Punctuated equilibrium
Punctuated Equilibrium is a theory that people like Stephen J Gould have developed to help explain how evolution operates in the real world.
It explains how most species spend most of their time NOT evolving, and how it is that you rarely find transitional species in the fossil record. Mostly they're at equilibrium, occasionally punctuated by change.
An example. Humans are surrounded by human stuff like houses and books and cars. They eat certain things like bread and chickens and "sweetened bovine secretions". This gives them no reason to be any different - no "selection pressure" as a Darwinian might call it. There's no point in humans being 10 foot tall; they wouldn't fit through their doors anymore. So they tend to stay as they are.
This is the negative feedback that keeps a species being that species. Sometimes it's called "steady state" or "dynamic equilibrium".
Lots of species are so good at controlling their immediate environment (their "niche" if you're an ecologist) that they haven't changed for millions of years. For example, horseshoe crabs are indistinguishable from their 360 million year old fossils. That's old. Really old. Half the time oxygen has been in the earth's atmosphere, six times older than when the dinosaurs were wiped out. Imagine the changes they've seen!
But things do happen, and when they do, they happen quickly by positive feedback. Humans have exploded on the earth, much of it in just the last couple of generations (see Simple population model), and they're having huge effects on everybody else's niches, causing all kinds of mayhem and extinction.
Of course, that will drastically change things for humans too, so who knows what's round the corner ...
