limbosis:
Nominal body size gets dumped all over by the theory of evolution, which maintains that average body size sometimes gets reduced when food is scarce (on islands and such). The problem with that is that it would necessarily apply to all animals, regardless of where they are, or how much food there is. Reduced body size would supposedly tend to stabilize a particular species most effectively. Of course, that would have led to diminutive dinosaurs, and elephants the size of grapes.
It's no problem for evolutionary theory at all. Selective pressures exist toward larger as well as smaller sizes. Context is all. Size will tend toward whatever norm confers the best reproductive and survival advantage on the creature in the environment.
Reduced size is indeed an advantage in some environments: reduced food supply, thick forestation, etc. But larger size is an advantage in others: oceans, open plains, etc. A smaller size enables better ability to hide from predators, but a large size confers a survival advantage against predators in environments where hiding places are few.
Push the slider toward either end of the size scale and something is gained, something is lost. It's always a tradeoff. The deciding factor is context: what works best in the environment?
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Edited by Archer Opterix, : HTML.
Archer
All species are transitional.