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Re: body size ad nauseum




On Fri, 15 May 1998, Colette H. Adams wrote:
> 
> Crocodilians have no doubt had a number of selective pressures keeping them
> large, but I must point out that if we look at the 24 living species of
> crocodilians, we find that they distribute something like this:
> 
> 0-50 kg: 9
> 50-100 kg:  2
> 100-150 kg:  5
> 150-200 kg:  0
> 200-250 kg:  4
> 250-300 kg:  1
> 300-350 kg:  1
> 350-400 kg:  2
> 
> I believe we will find a roughly similar exponentially decreasing size
> distribution in any taxonomic group. 

Non-avian dinosaurs of the Campanian and Maastrichtian, according to
Graeme Worth's "The Dinosaur Encyclopedia" sort out contrary to this.
This adds weight to the idea that something intrinsic to species creates
body-size rather than a universal rule!
0-.99m  1sp.
1-1.99m 5sp.
2-2.99  10sp.
3       13
4       5
5       6
6       8
7       2
8       5
9       6
10      6
11      3
12      7
13 and above 8

> We can no doubt find idiosyncratic
> reasons why the mean body size in one group differs from that of another.
> But the similarly shaped distribution in all taxa seems to demand a general
> explanation.

If the above data is accurate (I realize preservation bias might be an
issue), then maybe not.  Idiosyncratic reasons may be more important.

> Large
> size seems to be an evolutionary dead end.  Time and again we see
> increasing size within lineages, with the large species eventually going
> extinct without leaving descendants.

I'm not denying that such things as generation times, evolution rates, and
the ability of small things to find more niches are a prime influence on
the fitness of small things compared to large over geological time.  I am
saying though that body size selection can be very different from taxon
to taxon. I would also resist the idea that large size per se causes
extinction.

Thank you for your interesting ideas.
John Bois.