Evolution

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?