"Variations that confer an advantage to an individual organism will survive in a population" - Justify 

In species, variations that offer survival advantages are naturally selected. Individuals adjust to their environments with the help of these selected variations and consequently these variations are passed on to their progeny. Evolution of organisms occurs as a result of this natural selection. 

However, there can be some other variations, which do not offer any survival advantage and arise only accidentally. Such variations in small populations can change the frequency of some genes even if they are not important for survival. 

Consider the following example:

• A small population of beetles live in a bushy area with green leaves. Crows eat these beetles. As a result, variation occurs in these beetles due to sexual reproduction.

Case I: Due to the occurrence of variation, the colour of one progeny beetle changes from red to green.

What is the advantage of this variation to green beetles?

The green beetle can hide itself in leaves to escape from being eaten by crows. Thus, the variation provides a survival advantage to the beetle.

Hence, the red beetles are rendered more vulnerable. Their chances of survival to reproduce are lesser in comparison to green beetles. This leads to an increase in the population of green beetles.

Variation in population by natural selection

This type of variation, which increases the survival value of an organism, is naturally selected.

Therefore (as seen above), the population of green beetles increases because the red ones are eaten by the crows. Thus, because of natural selection, the beetle population evolved from red to green colour to fit better in their environment.

Case II: Due to variation, the colour of one progeny beetle changes from red to blue.

• This blue coloured beetle is also able to reproduce and form a small population among the red beetles.

• These blue beetles are equally vulnerable to crows. They are as easily visible as the red beetles.

• Let us assume that one day an elephant tramples the bushes where the red beetles live. Most of the beetles are killed, but those that were able to survive are mostly blue beetles. Now, the beetle population is mostly blue.

In this case, the colour blue offered no survival advantage to the beetles. However, the major population of beetles now consists of only blue beetles (or genes governing this colour). This is because of the accidental survival of blue beetles. However, had the beetle population been large, the elephant could not have destroyed the entire population of the red beetles. Thus, this accidental change in the frequency of genes in small population is referred to as genetic drift.