October 13, 2013
to be posted on nobabies.net
M. Paul Smith
Oxford University of Natural History
Parks Road, Oxford OX1 3PW
UK
paul.smith@oum.ox.ac.uk
Dear Paul Smith:
I was delighted to see the review of current thinking about the Cambrian explosion. (Causes of the Cambrian Explosion M. Paul Smith and David A. T. Harper SCIENCE vol. 341 no. 6152 September 29, 2013 page 1355) It well complements (Charles R. Marshall, When Prior Belief Trumps Scholarship page 1344 in the same issue reviewing Darwin’s Doubt by Stephen C. Meyer).
I am less than prepared to comment on your article except to say that it is clear and persuasive. However when I approach it from a different direction I get an idea. Starting with the obvious observation I think first made by Alfred Russel Wallace that effective selection must await speciation, selection is a race, and so is speciation. So far as I can tell speciation takes about two thousand generations of separation. After this some critical chromosome in one moiety cannot function properly with a corresponding chromosome from the other. Hybrid infertility ensues. So consider a population of a thousand under conditions of random mating. The daughters of one chromosome on average will not meet each other for two thousand generations. Unless there is some magical thing I cannot envision, they cannot then function together and the fertility of the whole population collapses. Populations of more than a thousand must be structured or the whole population dies. You could call it, “Extinction of the fittest,” since a successful population will grow. This is not the usual understanding, but I think the logic is impeccable.
I tested this with an experiment that was published this year. I attach it. It appears that there is a mechanism, doubtless epigenetic, that prevents a population from growing indefinitely. Sorry, Malthus. The mechanism had to post-date sexual reproduction. So when did sexual reproduction arise and when did the population limiting mechanism or mechanisms arise? Let me speculate. Looking at your excellent analysis “Times of Change,” it seems to me that sexual reproduction might have arisen in the Ediacaran era. I am sure it worked well, but something seems to have gone wrong. There was a dramatic fall in the number of taxa just as things seemed to be going swimmingly. Since speciation was new one could argue that it was imperfect; taxa shared their gametes to a degree. Then the mechanism I describe at some point overrode the rate of appearance of new forms. There was an enormous selective advantage to be gained by structuring those populations. So I propose that the first epigenetic mechanism to limit gene pool sizes emerged and was able to spread to different forms. The diversity of taxa went up again.
My work as outlined in the paper suggests that the mechanism in fruit flies, and probably all insects, is post-zygotic. Its emergence might have corresponded to the beginning of the Cambrian.
A post-zygotic mechanism works adequately, but it is rather forgiving. Populations can become quite large and then fall and recover repeatedly. That’s good for a simple organism because the loss of a species (through that mechanism I mentioned earlier) is not the loss of an enormous investment of time. Another kind of insect can be produced by speciation and selection in a relatively short time. But for more complex organisms it seems (Forgive me here. I don’t even follow this logic very well myself.) that a stricter limit on population size would be advantageous. This, I have suggested, is the case in humans and mice and as a first guess I would think all mammals. A pre-zygotic mechanism is unforgiving and would serve if combined with a pre-existing post zygotic mechanism; it would not just suppress but kill off a population that ventured above the gene pool size ceiling.
The appearance of this mechanism should have resulted in a burst of new forms some time during the Cambrian. And your “Times of Change” analysis shows more than one. In fact as I have flopped around in the murky waters of this issue I have often thought that there was more than one post-zygotic mechanism and/or pre-zygotic mechanism. (I’d be happy to be specific but it would be distracting right now. Ask me if you are curious.)
Wacky as it may sound, this notion is testable. If my hypothesis is right, then the first dramatic increase in diversity should have included animals that were rather like insects. Some later burst should have included chordates but not particularly those forms that had so flourished in the first explosion. In fact you may have the data right at your elbow.
Let me know what you think.
Sincerely,
M. Linton Herbert MD
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