Types of selection, selective breeding and antibiotic resistance

Last time

Before we start: last time, in Speciation and species concepts, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Three ways to shift a curve

Imagine the beak sizes in a population of finches plotted as a bell shaped curve. Natural selection can squeeze that curve, push it to one side, or split it in two. Each type is driven by a different environment. Farmers and doctors then meet the same ideas in selective breeding and in antibiotic resistance.

Stabilising selection

In stabilising selection, individuals near the mean have the highest fitness. Both extremes are selected against. The curve gets narrower, with less variation, but the mean does not change. It happens when the environment is stable. An example is human birth mass. Very small and very large babies are less likely to survive.

Directional selection

In directional selection, one extreme is favoured. The mean shifts towards that extreme. It happens when the environment changes in one direction. On the Galápagos island of Daphne Major, a drought left mainly large, hard seeds. Finches with bigger bills could crack them and survived, so the mean bill size of the next generation increased. Antibiotic resistance is another case: resistant bacteria are favoured, so the frequency of resistance alleles shifts upwards.

Disruptive selection

In disruptive selection, both extremes are favoured, and the intermediates are selected against. The curve splits into two peaks. In spadefoot toad tadpoles, small detritus feeders and large carnivores each do better than the intermediate type, in ponds with two food sources. If this goes on, with the two groups not interbreeding, it can lead to new species.