POPULATION
INTRODUCTION
Evolution is the sequence of gradual changes from simple life forms to complex life forms. Since, the evolution involves the living organisms. So we term it as organic evolution.
VARIATIONS IN A POPULATION AMONG GENETIC DRIFT
Here we can observe the changes in phenotype and genotype of the individuals in a population or between parents and their offspring. Let us imagine that there are 12 red beetles live in bushes with green leaves. By sexual reproduction, there is a growth of population. It helps to create generation. In a population of beetles, variation in genetic drift may occur through the following situations.
SITUATION 1
During reproduction, a color variation may arise, which may lead to the production of a green beetle. It passes the green color to its progeny.
As crows cannot see green colored beetles or green leaves. Therefore, crows do not eat them. Hence, crows continue to eat the red ones. In this way the number of green beetles increase in the population. In this case, variation gives survival advantage. The natural selection exerted by the crows will lead to the survival of green colored beetles.
SITUATION 2 GENETIC DRIFT
In another situation, a color variation arising during reproduction may result in a blue beetle instead of red beetle and all its progeny will be blue.
Crows can see both blue and red colored beetles. Therefore, they can eat both types of beetles. At the outset, there are a few blue and more of red beetles. All of a sudden, heavy weight of elephant on bushes causes killing of maximum beetles. By chance, the few beetles that survived were mostly blue. These blue colored insect will gradually increase in number. This change in frequency of some genes in a population which provide diversity without any survival disadvantage we term as genetic drift.
SITUATION 3
The bushes start suffering a plant disease and the amount of leaf material for the beetles has reduced due to expansion in population of beetles. As a result, the beetles are poorly nourishd and an average weight of adult beetle has decreased.
After a few years, the plant disease may get eliminated and there will be a lot of food for the beetles. Then, they will regain their normal weight. Here we cannot notice any genetic change. There may arise chance of phenotypic variation among the organism.
ACQUIRED AND INHERITED TRAITS INCLUDING GENETIC DRIFT
The characteristics developed during the lifetime of an individual that cannot be passed on to its progeny which we term as acquired traits or characters. The lifetime experiences of an individual cannot be passed on to its progeny and cannot direct evolution.
For example, if we breed a group of mice and remove the tails of these mice by surgery, the progeny would not be tailless because removal of the tail cannot change the genes of germ cells of the mice. So this procedure cannot direct evolution genetic drift. Thus it proves that changes in non-reproductive tissue cannot be passed on to the DNA of the germ cells in genetic drift.
Inherited traits are those characteristics which are received by off springs from their parents that means from one generation to another through specific genes like color of eye, and hair, shape of nose, earlobes etc.
SPECIATION IN POPULATION WITH GENETIC DRIFT
It is the development of one or more species from an existing speciation. It occurs when two populations are isolated in geographically and reproductively leading to almost no gene flow between the two population includes genetic drift.
However, inbreeding continues within the isolated population producing more and more generation. Eventually , the members of these two groups will be incapable of reproducing with each other even if they happen to meet. Over generations, genetic drift accumulates different changes in a sub-population. Natural selection may also operate variably in these different geographic location by genetic drift. For example, in the territory of one sub-population, crows may get eliminated by eagles. It is an example of genetic drift.
But this may not be the case in the other sub-population, whereas a result ,the crow numbers will become very high. As an outcome of this, the green variation will not be selected at the first site, while it will be strongly selected at the second.
Together, the processes of genetic drift and natural selection may make two isolated sub-populations more and more different from each other. New species will be formed by this process genetic drift.
These new species may have developed
- If the DNA changes are severe enough, such as a change in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other.
- If a new variation emerges in which females are only able to mate with few males due to their specific trait. This allows very strong natural selection for that trait by genetic drift.
HIGHER ORDER THINKING QUESTIONS ON POPULATION
Qs1: Traits are acquired during lifetime do not result in evolution, why? State two examples to justify your answer.
Ans1: A trait in an organism, which has been acquired during the lifetime we know as an acquired trait, like learning to swim, driving, cooking, skating, reading etc. the organisms are not born with such traits, but acquire them as they move through the life. Off springs cannot gain these traits from generation to generation. By sexual reproduction these traits transferred through gametes. So, inherited characters are passed through DNA or genes. So, acquired traits do not result in evolution.
Example 1: A son of a weight lifter does not necessarily have a muscular body as it is acquired trait.
Example 2: Cutting the tails of mouse is not inheritable. We can notice that next generation mouse contains complete tails.
Qs2: Distinguish between acquired and inherited characters with example.
Ans2:
| KEY POINT | ACQUIRED CHARACTER | INHERITED CHARACTER |
| DEVELOPMENT | These develop in the organisms during life time. | From the parents . off springs receive these characters. |
| CHANGES | No changes occur in the genes of organisms. | Changes in genes in organisms occur . |
| ULTIMATE FATE | With the death of individual, these characters are lost. | These are transferred to the next generation. |