Name and define the three types of natural selection.

Explanation

Three types of natural selection are :
(i) Stabilizing or Balancing Selection : It leads to the elimination of organisms having over specialized characters and maintains homogenous population which is genetically constant. It favours the average or normal phenotypes while eliminates the individual with extreme expression e.g., sickle cell anaemia in human beings. Also the phenotypic features of organisms coincide with optimal environmental conditions.
(ii) Directional or Progressive Selection: In this selection, the population changes towards one particular direction along with change in environment. As environment is undergoing continuous change, the organisms having acquired new characters survive, and others are eliminated e.g., Industrial mela nism.
(iii) Disruptive or Diversifying Selection : It is a type of natural selection which favours extreme expression of certain traits to increase variance in a population. It breaks a homo geneous population into many adaptive forms population is represented by both extreme phenotypes and results in balanced poly- morphism. e.g., Three types of snails in search simultaneously favours the individuals at both extremes of the distribution curve and eliminates the intermediate types.