What are the key different types of selection?

Directional selection occurs when a phenotype at the extreme end of a distribution is favoured and so over time the distribution shifts in the direction of the favoured phenotype. An example of this is the development antibiotic resistance in bacteria; the phenotype that has resistance survives the antibiotic and so the allele frequency of resistance increases, the distribution shifting towards a more resistant population.Stabilising selection occurs when a non-extreme phenotype is favoured, extremes at either end of a distribution being disadvantageous. An example of this would be fur thickness for a particular environment; if the fur is too thick the animal will get too hot, if the fur is too thin then the animal will get to hot. Selection favours the animals with fur close to a middling optimal thickness.Disruptive selection occurs when extreme phenotypes at both ends of a distribution are favoured whilst middling phenotypes are disadvantageous. An example of this would be the beaks of Galapagos finches; big beaks are useful to crack open large seeds, small beaks could easily pick up small seeds. Medium sized beaks have trouble with both large and small seeds and are therefore maladaptive. Disruptive selection is important as it can lead to sympatric speciation as the two extreme phenotypes become more and more different.