## TYPES OF SELECTION

Natural selection is that force which produces systematic heritable changes in a population from generation to generation, creating evolution. Thus it may become a directional phenomenon, producing changes in a definite direction, giving rise to new species. Natural selection may be very fast or very slow depending upon the environmental demands and rate of genetic changes. American naturalists K. Mather & J.M. Theoday (1953) divided natural selection into 3 categories, viz., stabilizing or normalising selection, directional or balancing selection and disruptive selection. 1. Stabilising or Normalising selection or Centripetal selection: Stabilising or Normalising selection or Centripetal selection: This type of natural selection operates in stable environmental conditions and in a short span of time, when species living in a particular environmental conditions are perfectly adapted to live in it. Thus individuals with extreme characters will be at a disadvantage as compared to the individuals having average characters and the latter would be favoured by natural selection. In is therefore a negative selection that weeds out continuously the less fit as well as more specialised genotypes from the population. Examples: H.C. Bumpus (1899) observed 133 sparrows killed in a storm and found that those birds that were killed possessed abnormally long or short wings, away from the average. Oppossum, an American marsupial, has changed very little over the past 60 million years. Similarly, in the case of Sphenodon, not much change has taken place during the past 150 million years due to almost stable environment in New Zealand. M.N.Karn & L.S. Penrose (1951) studied the survival rate of human babies in London hospitals and found that most of the babies that survived after birth had an average weight of 7.5 pounds. 2. Directional or Balancing selection or Orthogenesis: Directional or Balancing selection or Orthogenesis: This selection is always associated with environmental change, owing to which average characters become non-adaptive. Therefore, it favours individuals with non-average or extreme characters, which may prove useful in the changed environment. This is directional and progressive selection, which gives rise to new types from the original population, which have the ability to survive the change in the environment. By keeping the gene from disappearing in heterozygous condition, natural selection provides a species with reserves upon which it may draw when condition changes. Species that have lots of heterozygotes and plenty of variations are the fittest ones in changing environments. Examples: Evolution of horse is a good example of directional selection in which a small forest dwelling animal, Hyracotherium, had to undergo successive changes in its body when the environment changed from forest to grassland, giving rise to a tall, fast-running, grazing horse (see chapter ‘Evolution of Horse’. Evolution of black form of industrial melanic moth (Biston betularia) from grey 19thcentury ones in England in is also directional selection. Gradual replacement of susceptible mosquitoes by the DDT resistant strains due to excessive use of insecticide is an example of directional artificial selection. 3. Disruptive or diversifying selection: Disruptive or diversifying selection: This type of selection pushes some of the phenotypes away from the population average but at the same time maintains them in the population. Thus polymorphism appears and is maintained in the population. Uniformity of characters in young individuals and bimodal or trimodal diversity of characters in the adults is also considered to be due to disruptive selection. It enhances adaptability of a population. Megamutations may produce new types but they are not eliminated even when they are not advantageous.