force in evolutionary processes in his book, Wonderful Life: The Burgess Shale and the Nature of History. Without delving into great detail the book reviews the amazing fossils of the ancient Burgess Shale. 600 million year old rock containing not only soft part fossilizaiton but also a host of organisms unlike any other in the history of life. Whole unknown phyla just springing into existence. Or being snuffed out. Maybe evolution worked in a different way. At least that is Gould's interpretation. Simon Conway Morris, one of the discovers of the Shale formation, disagrees with Gould's hypothesis that evolution worked somehow radically different than it does now. That aside, Gould really is on to a key shaper of history. Namely that events-either biological or cultural/historical-don't happen in a vacuum but rather happen in context.
Psychologist, and historian Michael Shermer has defended Gould's radical contingency and also done some much needed work in fleshing out the theory. He lays out definitions quite succinctly in his book How We Believe: Science, Skepticism and the Search for God. He also points out how it is hard to predict what events in the present will have large scale effects on the course of history. We can post-dict by reviewing historical records what it was that shaped history. He also echoes Gould in noting that the earlier in the system you look the more impact a small event can have on the ontongeny of the system. This is because early in the system there is not much stability, or historical inertia. As stability accrues, the greater the magnitude of the event necessary to have large scale, long term effects. An example of this might be the history of mammals. Early in the system several mammal like reptiles arose alongside the archosaurs. No group really held ecological preemience so there was a great deal of variety. But for what ever accident of history, the archosaurs gained a foothold and the age of the Dinosaurs began, and a 150 million years of stability. Mammals didn't begin their own adaptive radiation until the dinosaurs followed an astroid into oblivion. Contingency is really Chaos theory applied to history. Were there anyone left in the sciences that still held to notions of progressivism then maybe Gould and Shermer's "radical contingency" really would be just that.
I think there is much to the idea of contingency but I don't think it is nearly as radical, or limiting as Gould or Shermer think it is. Nor does it limit certain types of progress in evolutionary processes. Co-evolutionary arms races are just one type of progress that is un-affected. There are more.
Clearly they are right in saying that evolution was not on a progressive march up to humanity, and that were we to "rewind the tape of life" and play it back it would be an unlikely event indeed that we would see the dominance of a big brained bipedal primate. But that being said, could we expect anything similar at all of this rewound tape.
The answer I think is yes. One of my favorite Stephen Jay Gould essays, and one of his most famous, was "The Panda's Thumb." In it he describes the "thumb" of pandas which is a tool for stripping bamboo. Of course pandas have no true thumb but a modified radial sesamoid bone (the radial bone is the top bone of the forearm). The thumb of the panda is not nearly as efficient as primate thumbs for gripping but it gets the job done. Michael Shermer uses this as an example of contingency. Gould used it to illustrate both that and the silliness of an intelligently designed biology. In How We Believe, Shermer cites Gould's Panda Principle, which states "The complex and curious pathways of history guarantee that most organisms and ecosystems cannot be designed optimally." Shermer adds, "Historical events that come together in an unplanned way create inevitable historical outcomes." To which I say optimal for what? From whose perspective is any organism optimally designed? To deduce the extent to which anything is optimal we had probably better start by looking at the problem from the organism's perspective.
Any design optima is going to be shaped by any number of factors, but that doesn't make an organism any less "optimal" in its given environment and its contingent history. Whatever Gould may mean by optimal, I don't get it. When you read Wonderful Life, or listen to some of Gould's lectures it becomes clear that his targets were human vanity, and his own bogeyman: an evolutionary biology overrun with adaptationists who saw in everything progress and optimality. Contingency was Gould's answer to the progressivism he thought was rampant in evolutionary biology. Shermer found the idea utterly intriguing and bristled at its critics, chiefly Daniel Dennett.
"In my opinion, Dennett, and others, who adhere to a strict Darwinian adaptationist program, may be trying to find in nature, a non-existing pattern that shows us-Homo sapiens- as the nearly inevitable result of evolution." So says Michael Shermer of Dennett and unnamed others. If there is a finer example of windmill tilting, I've not seen it. What evolutionary biologist holds this position? Certainly it isn't Dan Dennett. Or Richard Dawkins, or any other "orthodox" Darwinian of which I know.
So where does that leave the history of life and Gould's tape? We cannot replay the tape of course but we can look at the history of our planet and point out that while contingency figures prominently into evolution we can also see that, in a generalized way, there is much that repeats. Dennett called this phenomena "good moves in design space." It is estimated that eyes have evolved, independently at least forty times. Here is an example of something in which evolution has hit on again and again. Echo-location as a tool for navigation is not limited to the bats, but used by whales, and Oil-birds to name just two others. Use of electromagnetic fields as a sensory tool has been hit upon by such disparate organisms as platypus and morymid fishes. Intelligence, and extreme social organization has been hit on several times. Canids, Elephants, primates, and whales are excellent examples of the independent evolution of this useful adaptation.
A more dramatic example of ability of natural selection to again and again hit upon similar good moves in design space is the experiment of islands after the Cretaceous/Tertiary extinction event that wiped out the Dinosaurs. Natural selection managed to create functionally equivalent ecologies despite the contingencies imposed by given founder populations. Placental mammals diversified in the old and new world (along with many marsupials), in Australia it was the marsupials. What is interesting is that similar strategies were hit upon again and again. There were no monkeys in Australia, but the arboreal niche didn't go unfilled. Kangaroos fill it. Ungulates are thin on the ground in Australia, but kangaroo species fill that plains grazer niche too. This phenomena of similar strategies, and forms, is called convergent evolution and it positively abounds in biology. Borrowing Richard Dawkins favorite example the thylacine, or Tasmanian wolf, we see the phenomena of "good moves" writ large. Here a marsupial separated by at least 100 million years of evolutionary history has hit upon the telling characteristic form and function a dog. In fact one would be hard pressed at a glance to tell a thylacine-sadly now extinct- wasn't a dog.


New Zealand, though, is another matter altogether. A huge island, a thousand miles long, it lacked, until quite recently, any mammals at all. Its vertebrate colonizers? Birds. There were no large grazers, no squirrels, no large predators. Again a pattern repeats, as several niches found in other places are again rediscovered by the birds. Moas became large terrestrial grazers (the largest of these species weighing nearly 1000 lbs), kiwis the ecological equivalent of squirrels, and a ground parrot that would find itself welcome among rabbits. This is an awful lot of meat on the hoof, or claw as it were, to go unexploited, so it will be no surprise that the large predator niche was filled out by a huge eagle capable of bringing down the largest of the moa species (this to me is a frightening prospect!).
In some sense the experiment of the tape has been run over and over again, in parallel and sometimes chronologically after an extinction event. And the result seems to show that there are good moves in design space that keep getting rediscovered by natural selection. This doesn't discount contingency as a potent force in evolutionary history, but I think it must make us see that it isn't the terribly radical mover Gould and Shermer thought it was, at least not in evolutionary biology.

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