RE: Rate of evolution linked to metabolic rate

[evelyn sobielski <koreke77@yahoo.de>]

--- Guy Leahy <xrciseguy@sbcglobal.net> schrieb:

> This certainly could be part of the reason why
> mammals
> radiated so rapidly into large body ecologic niches
> after the K-Pg extinction event:
> 
> http://news.ufl.edu/2007/10/03/evolver/

The MR hypothesis is not very new, they've been trying
to prove it as a dominant mechanism since 1993:
MARTIN, A. P, and S. R. PALUMBI. 1993. Body size,
metabolic rate, generation time, and the molecular
clock. Proc. Natl. Acad. Sci. USA 90:40874091.

In birds specifically it has been looked into in
detail and quite firmly refuted (at least as a
mechanism of major significance), for example:

Mindel et al. (1996): Slow rates of molecular
evolution in birds and the metabolic rate and body
temperature hypotheses. Mol Biol Evol 13 (2): 422.
"Our findings regarding G. gallus do not support
predictions of the MR hypothesis using whole body
metabolic rate measures, as Gallus has slower rates of
mitochondrial and nuclear DNA sequence evolution
relative to four mammals, despite similarly high
specific metabolic rates. Our data regarding
cold-blooded reptiles are limited to that for the 12s
mitochondrial rDNA gene, but they are also counter to
the MR hypothesis predictions, as we found that C.
acutus does not have a slower rate of change for 12s
mitochondrial rDNA relative to birds or mammals."

Witt, Christopher C. (2004): Rates of Molecular
Evolution and Their Application to Neotropical Avian
Biogeography (Ph.D. dissertation)
"Second, I tested the metabolic rate hypothesis, which
holds that mutation rate in mtDNA is correlated with
mass-specific metabolic rate. This hypothesis predicts
that small-bodied lineages should evolve rapidly.
Previous studies verified this prediction, but none
utilized adequately large samples of independent
contrasts among appropriate taxa. The use of many such
contrasts from bird mtDNA sequences conspicuously
failed to corroborate the link between metabolic and
mtDNA rates. On the contrary, high rates of
nonsynonymous substitution were associated with large
body mass, implicating population size as a pervasive
cause of evolutionary rate variation."

Sergio L. Pereira and Allan J. Baker (2006): A
Mitogenomic Timescale for Birds Detects Variable
Phylogenetic Rates of Molecular Evolution and Refutes
the Standard Molecular Clock. Molecular Biology and
Evolution 23(9):1731-1740
"We found no support for the hypothesis that the
molecular clock in birds "ticks" according to a
constant rate of substitution per unit of
mass-specific metabolic energy rather than per unit of
time, as recently suggested."

Then there is the observation that in Testudines,
*some* factor (but certainly not metabolic rate) slows
the "molecular clock" by almost one order of magnitude
to small mammals and passerine birds. It's not
metabolic rate, because other turtles have more
"normal" rates of molecular evoution (various papers
verifying and refuting this phenomenon for different
Testudines; Google Scholar will help). The (on
average) slower rate compared to birds - not just
small ones - might be due to metabolic rates, but rate
variation *in* the Testudines cannot be explained
thus.

What I wonder is this: *Nobody* has ever tried looking
at the issue via generation times. There are
indications that this might in fact be the major cause
of "molecular clock" variation (e.g. the inability to
cook up a satisfying
biogeographical/paleoclimatological scenario for
radiations of large Falconiformes if one uses a
standard 2%/ma clock but good matches if rates of
1-0.5%/ma are used), let alone the basic fact that a
mutation can occur, be transferred or spread (in a
population) only from one generation to the next.

Still, the metabolic-rate hypothesis could explain
peculiarities in the data, such as in Cypselomorphae
(which are metabolically somewhat odd among birds, as
about every taxon in this group seems to be able to go
into torpor).

And then there is the "seabird problem": namely in
Procellariiformes, Pelecaniformes etc, metabolic
effects on DNA are... strange... in general (cf. the
telomeres that at least in Leach's Storm-petrel
actually become longer with increasing age)

So yes, it could well be a matter of statstics, as
Allen suggests. A phenomenon that by itself is weak,
but in a sufficiently large sample will peek through
the noise of other, more variable (but in themselves
stronger) factors.


Regards,

Eike


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