=Stabilizing selection= Within a population most variation can be fitted to a bell curve. For instance, in the image below, a group of people have been arranged based on height with the shortest on the left and the tallest on the right. You can clearly see that the majority of individuals are clustered somewhere in the middle and fewer individuals at each end of the range. [image:http://i.imgur.com/q54HGwV.png] [image:http://i.imgur.com/B5ZSgsP.jpg?1] Stabilising selection is probably the most common type of natural selection; it favours the most common phenotype as the best adapted. Stabilising selection reduces variation by selecting against alleles that produce more extreme phenotypes at either end of the phenotypic range. The resulting bell shaped curve is narrower. A good example is birth weight. Babies that are too light are often under-developed and therefore have a reduced chance of survival. Babies that are too heavy are usually larger and therefore they may be an increased risk of complications during birth. =Directional selection= [image:http://i.imgur.com/9MtATdN.jpg?1] This type of natural selection is most common during periods of '''environmental change'''. Directional selection favours alleles that produce phenotypes at one extreme of a phenotypic range. Selection reduces variation at one extreme of the range while favouring variants at the other end. The resulting bell shaped curve shifts in the direction of the selection. For instance a population of snails may exhibit some variation in shell colour. Directional selection might act against the lightest coloured individuals, reducing the frequency of alleles that code for lighter colours. =Disruptive selection= [image:http://i.imgur.com/gYKA0mZ.jpg?1] Disruptive selection favours alleles that code for phenotypes at both extremes of a phenotypic range. The bell shaped curve acquires two peaks. Disruptive selection may occur when environmental conditions are varied or when the population covers a large area. For instance, a population of snails might live in a region that has areas with white rocks and areas with black rocks. Disruptive selection would act against the individuals with intermediate colours (grey or beige individuals), reducing the frequency of alleles that code for these colours. Disruptive selection can result in two distinct groups and if they become adapted to a different way of life (niche) they could eventually evolve into separate species (speciations).