June 24, 2012
to be posted on nobabies.net
Rick Shine
Professor in Evolutionary Biology
Phone: +61 2 9351 3772
Fax: +61 2 9351 5609
Email: document.write('' + 'rick.shine' + '@' + 'sydney.edu.au' + ''); rick.shine@sydney.edu.au
Location: Room 209, A08 - Heydon-Laurence Building, The University of Sydney, NSW 2006, Australia
Dear Rick Shine:
Forgive me for starting out by rattling on about the weather, but I don’t know quite how to approach this. As I write it is raining. We have had more rain today than ever before in the thirty years I have lived here. We haven’t even had a hurricane dump this much water. The wind picked up my neighbor’s trampoline, crumpled it and dropped it off my seawall. That’s the second trampoline he’s lost to the wind in a year. I have had two phone calls today from friends who would take me in if there was a flood here.
I was enjoying the charming article on you (Sarah Zelinski The Reluctant Toad Killer SCIENCE vol. 6087 no. 6087 June 15, 2012 page 1375) when I was struck by the fact that the advance of pesky cane toads across Australia was accelerating. While I cannot say I can offer you anything that will help I do think I can offer a perspective.
It was an Australian, A. J. Nicholson who did an elegant series of experiments counting flies to establish factors that controlled their population size, generally different feeding regimens. One regimen he did not try was simply trying to give them more food than they could possibly exploit and then watching what happened. He makes it quite clear why in a paper, I think it was at Cold Spring Harbor. He believed that given food and space the flies would increase without limit. He gives the reasons for this belief, most of which come down to, “It’s obviously true,” but no evidence. Had you been his mentor, I think he would have checked it out. You point out that guessing what will happen in an experiment is a good way to be surprised.
You can see his logic. Darwin was right. Darwin based his thinking on Malthus. So Malthus had to be right. Only he wasn’t.
If you haven’t decided I am deranged yet, let me try to forestall that moment by pointing out an oddity in my use of the word “population.” I generally mean a defined group, like Nicholson’s flies, and usually not a terribly large group. So when I say a “large random mating population,” I am likely to mean a population in the hundreds or thousands. Most people think of a “large” population to be a significant fraction of a billion (American meaning) or multiple billions.
Long after Nicholson had done his work a paper was published (On the Regulation of Populations of Mammals, Birds, Fish and Insects, Richard M. Sibly, Daniel Barker, Michael C. Denham, Jim Hope and Mark Pagel SCIENCE vol. 309 July 22, 2005 page 609) that found that wild animal populations grow ever slower as they get bigger. (Yes, I know. Cane toads. I’m getting there.) Instead of growth stopping at some level of presumptive environmental limit, it continues to fall ever more slowly even after the population has entered negative growth.
It is off topic here, but the same thing happens in humans. (An Association between Kinship and Fertility of Human Couples Agnar Helgason et al. SCIENCE vol. 329 no. 5864 February 8, 2008 page 813 – 816 AND Human Fertility Increases with marital radius. Rodrigo Labourian and Antonio Amorim. GENETICS volume 178 January 2008 page 603 AND Comment on “An Association Between the Kinship and Fertility of Human Couples,” Rodrigo Labouriau and António Amorim SCIENCE vol. 322, page 1634b December 12, 2008 looking at fertility in modern Iceland and Denmark) Average kinship and population size are of course just different ways of looking at the same thing.
So it isn’t starvation because it happens in rich socialist countries and it isn’t saving for college or worries about the world because it happens in animals. There must be a biological mechanism.
The need for such a mechanism may not be obvious. But consider that speciation takes a number of generations, say two thousand. Divide a population for two thousand generations and when they get back together their hybrids are infertile. There is at least one pair of chromosomes that cannot do business with each other. So take a random mating population of a thousand. That’s two thousand chromosomes of that type. Two chromosomes separated by a single meiotic event will take on average about two thousand generations to get back together. They will not be able to do business. The whole population dies.
There are a couple of ways out. You can turn off speciation. But in a real environment the forms that can undergo speciation will out maneuver those that cannot. The other thing is to keep all populations of the species below some threshold, and that threshold needs to be safely under a thousand.
The results in the papers above simply reflect the mechanism that evolution has mandated to keep populations small enough for long term survival.
The notion of an optimal population size is not new. It has been found that “major histocompatibility complex” genes demonstrate this. Too little variation, too much homozygosity, reduces immune fuction. Too much variation reduces immune function. Patrick Bateman, who wrote Mate Choice, has called this “optimal outbreeding,” the degree of outbreeding that produces the best result. It clearly holds for fertility as well as for immunity.
The founding population of cane toads brought in with the hope of controlling beetles was in the thousands. Even without knowing whether they all came from the same area, that is already too many for optimal outbreeding.
There is something they call “founder effect.” When a population buds off a new smaller population, the new population will have less overall genetic diversity than the original. So with the toads. As they expanded, founder effect would have driven them in the direction of optimal outbreeding, not optimal for anything else living in Australia, of course.
I suspect that optimal outbreeding mediated by founder effect was why pioneers had so many babies and Native Americans so few. A similar phenomenon has been described in Quebec.
I know there are other causes of the acceleration of the toad advance described in the article. But this could be part of it. Checking it out would be straight forward, homozygosity, fertility and rate of advance should all track along together. As for turning this against the toads, I confess I have no good idea. Shipping them around enough to have a significant impact would require catching a significant fraction of them. Captive breeding and release far from home would win no friends. But there are brighter people than I am, so maybe somebody could come up with an idea.
Please let me know what you think about this.
Sincerely,
M. Linton Herbert MD
The professor was prompt and courteous to respond. My heartfelt thanks. Here, with his permission, is his note:
Dear Dr Herbert,
Thanks for your interesting ideas. I am a fan of AJ Nicholson (and his protagonists at the time –I worked with Charles Birch). I agree that population sizes can affect evolutionary trajectories and long-term success, but as an avowed individual selectionist, I prefer to focus on mechanisms with immediate fitness consequences for individuals. Apart from anything else., that focus makes for tractable experimental work. I'll ponder your thoughts some more, however – interesting stuff!
Cheers, Rick Shine
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