In most cases, the source of selective pressure is unknown.

The new versions were maintained, an instance of positive selective pressure.

This may be due to the opposite selective pressure involved.

About 5,000 to 10,000 years ago, this haplotype came under very strong selective pressure.

The lack of diversity created can make the population at risk to other selective pressures.

Hence, a greater sexual selective pressure on women to lose body hair.

With a short enough generation period evolution can happen extremely quickly in the face of strong selective pressure.

The second copy of the gene is often relatively free from selective pressure.

For thousands of years, states have evolved under other selective pressures.

The cause of this selective pressure has not yet been identified.