[PDF Notes] Short essay on Acquired and Inherited Traits

A trait (or characteristic) of an organism which is “not inherited’ but develops in response to the environment is called an acquired trait. For example, if a beetle does not get sufficient food for a considerable time, its weight will be reduced due to starvation.

The Tow weight’ of this beetle is an acquired trait of the beetle which has been acquired in response to the environment which contained insufficient food. Again, suppose the tail of a mouse gets cut. The ‘cut tail’ of this mouse is also an acquired trait which has been brought about by some agent in its environment.

A man may know how to swim or roller skate or speak French or may have a scar on the face from a cut he got in an accident. All these are acquired traits (or characteristics) which the man has picked up (or acquired) himself as he goes through life. The man is not born with these traits and he cannot pass on these traits to his children. The acquired traits of organisms cannot be passed on to their future generations. The reason for this is discussed below.

We have already studied that the traits (or characteristics) of parents are passed to their offsprings through genes in reproductive cells (or gametes) during the process of reproduction. So, for the trait of an organism to be passed on, it must have been caused by a change in the genes (or DNA) present in the reproductive cells of the organism.

In other words, only those traits can be transmitted to future generations in which changes have occurred in the genes (or DNA) present in the reproductive cells (or gametes) of parent organisms. The changes in the non-reproductive body cells of an organism cannot be inherited by its offsprings. This will become clear from the following examples.

When the weight of a beetle is reduced too much due to starvation, then though there is a change in the normal body cells of the beetle but no change takes place in the genes (or DNA) present in its reproductive cells (or gametes).

And since there is no change in the genes (or DNA) of gametes, this acquired trait (of low weight) of beetle cannot be inherited by its offsprings. So, if some generations of beetles are low in body weight because of the availability of less food, then this is not an example of evolution because this change cannot be inherited over generations. Whenever these beetles will get sufficient food, they will become healthy again and the trait of ‘low body weight’ will disappear.

Let us discuss the other example now. If we breed some mice, all the progeny of mice will have tails, just like their parents. Now, if we cut the tails of these first generation mice surgically and breed them, we will get new mice, all with full tails.

It has been observed that even after cutting the tails of mice for a number of generations, a tail-less mouse is never born. Actually, the cut tail of mice is an acquired trait which is never passed on to their progeny. This is because cutting the tails of mice does not change the genes of their reproductive cells (or gametes) cannot be passed on to its progeny, and hence cannot lead to evolution (because they are not caused by the change in genes).

A trait (or characteristic) of an organism which is caused by a change in its genes (or DNA) is called an inherited trait. Inherited traits can be passed on to the progeny of the organism because they have produced changes in the genes (or DNA) of the organism. Suppose there is a population of red beetles in the green bushes.

Again suppose that a colour variation arises during reproduction so that there is one beetle which is green in colour (instead of red). This change of green colour in the beetle has been brought about by a change in the genes (or DNA) of the reproductive cells.

The green colour of this beetle is an inherited trait which can be passed on to the next generations. The change from red beetle to green beetle can be considered to be an example of evolution because it helps in its survival by mixing with green bushes.

Inherited traits actually mean the characteristics which we receive from our parents. This point will become more clear from the following example. Suppose a father has red curly hair, brown eyes, a snub nose and a cleft chin. Again suppose that the mother has straight black hair, blue eyes, a long thin nose and a pointed chin.

The children in the family inherit some characteristics from each of their parents. For example, two children have red hair like father but one of them has straight red hair while the other one has curly red hair. The two children have black hair like the mother.

Again, two children have brown eyes like father but the other two have blue eyes like the mother. And finally, two children have snub nose and cleft chin like father whereas the other two have a long thin nose and a pointed chin.

Evolution :

There is an enormous ‘number’ and ‘types’ of living organisms (plants and animals) on this earth. In addition to this wide variety of living organisms, the remains of the dead organisms which lived in the remote past (called fossils) are also known. An important question now arises: How and from where has such a great variety of living organisms come to exist on this earth?

Also, how the human beings have evolved on this earth? All these things are studied in the branch of biology called ‘evolution’. The word ‘evolution’ has been derived from the Latin word ‘evolvere’ which means to ‘unroll’ or ‘unfold’.

Evolution is a kind of gradual unfolding (or formation) of the new organisms from the pre-existing primitive organisms through slow and steady changes. We can now define evolution as follows: Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced. Since the evolution is of the living organisms, so it is also called ‘organic evolution’.

It is through the constant process of evolution taking place in the organisms since the origin of life that such an enormous variety of plants and animals have come to exist on this earth at present. All the plants and animals (or organisms) which we see today around us have evolved from some or the other ancestors that lived on this earth long, long ago.

The process of evolution will become clear from the following example of ‘pterosaur’. Pterosaur is an ancient flying reptile which lived on the earth about 150 million years ago. The development of pterosaur is an example of evolution. It began life as a big lizard which could just crawl on land.

Over millions of years, small folds of skin developed between its feet which enabled it to glide from tree to tree. Over many, many generations, spread over millions of years, the folds of skin, and the bones and muscles supporting them grew to form wings which could make it fly. In this way, an animal which crawled on ground evolved into a flying animal. This evolution led to the formation of a new species (of a flying reptile).

Evidences for Evolution :

Various biological studies tell us that since their origin, living organisms have been undergoing changes in their organisation to evolve into new forms. A number of common features of different kinds of organisms provide evidence in favour of evolution because they can be considered to have evolved from the common ancestor.

The more characteristics (or features) two species have in common, the more closely they will be related. And the more closely they are related, the more recently they will have had a common ancestor.

We will now give some of the evidences which indicate the occurrence of evolution. These evidences reinforce the view that the living organisms have evolved from common ancestors. Some of the important sources which provide evidences for evolution are:

(i) Homologous organs,

(ii) Analogous organs, and

(iii) Fossils.

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