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
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:
- 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.
- 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 .
- 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?