Now we know how variation takes place:

  • Due to errors in copying DNA
  • Due to Sexual Reproduction

Let us see what happens when the variation gets accumulated:

  • Evolution is a long-term consequence of the accumulation of variations .
  • It is a naturally occurring slow process.
  • Evolution is a continuous and irreversible process of change .

Evolution can be easily understood by looking at how pre-existing organisms gradually changed over the years since the beginning of life , to become the organisms we see today.


Let us understand evolution by observing the following case study:

  • Imagine a green bush in which 12 red beetles live.
  • Crows feed on the beetles

Case 1:


Case 2:


Case 3:


Note - Evolution is a very slow process and happens over many generations

Analysing the case study

  • In case 1 and 2 ,
  • The frequency of the inherited trait or gene changed over the years.
  • This means, a rare variety of beetle introduced during reproduction, became a common characteristic in the population over generations .
  • This is due to evolution .
  • But in Case 3 , the situation is different

Let us take a closer look:

Case 1:

  • The variation became a common characteristic due to its survival advantage .
  • The crows could see red beetles better than green beetles against green bushes resulting in more proportion of green beetles.
  • This is known as Natural Selection - An adaptation to fit in the environment better.
  • Natural Selection as a concept was introduced by Charles Darwin.

Case 2:

  • In this case, the red and blue beetles were equally visible to the crows.
  • But when the elephant stepped on the population of beetles, beetles of one characteristic accidentally survived .
  • Thus this is not a survival advantage .
  • Accidents in small populations can change the frequency of some genes in a population.
  • This is known as Genetic Drift .

Case 3: 

  • Unlike the other 2 cases, here there is no genetic change occurring; there is no change in the reproductive cells, change is only in the non-reproductive cells.
  • Reduction in weight is an acquired trait .
  • Due to environmental stress (availability of less food), the weight of beetles reduced.
  • Once conditions became normal again, the initial weight was restored .
  • There is no characteristic evolution in this case.

Note - Case 3 is an example of acquired traits not being passed from one generation to another.

  • These small and significant changes in a species that get accumulated over generations and appear as a common characteristic is known as microevolution .
  • But what would happen if these beetles evolve in a way that they form 2 different populations that cannot reproduce with each other?

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo