Class 10 heredity-evolution notes
In the lesson on reproduction, you read about DNA replication’s role in variations. You have read that variations occur even during asexual reproduction but the instances of variations are much more during sexual reproduction. Variations are usually inconspicuous when observed between two continuous generations. But the accumulation of variations over successive generations gives rise to the evolution of a new species over a period of time. In this lesson, you will learn about the inheritance of characters and variations. You will learn about the great contribution made by Mendel in the field of genetics. You will also learn about evolution and about the mechanism of evolution of new species. Moreover, you will learn about the correlation between evolution and classification of living beings. At the end of the chapter, there is an interesting debate about evolution vis-à-vis progress. The much interesting topic of human evolution has been explained in brief at the end of this lesson.
This figure shows a diagrammatic representation of three generations of an imaginary organism. Let us assume that this organism reproduces asexually. In spite of asexual reproduction, variations are bound to occur because of discrepancies during DNA replication. In the second generation, each individual shows some variations compared to the parent. Such variations can also be seen between two individuals of the second generation. However, these variations are small in number. Let us analyse what happens in the third generation. You will notice that each individual of the second generation produces its own progenies in the third generation. Progenies from different individuals would hence show a number of variations; compared to what happened in the second generation. These variations appear to be more pronounced because of accumulation of many variations.
Before proceeding further, let us try to answer an important question. Do all variations stand a chance to survive? The answer would be no. Let us take the example of two bacteria. One is having heat resistant property, while another does not. It is obvious that the bacterium with heat resistant property would survive the conditions of a heat wave while another bacterium would perish in that condition. This example shows that only useful variations are able to survive while useless variations perish in the process. In other words, organisms with useful variations are able to survive under changed environmental conditions. But organisms with useless variations are unable to survive under changed environmental conditions.
Do You Know?
Each zebra has a different pattern of stripes and each pattern is unique like fingerprints of humans.
Example:
If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier? (NCERT)
Solution:
The trait which exists in a lesser portion of the population is likely to have arisen later than the trait in a greater portion of the population. The Older trait has spread to more individuals.
Example:
How does the creation of variations in a species promote survival? (NCERT)
Solution:
Variations are usually produced in response to changes in external environment. Useful variations help an organism to survive in changed conditions. Thus, variations in a species promote survival.
Example:
The appearance of variations is too slow in most of the case. Why?
Solution:
Irregularities in DNA replication creates variations. But if variations in genotype are too much; then the resulting DNA will not be able to cope with the mechanism of the cell. It will result in cell death. On the other hand, gradual variations are too small to interfere with the mechanism of the cell. Hence, the appearance of variations is too slow in most of the case.