evolution rates

["David Marjanovic" <david.marjanovic@gmx.at>]

Just found nobody answered to this, and I think it is of some general
interest...

----- Original Message -----
From: <h.b.j.ensing@kpn.com>
To: <david.marjanovic@gmx.at>; <dinosaur@usc.edu>
Sent: Wednesday, July 10, 2002 12:43 PM
Subject: RE: (extinction) (long)


> Hello,
>
> You wrote:
>
> > -- horses and rhinos began small and therefore evolved fast at
> > first, then they grew and their evolution slowed down.
>
> Just wondering .... Is this a "law" in evolution theory ?
> I'm certainly not an expert but I always thought that speed of evolution
> is more dependant on population size (rapid when there are not so many
> of them) than on body-size.

Population size is definitely a factor. But so is generation time, which
grows with body size. More generations per time at the same mutation rates
should produce faster evolution, and I think occurrences of this are more
common than population bottlenecks, and should outweigh the effects of the
usually small population size of big animals. I was definitely
oversimplifying, but I think as an approximation this holds.
        Fastest molecular evolution of all placentals: Mice and rats. Have
commonly appeared at the base of Placentalia in early molecular cladograms
because of long-branch attraction.

> If I'm not mistaken elephants and rhino's show
> rapid evolution in some cases.

No idea about rhinos, but elephants do evolve faster than expected because
they keep their testicles internal. The higher temperature inside the body
somehow produces more mutations. All this holds for the much smaller
tenrecs. (This character doesn't do me the favor of being the first
morphological synapomorphy of Afrotheria, does it? :-) )