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Theory:

As discussed in the topic of reproduction in organisms, both because of faults in DNA copying and sexual reproduction, there is an innate tendency for variation during reproduction.
 
Hence, from this, reproduction can be defined as follows:
It is the series of gradual, irreversible changes in primitive creatures over millions of years to form new species today.
Variations that led to new species' emergence were mainly caused by DNA copying and sexual reproduction faults.
 
In the sexual reproduction of animals, including human beings, the number of successful distinct variations among individuals is high. Even in asexual reproduction, there are some amount of variations.
 
The long-term consequences of the accumulation of variations occur due to the transfer of traits for many generations. This accumulation of variations is the basis for the occurrence of evolution. The following illustration can explain variations in a population.
 
A group of twelve red beetles live in green bushes and reproduce sexually, so they are likely to develop variations. Consider that crows eat these beetles. Crows consume many beetles; therefore, there aren't many left to reproduce. Let us now consider a variety of scenarios that could arise in this beetle population.
 
ariations-in-a-population-Fig-97-Ch-9-Class-X-Science-NCERT-2009.png
 
First scenario:
  
In this scenario, due to the colour variation that arises during the reproduction, one green beetle is produced instead of red ones. This green beetle passes down its colour to its progeny so that all its entire progeny is green in colour. The crows cannot see these green-coloured beetles on the green leaves of the bushes, and therefore cannot eat them. Hence crows eat the red beetles because red ones are easy to find on green plants.
As crows cannot see the green beetles, their number continues to grow while the population of red beetles decreases. This form of diversity provides an advantage in terms of survival.
 
Second scenario:
  
In this scenario, again, one blue beetle is produced due to a colour variation during reproduction. This blue beetle may also pass down its colour to its progeny so that its entire progeny is blue in colour. Crows can see blue-coloured beetles on the green leaves of the bushes as well as the red ones, and therefore can eat them. During this, an elephant kills red beetles by stamping on a bush, resulting in the survival of blue beetles. The beetle population gradually grows again, but the majority of the beetles are now blue.
From both these scenarios, we can conclude that the rare initial variations finally became a common characteristic in the population. To put it in other terms, the frequency of an inherited trait changes over generations. The frequency of specific genes in a population changed across generations, as previously described genes. This change in frequency is the essence of evolution.
Along with the similarities between the two scenarios, there are differences too.
From the first scenario, we can say that the variation among traits became a common factor as it served as an advantage for survival. In other terms, it was naturally selected, as we can see that the crows selected the beetles naturally. An increase in the number of crows would decrease the number of red beetles and an increase in the number of blue beetles. Hence, we can conclude that natural selection is a factor that triggers evolution in the beetle population, thereby resulting in adaptations in the beetle population to fit their environment better.
 
In the second scenario, variation of colour played no role in the survival of beetles. Instead, the change in the characteristics of the population was due to the havoc created by the elephant. This disturbance caused by the elephant was a major one because the large beetle population was reduced.
 
We can conclude that accidents can alter the frequency of genes even without survival advantage. This is known as genetic drift, which leads to variations.
Third scenario:
 
In this case, the beetle population begins to expand, but the bush is affected by a plant disease due to which its leaf material on which the beetles feed is reduced. This leads to the poor nourishment of beetles. The average weight of the beetles decreases without any genetic change.
 
After a few years, as the plant disease is eliminated and enough food is available for the beetles, they return to their standard size. This type of change is not inherited.