(This title is wrong
. I think ecologist
s have a proper
name for this principle.)
In naturally evolving environments, given stable external
conditions, the system of different entities populating the environment will evolve to a stable equilibrium.
This principle says that certain patterns are likely to occur in which various entities in the system have
certain predictable functions. For example, when a creature becomes more agile on land, it's probably going to develop legs. And when a whole ecosystem develops to stability, it's probably going to have large carnivorous predators at the top of the food chain. If there is a niche to be filled there, something will do so (probably changing the whole system as a result)
This explains convergent evolution.