Sunday, December 5, 2010
Sunday Spinelessness - The origin and extinction of species
I don't use these pages to write about my own work very much, partly because it's not yet published and partly because I write about that all day as it is. The shortest answer I can provide to the question "what do you do" is "I use genetic tools to study evolution" and I guess that makes me an evolutionary geneticist. You can split the people that work in our field into two groups: there are biologists that are really interested in a group of organisms and have learned some genetics to help their study of them, and there are people who are interested in a particular question and have chosen their study organisms to suit. I'm very much of the second sort, and like most people in that group I've caught myself saying "I'm interested in the questions, not the animals". Paraphrased, that becomes something like, "Oh sure, I study Pacific land snails, but for all I care they're just little bags of genes that help me answer questions". But that's a lie. You can't work on animals without having them effect you. When I started my PhD I had no particular love of snails, but now I'm a complete snail fan-boy and I frequently find myself preaching on the wonders of life as a terrestrial mollusc to people whose only mistake was to ask me what I do for a living. Did you know most slugs retain the remnants of their shells? Or that almost all snail shells coil to the right? Or that mating in many land snail species only proceeds after one snail has stabbed the other with a "love dart"? A couple of weeks ago I was recounting the the sad tale of The Society Islands partulids to someone I'd met three minutes earlier, and today I'm going to tell you that story (though, of course you have an advantage over the first recipient of the story, since you don't have to read this crap)
Believe it or not, land snails are one of the characteristic animals of Pacific Islands. Anak krakatau is so young it's still smoldering, and it has a native land snail species and Rapa nui (Easter Island), which is arguably the most isolated island in the Pacfic, had its own land snail fauna back when it had forests. It's not entirely clear how these unlikely colonists get to islands. Darwin was so interested in the question* he, ever the experimentalist, stuck snails to ducks' feet to see if they'd survive an inter-island journey. Birds have been shown to carry snails great distances, but wind blown leaves are probably a more common mode of conveyance. We might not know exactly how snails get to islands, but we know what happens once they establish themselves. The land snails of the Pacific include some of the most outrageous explosions of diversity in the biological world. Chief among these evolutionary radiations were the partulid snails of The Society Islands (the French Polynesian archipelago that includes Tahiti). Partulids are very elegant tree snails that form part of the land snail fauna across most of Polynesia, in the Societies they made up most of the land snail fauna. In total, the tiny islands had 58 species of these snails with each of the main islands have their own endemic forms.
A plate from Crampton's monograph on the partulids of Moorea
The Society Islands' land snails were a marvel all by themselves, but they were also an extraordinary resource for scientists. The first person to seriously take up their study was the American embryologist and evolutionary biologist Henry Crampton. Crampton was working at the turn of the 20th century, a time in which the mechanisms underlying genetics and evolution were very much up for debate, and he hoped Tahitian and Moorean partulids could help set the story straight. Crampton's monogrpahs are famous (at least among people that spend thier lives thinking about snails) for their detail. He collected and measured over two hundred thousand shells, then calculated summary statistics for each species, each site and each measurement. By hand. To eight decimal places.
Those massive tables (there are more than 100 pages of them in the Moorean monograph) might seem like an old-fashioned, descriptive, way to do biology. But in many ways Crampton was ahead of his time. For one, he was a Darwinist when not every evolutionist was. By the end of the 19th century Darwin had convinced the world of the fact evolution had happened, but relatively few naturalist bought his theory of how evolutionary change happened. The anti-Darwinian theories that prospered during the so called "eclipse of Darwinism" placed very little importance on the variation within species. The orthogenesists and the lamarckians thought evolution had a driving force, pushing species towards perfection. In their scheme variation within a species was deviance from the mainstream of evolution and was quckly stamped out by natural selection (which they didn't deny, they just said it couldn't be a creative force). Similarly, saltationists thought large-scale evolutionary changes occurred in a single generation, and the small changes you see in populations were of no consequence in the grand scheme of evolution. Crampton realised that, in a Darwinian world, variation within populations was the raw material of evolution. He was obsessive about measuring his shells because he knew could use the data he was recording to understand where species came from. In particular, we was able to show that isolated populations of the same species varied from each other. That finding that makes sense in light of Darwin's theory, since species arise from populations evolving away from each other; but is harder to fit into progressive theories of evolution, in which you'd expect different populations of the same species to follow the same trajectory.
Crampton's results influenced people like Dobzhanky, Mayr and Huxley who helped to re-establish Darwinism as the principal theory of evolution in the Modern Evolutionary Synthesis. But Crampton also predicted arguably the most important development in evolutionary theory since the modern synthesis. In the middle of the 20th century evolutionary genetics was defined by a single debate. The "classical" school held that populations in the wild would have almost no genetic variation, because for every gene there would be one 'best' version and every member of the population would have two copies of that gene. Arguing against the classical school, the "balance" school argued that, quite often, there would be no single best gene and organisms would do better having two different versions of the same gene**. The ballancers thought natural selection would keep lots of different versions of maybe 10% of a species' genes. Both schools assumed natural selection was such a pervasive force that selection would dictate the way populations were made up, they just disagreed on what would result from it. Here's the funny thing, they were both spectacularly wrong. When scientists started being able to measure he genetic diversity of populations in the 1960s it became clear almost every single gene had multiple different versions. Now, in the post-genomic age there is a database with 30 million examples of one sort of genetic variant amongst humans.
