Directional Selection

For many characteristics, such as body mass or height, a population shows a range of values that forms a roughly normal distribution around a mean. Natural selection can change this distribution in different ways.Directional selection happens when individuals at one extreme of the range have a selective advantage. This usually occurs when the environment changes, so the previous mean is no longer the best value.The steps are:1. The environment changes, creating a new selection pressure.2. Individuals with the characteristic at one extreme are more likely to survive and reproduce.3. They pass on their alleles, so the frequency of alleles for that extreme increases.4. Over generations the whole distribution shifts in one direction, and the mean changes.Worked example. In a population of mammals, fur length is normally distributed with a mean of 20 mm. Over many years the climate becomes colder. Animals with longer fur lose less heat, so they are more likely to survive the winter and breed. Over many generations the mean fur length rises, for example to 26 mm, and the curve moves to the right.Other examples include the peppered moth (dark colour favoured in polluted areas) and the evolution of antibiotic resistance in bacteria, where the most resistant bacteria survive.Directional selection is different from stabilising selection, in which individuals close to the mean are favoured and the extremes are selected against. Stabilising selection keeps the mean the same and narrows the range, and usually happens when the environment is not changing.