Science Forums 2

[Boerseun]

I'd say that evolution is a species' response to environmental pressure. I suppose that'll make it external, although I don't quite get the gist of your question. Evolution is also a species' response to predators, whereby predators gain new skills which will act as an evolutionary sieve on the prey, and the survivors will adapt new techniques in evading the predators - who in turn would have to invent new strategies to catch the prey, which would then have to invent new defenses, etc.

Plants have evolved certain toxins so animals wouldn't eat them. So us slinky bastard animals have invented livers to get over that hurdle. It's a continuous process, but I'm still not sure I get your question.