Faced with the overwhelming variation he recorded in partulid shells, Crampton had argued natural selection didn't have a damn thing to do with it. Snails isolated from each other by a mountain weren't adapting to their local habitat, they just varied with respect to traits that had no influence on their survival. The fact two populations were isolated meant each would follow its own path and two populations could drift apart from each other. Faced with the overwhelming genetic variation coming from studies in the 1960s Motoo Kimura proposed the neurtral theory of molecualr evolution. Kimura's explanation was the same as Crampton's, almost all of the variation we see at genetic level has no bearing on the success or failure or organisms so the frequency of different variants drifts around at random. The neutral theory is at the heart of a lot of modern evolutionary genetics, and Crampton had understood the underlying principle 50 years before we knew we needed it!
At the end of his monograph on the partulids of Moorea, Crampton said he'd got as far as his measurements could take him, and it was time for someone to study their genetics. In took a bit longer than Crampton might have hoped, but in the 1960s two leading geneticists took up the study of his snails. James Murray from Virginia and Bryan Clarke from Nottingham spent almost 20 years working in what they called, in more than one paper, the perfect "museum and laboratory" in which to study the origin of species. Their work helped scientists understand, among other things, how ecology can contribute the formation of new species and what happens to species when they hybridise with others from time to time. Then, in 1984, Murray and Clarke had to write the most heart-breaking scientific paper I've ever read. It's written in the careful prose scientists use to talk to each other, but the message it delivered was devestating:
In an attempt to control the numbers of the giant African snail, Achatina fulica, which is an agricultural pest, a carnivorous snail, Euglandina rosea; has been introduced into Moorea. It is spreading across the island at the rate of about 1.2 km per year, eliminating the endemic Partula. One species is aiready extinct in the wild ; and extrapolating the rate of spread of Euglandina , it is expected that all the remaining taxa (possibly excepting P. exigua) will be eliminated by 1986-1987.
The bad guys: Euglandina on the left, Achatina on the right.
Euglandina rosea is better known as the Rosy Wolf Snail. It senses the mucous trails of other snails, tracks them down and eats them. It's not clear if the wolf snail had any effect on the pest species it was introduced to control, but it had huge impact on the partulids. By the time Murray and Clarke wrote their paper, E. rosea had already done for one species and it was too well established to control. All they could do was watch as human stupidity and molluscan hunger slowly (1.2 km per year) destroyed the species they'd been studying for 20 years and Crampton had dedicated 50 years of his life to. The same slow torture played itself out in Tahiti and then the rest of the Society Islands. Where there were 58 named species, there are now 5 alive in the wild. Crampton's hundreds of pages of tables should have been the starting point from which the evolution of the partulids could have been tracked. Murray and Clarke's natural laboratory should still be open and be taking advantage of a new generation of technologies that might be able to reveal the genetic and genomic changes that occur when a new species arises. Extinction is a natural part of life, and the fate of all species eventually, but when it's driven by human short-sightedness and robs us of not just a wonderful product of nature but a window through which we might have understood nature's working it's very hard to write about.
I should end by saying there is just a tiny scrap of good news in this story. The partulids are no longer an iconic species in the study of evolution, but they have become the pandas of invertebrate conservation. Murray and Clarke were able to get 15 of the species of the islands and into zoos and labs across the Northern Hemisphere. Breeding programs have been succesful, and new lab-based studies come out form time to time. The relict populations back in the Societies don't have nearly the range they used to, but it appears they've held on to most of their original genetic variation. Perhaps, one day, Eulglandina can be taken care of and some of the partulids can have their islands back.
*Darwin had to be interested in dispersal. Before evolution was widely accepted naturalists thought creatures were created for their habitat (the modern creationist notion of a post-flood diaspora explaining the distribution of animals is almost entirely an invention of Seventh Day Adventists, no, really, it is), Darwin's theory did away with special creation but still needed to explain how life came to live everywhere
** A concept similar to "hybrid vigour", in which crosses between relatively unrelated strains/cultivar bring together different genes and do well as a result. You've seen evidence of this phenomenon any time you've eaten yellow and white "honey and pearl" corn. That corn is a hybrid between a white and yellow cultivar and if you count up the kernals you should get close to the 3:1 ratio Mendel preficts for a dihybrid cross.
Some further reading:
Stephen Jay Gould, who was a snail man himself, wrote and essay on Crampton and the Society Island partulids in which made a humanistic argument for the importance conservation. I resisted the urge to re-read it in researching this piece so anything I stole from him I stole sub-consciously!
- Gould, S.J., 1994. Unenchanted evening in Eight Little Piggies: Reflections in Natural History
Crampton's monograph on the Mooeran partulids (from which the figures above are taken) is available online
- Crampton, H.E., 1932. Studies on the variation, distribution, and evolution of the genusPartula. The species inhabiting Moorea. Carnegie Institution of Washington, 410, 1335.
Finally, the paper in which Clarke and Murray told the world about the demise of their snails:
- Clarke, B., Murray, J. & Johnson, M.S., 1984. The extinction of endemic species by a program of biological control. Pacific Science, 38(2), 97-104.
The only time I learned much about land snails was as an undergraduate when I took an elective class on Zoogeography. It was my first class in which the lecturer used a projector to illustrate his entire lecture with slides. Perhaps snail people tend to be before their time. In this case, though, it was very strange because the lecturer was Geerat Vermeij and although not being able to see them, he described the take home lesson from each slide in great detail. Great class.
PS - Very Darwinian of you, with your post title, to avoid defining a species.
I think the thing with the modern synthesis was the same people that made it wrote the history of it. So read Mayr and you sort of think it went Stupid People -> Darwin -> More Stupid People -> Darwin Restored!
And sort of become the story, but it's much more interesting to dig into the history of these ideas. And yes, I've actually just written an manuscript in which I do my very best to not define species at the same time as delmiting them, so we'll see how that goes!