Human kind, and a whole lot of other animals have been experiencing just the type of scenario which leads to relatively rapid evolution. The circumstances of weather pattern changes, mostly due to the movement and collision of contintents and the associated ocean current changes, have led to a series of ice ages.
Every 20-30 thousand years or so the great ice sheets have retreated. This means that there is expanding green space. Each person's children had a good chance of finding new land with abundant game. In such an environment the population rises, and the number of novel genes and combinations rise with the population. This continues for 1000 generations of relative prosperity.
Then the cycle reverses, The ice sheets begin to grow. Each year there is less land free from the ice and the game and hunters all get crowded back. There is not enough for all, so over the next 1000 generations more die than are replaced. A gradual but steady selection takes place. Now those that can run faster, or speak more clearly to their allies, or digest milk products better, are favored and come to make up the majority of the shrinking population.
Again the cycle reverses and times get better, and families can grow larger with more surviving offspring. What makes this time different is that all of the now growing population is descended from those who won the struggle for survival, and not just a random sampling of the original population.
These boom and bust cycles speed up change by allowing both a period where living is easy and different genes and strategies can survive, and a period where beneficial characteristics will predominate over neutral or harmfull characteristics. The process is slow enough to allow thorough spreading and mixing of the genes under selection.
During periods where all land was in one mass, things were very stable for very long periods, and so were the species.
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