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 is the process by which the better genetic variations become more common in successive generations of a population.
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 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. On the other hand, the rate of the transmission of unfavourable 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.
In the example mentioned next, we can see a mouse population with variation in skin colour. Cat preys upon light and medium coloured mouse. In first generation, light coloured mouse is preyed 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 makes generation. If this happens in many generations, we will see only the dark coloured mouse in the population.
As a result of natural selection, the allele that gives more fitness of characteristics than other alleles becomes more common within population. So, the individual with favourable variations becomes a major part of population while the individuals with harmful or unfavourable variations become rarer.
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