Darwin ‘s theory of evolution can be described as follows

Charles Robert Darwin gave the theory of evolution in his famous book ‘The Origin of Species’. The theory of evolution proposed by Darwin is known as ‘The Theory of Natural Selection’. This theory is called the theory of natural selection because it suggests that the best adapted organisms are selected by nature to pass on their characteristics (or traits) to the next generation. Darwin’s theory of evolution applies to plants as well as animals.

Darwin ‘s theory of evolution can be described as follows:

1. Within any population, there is natural variation. Some individuals have more favourable variations than others.

2. Even though all species produce a large number of offsprings, populations remain fairly constant naturally.

3. This is due to the struggle between members of the same species and different species for food, space and mate.

4. The struggle for survival within populations eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called natural selection (or survival of the fittest).

5. The individuals having favourable variations pass on these variations to their progeny from generation to generation.

6. These variations when accumulated over a long period of time, who gave the theory of evolution, lead to the origin of a new species.

We will now understand Darwin’s theory of evolution by ‘natural selection’ by taking an example. No two animals are ever exactly alike. So some changes always appear when animals produce their progeny by sexual reproduction.

For example, one of the progeny may be tall (having long legs) than the other progeny. Thus, there may be a variation of height in the progeny. Now, the advantage of long legs to the progeny is that when no food (grass, etc.) is available on the ground, then this progeny having long legs can reach the leaves on tall trees, eat them as food and survive.

On the other hand, the progeny which have short height (due to short legs) cannot reach the leaves on tall trees, they will not get any food, they will starve and hence die. Thus, nature has selected the animal with long legs to survive (because it is the fittest animal under these circumstances).

Now, since long legs help in survival, the long-legged animals will live long enough to produce their offsprings. The offspring will inherit long legs. So, all the future generations will have long-legged animals. In this way, the animals having short legs have evolved into animals having long legs due to variation. This is an example of evolution.

We can now define natural selection as follows: Natural selection is the process of evolution of a species whereby characteristics which help individual organisms to survive and reproduce are passed on to their offspring, and those characteristics which do not help are not passed on.

Though Darwin’s theory was widely accepted, but it was criticised on the ground that it could not explain ‘how the variations (which lead to origin of new species) arise’. With the progress in genetics, the source of variations was explained to be the ‘genes’.

Genes vary in natural population. Genetic variation is the raw material of evolution. So, the Darwin’s theory was modified accordingly. These days, the most accepted theory of evolution is the Synthetic Theory of Evolution in which the origin of species is based on the interaction of ‘genetic variation’ and ‘natural selection’.

Sometimes, a species (a type of animal or plant) may completely die out. It may become extinct. Dodo was a large flightless bird which has become extinct (see Figure 36). Once a species is extinct, its genes are lost for ever.

It cannot re-emerge at all. The small numbers of surviving tigers are a cause of worry from the point of view of genetics because if they all die out and become extinct, their genes will be lost for ever. Our coming generations will not be able to see tigers at all. We should, therefore, make all out efforts to protect tigers (and other endangered species) to prevent them from extinction.

Speciation :

A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring. Species can be of plants or of animals. Wheat, paddy, sunflower, lotus, mango, neem, humans, tiger, dog and cat, etc., are all examples of various types of species.

The human beings who look so different from each other in terms of size, colour and looks are said to belong to the same species (Homo sapiens) because they can interbreed to produce fertile offsprings (sons and daughters). The process by which new species develop from the existing species is known as speciation. In simple words, the formation of new species is called speciation. We will now explain how new species are formed from the existing species of various populations.

In most of the cases, new species are formed when the population of same species splits into two separate groups which then get isolated from each other geographically by the barriers such as mountain ranges, rivers or the sea.

The geographical isolation of the two groups of population leads to their reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated populations producing more and more generations.

Over the generations, the processes of genetic drift (random change in gene frequency), and natural selection operate in different ways in the two isolated groups of population and make them more and more different from each other. After thousands of years, the individuals of these isolated groups of population become so different that they will be incapable of reproducing with each other even if they happen to meet again. We say that two new species have been formed.

From the above discussion we conclude that the important factors which could lead to the rise (or formation) of a new species are the following:

(i) Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population.

(ii) Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.

(iii) Variations caused in individuals due to natural selection.

It should be noted that geographical isolation is the major factor in the speciation of sexually reproducing animals because it interrupts the flow of genes between their isolated populations through the gametes.

The geographical isolation, however, cannot be a major factor in the speciation of a self- pollinating plant species because it does not have to look to other plants for its process of reproduction to be carried out. Geographical isolation also cannot be a major factor in the speciation of an asexually reproducing organism because it does not require any other organism to carry out reproduction.

Evolution of Eyes :

The eye is a very important organ for animals. The eye is a complicated organ which cannot be generated by a single DNA change. The complex body organs of animals such as eyes have been created in ‘stages’ over many generations. First of all the rudimentary eye (basic eye) like that of a flatworm (Planaria) was formed.

The eyes of flatworm are very simple that are actually just ‘eye-spots’ which can detect light. Even these rudimentary eyes provide a survival advantage to flatworm. Starting from this basic design, more and more complex eyes were then evolved in various organisms.

Most of the animals have eyes. For example, the insects, octopus and invertebrates, all have eyes. The structure of eyes in each of these organisms is, however, different. A flatworm (Planaria) has very which suggests their separate evolutionary origins. The evolution of simple eyes called rudimentary eyes, eye is an example of evolution by stages.