Three Types of Natural Selection
Introduction to three types of natural selection
Natural selection means differential reproduction, i.e. some members of a population reproduces at a higher rate and leave more surviving offsprings in the next generation than others. Those that produce more offsprings contribute proportionately greater percentage o genes to the gene pool pf the next generation than those that produce fewer offsprings.
There are three types of natural selection, directional natural selection, disruptive natural selection and stabilizing natural selection.
- Directional natural selection: In this type of natural selection, a previous normal trait of the individuals of a population is altered in a response to a changing environment as the number of better adapted individual increases.
Example: Industrial melanism illustrates directional selection. With in ht e last centaury in the industrial regions of England, a grey colours peppered moth Biston betularia has given rise to dark melenic spaces Biston carbonaria by change o colour through a dominant mutation in a single gene. The grey coloured species is well adapted to rest during the day one lichem covered trunks of trees. Here it is not seen by the predatory birds. In the industrial areas, the falling smoke particles killed the lichens and darken the background of tree trunks. Against a dark background, the grey coloured moths are conspicuous and are picked up by the predatory birds. For the survival of moths, nature has produced by mutation a dark coloured melenic species (B. Caroneria) from the grey coloured species (B.betularia). The former has a much better chance of survival under the new conditions created by pollution. Within a period of years in industrial or polluted areas, the dark species has almost replaced the light species. This is industrial melanism.
2. Disruptive natural selection: In This type of natural selection both the extreme expressions of a trait have a survival advantage.
Example: A population of marine snails inhabit brownish rock covered with white barnacles. White snails are invisible when among the barnacles, and black snails are concealed when on the bare rock. The snails which are not white or brownish or which rest on opposite coloured background are spotted and eaten by predatory shore birds.
3. Stabilising natural selection: In this type of natural selection the intermediate phenotype is more adaptive and has greater survival value than the two extreme phenotypes.
Example: Infants having a birth weight of 2.27 to 4.4 Kg are more likely to survive than the infants less than 2.27 Kg or over 4.54 Kg.
