h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think there's a 20 generation step for each rung of the evolutionary ladder in the case of humans. That's 400 years.
In other words when something happens requiring an evolutionary change it will generally come about within 20 generations.
For example, people living in an obese society where legs are no longer required will find that these limbs will be useless in 400 years and that their stomachs will be twice as big as they are today. And so it goes.
Evolution must be combined with adaptation to the environment. Polar bears must now evolve to live on land because living on the icecap is proving to be too dangerous due to the rapid melt and many offspring are drowning or starving to death.