Evolution
We have seen that there is an inbuilt tendency to variation during reproduction, both because of errors in DNA copying and as a result of sexual reproduction.
- A rare variation can become a common characteristic in a population when the frequency of an inherited trait changes over generations. Since genes control traits, we can say that the frequency of certain genes in a population changed over generations. This is the essence of the idea of evolution.
- A particular variation may become common if it gives a survival advantage to the population. This is known as Natural selection which can direct evolution in a population. It results in adaptations in the population to fit their environment better.
- Accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage. This is the notion of genetic drift, which provides diversity without any adaptations.
Example 1.
Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually?
Answer:
Sexual reproduction gives rise to more viable variations as compared to asexual reproduction. It is because there is an inbuilt tendency to variation during reproduction because of errors in DNA copying. In asexual reproduction, genes cannot be separated from the parent organism.
The resulting variation in traits out of sexual reproduction not only helps in the survival of species but also support diversity in long run. Drift in genetic traits gets accumulated spanning across generations, this gives rise to the formation of new species.
Example 2.
Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
Answer:
A small number of surviving tigers is the cause of worry from the point of view of genetics because the size of its population plays a dominant role for its evolutionary expansion. For a small population of tigers, diversity of traits will be adversely affected as gene pool will be less and hence there will be less variations. In the event of a disease or natural disaster possibility of survival against numbers will be greatly reduced. Small number of surviving tiger will affect diversity negatively and will result in an ecological imbalance.
Acquired and Inherited Traits
Acquired Trait is a phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weight lifter).
For evolution to take place, there must be changes in the DNA of the germ cell. Change in non-reproductive tissues cannot be passed on to the DNA of the germ cells. Therefore the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution.
Example: If we breed a group of mice, all their progeny will have tails, as expected. Now, if the tails of these mice are removed by surgery in each generation, then these tailless mice do not produce tailless progeny because removal of the tail cannot change the genes of the germ cells of the mice.
Differences between acquired traits and inherited traits:
| Acquired Traits | Inherited Traits |
| 1. These traits or characteristics are not transmitted from one generation to the next | 1. These traits are transmitted from parents to their progeny. |
| 2. These traits do not bring about any changes in the germ cells or DNA as these are changes in non-reproductive tissues. | 2. Genes of inherited traits are present in the germ cells or DNA |
| 3. These cannot direct evolution as these are experiences of an individual acquired during its lifetime. | 3. These may direct evolution as these bring about changes in the germ cells or DNA. |
| Example: Acquiring a new hair style | Example: Hair texture |
Origin of Life on Earth
J.B.S. Haldane suggested in 1929 that life must have developed from the simple inorganic molecules which were present on earth soon after it was formed. He speculated that the conditions on earth at that time, which were far from the conditions we see today, could have given rise to more complex organic molecules that were necessary for life. The first primitive organisms would arise from further chemical synthesis.
In 1953, Stanley L. Miller and Harold C. Urey assembled an atmosphere similar to that thought to exist on early earth (this had molecules like ammonia, methane, and hydrogen sulphide, but no oxygen) over water. This was maintained at a temperature just below 100°C and sparks were passed through the mixture of gases to simulate lightning. At the end of a week, 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules.