Evolution — BIOLOGY STD 12 Science — Question

Natural selection is the process by which comparatively better adapted individuals out of a heterogeneous population are favoured by the nature over the less adapted individuals. There are three different types of natural selections observed which are as follows :
1. Stabilizing selection or balancing selection : This type of selection favours average sized individuals which eliminates small sized individuals. It reduces variation and hence does not promote evolutionary changes however it maintains the mean value from generation to generation if graphical representation is made for the population it comes to be bell shaped.
Example : Infant Mortality : Birth weight in human infants is also an example of stabilizing selection. The optimal weight of new-born infants is 7.3 pounds. Those who weigh less than 5.5 pounds or more than 10 pounds have a higher mortality rate.
2. Directional Selection : This type of selection is related to changes in the environment. It motivates the genetic structure of the population in the direction by selecting the extreme organisms. This type of selection makes a large number of organisms located at a level other than normal or average extinct and on the other hand, it increases the number of living organisms. It selects such individuals which is most suitable according to the changed environment. In this way, it changes the mean value of the population in a certain direction, that is, it produces a change in the frequency of living organisms. It causes the local character and is followed by environmental changes.
Example : (i) Development of resistance in insect against DDT and (ii) industrial mechanism in Bistonbetularia.
3. Distractive Selection : This kind of selection is rare in nature but it is very important in bringing about evolutionary changes. This type of selection favours both small sized and large sized individuals.lt eliminates most of the members with mean expressions so produces two peaks in the distribution of the traits that may lead to the development of two different populations or sub populations, if there occurs no gene flow between these two sub population than each population may evolve as new species.
Example : There are two types of shells found in marine molluscs (Limpet)-white or brown. White colored molluscs live on white colored barnacles and brown coloredmolluscs live on brown colored rocks. Both survive because they are most suited to their respective environments and their numbers remain constant in the population.