Natural selection: The primary factor controlling the course of evolution is natural selection. Darwinian concept of natural selection took into account a population more or less numerically stable with a reproductive rate far higher than necessary to ensure the maintenance of the population size. Because there is an enormous amount of variations in the population, deaths occur more frequently among the less adapted individuals and the better adapted types survive. This is natural selection or the survival of the fittest.
The modern concept of natural selection involves a subtle change in emphasis from differential survival to differential reproduction. From the standpoint of evolution it matters little whether an individual survives to the age of 2 or 102 if he dies without offspring (his genes are lost from the population). Traits which bring about differential reproduction are the traits favoured by natural selection. Some of these traits could be survival and longevity, fertility and fecundity, competition and cooperation, disease and parasite resistance, physiological tolerance, colour patterns, behaviour patterns and so on and so forth. The favourable traits will increase in frequency while the less favourable traits will decline in frequency each generation. The net effect is the production of organisms well adapted to survive in their particular environments.
Since many selective pressures operate, the organism must make some adjustments to all of them. For example in parts of Africa people die of sickle cell anaemia and many more, otherwise healthy, die after contracting malaria. It is observed that people who are carriers for sickle cell anaemia don’t get malaria. And thus they are affected by neither of the diseases and are better off than those who are absolutely free from sickle cell anaemia. This is a fine example of two selection pressures. By all these we then see that natural selection brings about adaptations, maybe to a changing environment or improvements to a fairly stable environment. Evolution may thus be thought of as progressive adaptations.
Since many selective pressures operate, the organism must make some adjustments to all of them. For example in parts of Africa people die of sickle cell anaemia and many more, otherwise healthy, die after contracting malaria. It is observed that people who are carriers for sickle cell anaemia don’t get malaria. And thus they are affected by neither of the diseases and are better off than those who are absolutely free from sickle cell anaemia. This is a fine example of two selection pressures. By all these we then see that natural selection brings about adaptations, maybe to a changing environment or improvements to a fairly stable environment. Evolution may thus be thought of as progressive adaptations.
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