Ans. Mechanism of Evolution-Natural Selection
Almost every population contains several variations for the characteristics of its members. In other words, there are morphological and physiological variations in all populations.
Natural Selection
Natural selection is the process by which the better genetic variations become more common in successive generations of a population:
Explanation .
The central concept of natural selection is the evolutionary fitness of an organism. Fitness means an organism's ability to survive and reproduce. Organisms produce more offspring than can survive and these offspring vary in fitness. These conditions produce struggle for-survival among the organisms of population. The organisms with favourable variations are able to reproduce and pass these variations to their next generations. Oh the other hand, the rate of the transmission of unfavourable variations to next generations is low. We can say that the favourable variations are "selected for" their transmission to next generations, while the unfavourable variations are "selected against" their transmission to next generations. ,»
Examples
1) We can see a mouse population with variations in skin colour. Cat preys upon light and medium coloured mouse. In first generation, light coloured mouse is preyed upon by cat. Only medium and dark coloured mouse can make their next generations. In next generation, population again contains light, medium and dark coloured mouse. Cat preys upon the light and medium coloured mouse. Now only the dark coloured mouse make new generation. If this happens in many generations, we will see only the dark coloured (favourable variation) mouse in the population.
As a result of natural selection, the allele that gives more fitness of characteristics (favourable variations) than other alleles becomes more common within population. So, the individuals with favourable variations become a major part of population while the individuals withn harmful or unfavourable variations become rarer
2)In England, the moths had two variations i.e. dark and white coloured moths. The moths used to rest on the light colourd tree trunks (on which white lichens had grown). In the 19th century when industries were established in England, the lichens on tree trunks died (due to polluted air) and the naked tree trunks turned dark. Now the white moth variation became harmful because a white moth resting on a dark tree trunk was easily visible to the predatory birds, the natural selection selected dark moths to reproduce. In this way dark coloured .moth became more common and at last the white moths disappeared from population. In this case, the dark colour variation in moth may be considered an adaptation to environment.