Question

What fact must exist about a population of a species for the theory of natural selection to be able to explain evolution occurring in that population?

Answer 1

Evolution refers to the change in the inherited traits in a population through various generation. The mutation or variation in the genes or allele are able to produce a new phenotypic trait. These altered traits are rare at first but once they are fixed i.e becomes more common through no of generations then the species is said to be evoluted. These altered traits can be herited in a population either non randomly through natural selection or randomly through genetic drift. So natural selection is the process of inheritance of only those traits which are required for the species to survive or be the fittest. Therefore, natural selection is also known as survival of the fittest. The mechanism of natural selection relieson five steps : variation, inheritance, selection, time and adaptation. As natural selection is distinct from genetic drift in a fact that it is non random which means that selection is the most important criteria in the natural selection procedure for evolution to occur. There are various ways for the selection of the helpful traits and removal of harmful altered traits:

1. Stabilizing selection: in this type of selection a decrease in the genetic variation occurs as the natural selection favours the average or normal trait against the altered or new one to stabilize the population.

2. Directional selection: when the environmental conditions change then through natural selection the population chooses a single phenotypic trait which causes the allelic frequency to move in one direction.

3. Diversifying selection: in this type of selection the new altered trait which are extreme are selected over the normal ones.

4. Frequency dependent selection : in this type of mechanism the sitness or selection of a trait depends on its allele frequency relative to the other phenotypes i.e the common trait can also be selected if there frequency is greater ( positive frequency selection) or the extreme or new altered traits can be selected as well( negative frequency dependent selection)

5. Sexual selection: as we know that natural selection relies on non random selection i.e it only selects a particular trait which is useful for the population. For this to occur in sexual selection the mating also occurs non randomly i.e the members of a population choose who they mate with which is always for the useful trait.  

So for natural selection to occur the population must select or mate non randomly for evolution to occur only after genetic variation occurs.