According to the initial post in the OOC, the amount of influence in a taxon is proportional to the speed it takes for the desired biological change to happen. And once you put influence into something, you can't remove it until that taxon is extinct (and even then it only gradually comes back).

So, if you need your taxon to evolve quickly, then invest a large percentage of your influence into it. But if you'd prefer slower evolution, and if you want to keep some influence on hand in case of an emergency or opportunity, then you should save it.

Wish I'd thought about this before using all my points up...