Directional, stabilising and disruptive natural selection

Natural selection leaves a certain gene pool’s allele make-up quite different depending on selection pressures. The results are quite easy to understand. Take a population of toads. Directional selection would happen if smaller toads avoided predators better, hence larger toads would be gobbled up more frequently. The average toad size in the following generations would shift in the DIRECTION of the smaller extreme.

Stabilising selection, on the other hand, shifts extremes towards the middle values. Say small toads have less reproductive success, and large toads are more appealing to predators. Average-sized toads will be best adapted to that situation (of course, unless they too happen to have some “issue”, in which case, bad news for the toad empire).

Finally, perhaps the most bizarre of these is disruptive selection. It occurs as a result of extreme phenotypes (noticeable traits) being better adapted to the environment than mid-range values. Say smaller toads survive better due to weather conditions during a certain period of time, while larger toads survive better during other times, but average-sized toads have no advantage at any given time.

This stuff becomes so much more interesting when applied to humans. Where you live, which type of selection do you think is taking place?

…if any.

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