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body size ad nauseum
Just a few last comments about body size.
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. 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
I realize I have been mixing notions about body size trends within lineages
with body size variation within taxonomic groups all along. The fact is,
body size tends to increase in many lineages, while the negative
exponential distribution of body size remains constant within taxa. 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.
Why did hadrosaurs and ceratopsians evolve from smaller dinosaurs? There
were big sauropods around in the Cretaceous. Why didn't they evolve into
other herbivorous dinosaurs? Because large size slows down evolution,
while at the same time making you vulnerable to extinction. For the same
reason, elephants are never likely to evolve into anything dramatically
different. But mice? Just give them a few million years.