Stabilising selection

• Continuous variation characteristics show a range of values.

• In a certain environment, it could be the average phenotype that provides the advantage compared to either extreme. In this case, the extreme values are selected against.

• The normal distribution curve has a smaller and smaller standard deviation over generations but a higher peak. That’s to say, the average has stayed the same, but more individuals have that value.

Directional selection

• Changing environment, an extreme phenotype may become advantageous.

• Then, other values are selected against and, over time, the average phenotype changes.

Disruptive selection

• In some situations, the average phenotype does not provide an advantage and is selected against.

• Over several generations, a lower and a higher value are selected → curve displaying proportions of alleles is bimodal (has two peaks!)

• Example: Coho salmon. Males are large AND small. Large males gain access to females by fighting whereas small gain access by “sneaking.” Medium sized males can’t use either strategy so are selected against.