Natural selection is about survival and reproduction. Actually, natural selection is an issue of differential reproduction. This means that individuals bearing certain alleles produce more offspring than other individuals with different alleles.
Natural selection does not occur because genetic changes in individuals act on individuals but evolution occurs in populations. Evolution is a result of alternation of allele frequencies of a population, owing to differential reproduction among organisms bearing different alleles. Evolutionary changes are reported to be not good or progressive in any absolute sense. Natural selection is known to influence population in three major ways:
1. Directional selection favours individuals with values for a trait at one end of distribution range of a particular trait and selects against both average individuals and individuals at opposite extreme of distribution.
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2. Stabilizing selection favours individuals with an average value for a trait and selects against individuals with extreme values.
3. Disruptive selection favours individuals with relatively extreme values for a trait at expense of individuals with average values. Disruptive selection favours organisms at both ends of distribution of trait (Audesirk and Audesirk, 1999).
Natural selection is the result of interactions between living organisms and their environment. Sexual selection and altruism are two types of natural selection within a species. Interspecific interaction takes place in form of competition, predation and symbiosis.
Objections to Theory of Evolution by Natural Selection are:
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a. Darwin’s theory stresses upon small fluctuating variations, which are largely no heritable and play no part in evolution.
b. It does not explain the effect of use and disuse and presence of vestigial organs.
c. Darwin failed to differentiate between somatic and germinal variations and considered all variations as heritable.
d. It explains survival of the fittest but not arrival of the fittest.
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e. Natural selection does not account how an organ can be useful in initial stages.
f. Natural selection could not explain the evolution of terrestrial animals from aquatic forms.
g. Darwin proposed “artificial selections” for improving races of domestic plants and animals but these could never lead to permanent variation.
h. It could not tell whether instincts are acquired and modified through natural selection or not?