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Re: Rate of evolution linked to metabolic rate

WARNING: wildly speculative idea

Well, metabolic rate and also body size: the study reported concluded that evolution (of new proteins-- it sounds as if the research had to look at masses of data, so only a fairly "raw" input of changes to sequences could be handled, with no analysis of the functions, if any, served by the changes) is faster in small-bodied animals than in larger.

Leading to a WILDLY SPECULATIVE idea I've had for a while. Every so often "Cope's Law" (crittres in a clade tend to increase in average size over the evolutionary history of the clade) gets discussed, often in the context of arguments whether it is any more of a "law" than, say, Bode's Law in astronomy.

Thought. Suppose that Cope's Law is true as a statistical description: on average, later representatives of a clade are bigger than earlier. (Is it? My sense is that the studies that have tried to address the question have been inconclusive.)(*) Does this mean there is a "direction" to evolution, that for some reason the process of evolution favors size-increase more than size-decrease? NOT NECESSARILY. Suppose small critters evolve faster than larger. In that case a survey of small critters is likely to turn up the "founders" of more new clades than a survey of big ones. Once a new clade is established, evolutionary size-change could be totally random: as likely to be size-increasing as size-decreasing, but only within the permissible size "Band" for the clade. (So, e.g., mammalian evolution would be equally likely to lead to smaller and bigger species, but-- since there are physiological limits to what is feasible for mammalian physiology-- won't continue downwards to produce something fly or ant or mite or bacterium size.)

The result of this, it seems to me, might be that we would observe something like Cope's Law in the data: that on average the later species of a clade would tend (statistically) to be bigger than the earlier, but this would NOT be because of a "preference" on the part of evolutionary "forces": it would be due only to the "founder effect" that the early species in a clade would be closer to the bottom than to the middle of its size band.

Allen Hazen
Philosophy Department
University of Melbourne

(*) So, observationally, are the "founder" species of large clades typically small for species of that clade? Works for mammals: Morganucodon and Hadrocodium are shrew-size. Doesn't seem to work for, say, tetrapods: Tiktaalik and Acanthostega are both, I think, bigger than the median tetrapod. Among dinosaurs... Well, we've recently been told that birdoformimorphs (do I mean Eumaniraptora? anyway, the clade of birds and near-birds out far enough to include Dromaeosaurs and Troödontids) started small, with size-increases in four sub-clades. On the other hand, birds sensu a bit strictu-er don't seem to have started particularly small: given the number of small Passerines, I suspect Archaeopteryx was well above the median size for birds. Clearly there is more work to be done to see if there is even a phenomenon out there for my speculative idea to apply to!