The patterns of selection are specific mechanisms which help to explain how certain genes shift and become favoured and prevalent in a population. There are three main patterns of selection which are as follows:
- Stabilizing Selection: This is the most common form of selection and is based on the idea that populations do not change much from one generation to the next. Average phenotypes are most common under this mechanism and extreme phenotypes become knocked out each generation. These populations tend to stabilize around particular traits and have lower genetic diversity. For example, in nature birds whose eggs hatch too early or too late have lower survival success than birds whose eggs hatch at the average time. Parents might not be as good at protection too early in the season and food resources may become less optimal later in the season.
- Directional Selection: In directional selection, it is not the average phenotypes which are favoured, but a single phenotype. A certain allele will become fixed in a population due to it being advantageous. Usually this results from environmental changes. Those with the optimal phenotype will have higher reproductive and survival success. For example, directional selection was evident with Darwin’s finches in the Galapagos Islands. The drought wiped out all of the soft seeds, leaving mainly large and hard seeds. Thus, only birds with deep and strong beaks were able to survive and this trait became favoured.
- Disruptive Selection: In this type of selection the average phenotype dies out over time and two or more extreme phenotypes become favoured. This is also thought of as a binomial distribution. For example, for a specific species of bird, females seek two qualities in males, plumage colouration and good territory. Thus, in this case intermediate phenotypes will lose out.
