Re: Rate of evolution linked to metabolic rate

[Allen Hazen <allenph@unimelb.edu.au>]

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!