Heredity and Evolution
A recognisable feature of a human being (or any other organism) like height, complexion, shape of hair, colour of eyes, and shape of nose and chin, etc, are called ‘characters’ or ‘traits’. The transmission of characters (or traits) from the parents to their offsprings is called heredity. In most simple terms, heredity means continuity of features from one generation to the next.
Two parents, a male and a female, are involved in sexual reproduction. The sexually reproducing organisms produce sex cells or gametes. The male gamete called sperm fuses with a female gamete called ovum (or egg) to form a zygote which gradually develops into a young one (or offspring), showing some similarities with the parents. Actually, the hereditary information is present in the sex cells (or gametes) of the parents. Thus, gametes constitute the link between one generation and the next, and pass on the paternal (father’s) and maternal (mother’s) characters or traits to the offspring. This relation that continues to exist between successive generations is referred to as heredity.
Variations:
Although the offsprings inherit the characters (or traits) of the parents and resemble them very closely, but the resemblance is not complete in all respects. The offsprings are never a true copy of the parents.
In fact, no two individuals are exactly alike and the members of any one species differ from one another in some characters (or traits) or the other. These differences are known as variations. So, from the biological point of view, variation is the occurrence of differences among the individuals of a species.
For example, people have different heights. Their complexion, type of hair, colour of eyes, shape of nose and shape of chin also show differences. The differences in the characters (or traits) among the individuals of a species is called variation.
For example, human height is a trait which shows variation. This is because some people are very tall, some are less tall, some have medium height, some have short height whereas others are very short.
Here is another example of variations in human beings which involves our ears. The lowest part of our ear is called earlobe. In most of the people, the earlobe is ‘hanging’ and it is called free earlobe. In some people, however, the earlobe is closely attached to the side of the head and it is called attached earlobe. Thus, most people have free earlobes whereas some people have attached ear lobes. So, the free earlobes and attached earlobes are the two variations found in human population.
Some amount of variations is produced even during asexual reproduction but it is very small. The number of variations produced during sexual reproduction is, however, very large.
For example, the sugar cane plants reproduce by the process of asexual reproduction, so if we observe a field of sugar cane, we will find very little variations in various sugar cane plants. All the sugar cane plants almost look alike. But in animals (including human beings) which reproduce by the process of sexual reproduction, a large number of variations are produced.
It is due to these variations that no two human beings look alike (except identical twins). From this discussion we conclude that the number of successful variations is maximised by the process of sexual reproduction. The variation is a necessity for organic evolution.
