Evolution of Organisms: Genetic Variation, Frequency of Genes in a Population and Acquired and Inherited Traits!
Evolution refers to the process by which early organisms of the earth diversified into various new forms through slow but continuous variations.
Ever since the appearance of the first living beings on the earth some 3.5 billion years ago, new forms have continuously originated.
And, the different forms have undergone modifications and given rise to new forms.
The newer forms are sufficiently different to be recognized as new species. They breed amongst their own members and not with the ancestral forms or any other forms. The newly formed species may give rise to still newer species over a period of time.
This process is called descent with modification. This is the main theme of evolution. Evolution occurs due to the survival of advantageous variations produced in reproduction.
Sources of Genetic Variation:
You know that there are two main types of variations—somatic and germinal (genetic). Genetic variation arises due to mutation and it can account for the creation of a new species. Mutation is any change in the structure of a gene. Mutation may lead to a change in the expression of a gene. Such a change may even produce harmful effects in the organism.
Another source of variation is genetic recombination. It is a natural process due to which the arrangement of genes in the progeny is in a combination that differs from that of the parents. This is because the offspring receive genes from both the parents, and this ensures the transmission of some genetic variability from the parents to the offspring.
Mutation and genetic recombination may give rise to new characters due to change in genes. These new characters may help the individuals to adapt to their environment. Sometimes the new characters may not help individuals to adapt. Disease, competition, etc., can eliminate those less well-adapted individuals.
The survivors pass on their advantageous characters to their offspring. This enables the offspring to adapt well to their environment. Thus nature selects new characters by favouring some of them and eliminating others. In this way natural selection may lead to the evolution of a new species with new characters. Let us see how variations in a population lead to evolution.
Frequency of Genes in a Population:
The proportion of a particular allele in a population is called gene frequency. How does gene frequency in a population change? Let us consider the genes of a particular species. All the genes in a population of a species at a given time form its gene pool. The frequency of certain genes in the population of an area can change due to certain environmental factors.
Let us take an example and observe the results in different situations (see Figure 7.6). Suppose there is a population of red beetles living in some bushes in a particular area of a forest. Let us assume that they can generate heritable variations during their sexual reproduction.
In the first situation, a heritable colour variation occurs so that a green offspring is born to its red-coloured parents. The green beetle then passes its green trait on to its offspring.
These green beetles living in the green leaves of the bushes escape the notice of crows, while the red beetles, because of their bright colour, are easily spotted and eaten by the crows. As a result, the red beetles are soon eaten up by the crows, while the green beetles survive, reproduce and increase in number.
In the second situation, a blue-colour variation arises during the reproduction in red beetles. The blue beetle also gives birth to and more blue beetles. This change in colour, however, gives no survival advantage over the red variety since the crows easily find and eat both blue and red beetles.
Now, suppose a bush fire occurs suddenly and kills a large number of beetles, and all the surviving beetles are, by chance, blue. The progeny of the blue beetles are also blue.
The survival of these blue beetles is, however, not a case of natural selection, unlike the survival of the Heredity and Evolution green beetles in the first situation. This is a case of genetic drift, that is, a random change in the gene frequency.
In the third situation, many of the bushes dry up due to a prolonged dry period. The bushes that’ survive the drought have smaller leaves. This leads to a food shortage for the red beetles. Incidentally, some beetles in the population are smaller in size on account of a heritable variation.
They manage to survive, as they require less food, while most of the large beetles die of starvation. Some young beetles of the large variety survive, but these cannot grow to their full size due to undernourishment.
Hence, there are only small beetles. When the drought ends and there is enough food for all the beetles, large beetles reappear, and there are both large and small beetles in the population.
The genetically small beetles remain small even when they have more food, but the undernourished beetles, which did not undergo any genetic change, grow to their normal size.
In the above situations, we can examine where genetic drift has occurred. In the first situation, natural selection played a part in preserving a certain heritable variation. In the second situation, an accident caused genetic drift. In the third situation, an environmental factor led to the production of smaller beetles although some were also produced due to heritable variation.
Acquired and Inherited Traits:
From very early times scientists have been trying to explain the origin, evolution and diversity of life forms. In the nineteenth century, however, the idea that complex animals and plants developed by gradual change from simpler forms began to be taken seriously.
The mechanism of the origin of new species from the existing species was explained first by Jean Baptiste Pierre Antoine de Monet Lamarck (1744-1829), a French biologist, and then by Charles Robert Darwin (1809-82), a British naturalist.
Having accepted the fact that new species have arisen from pre-existing species with modifications, a number of scientists have tried to explain the mechanism by which this might have occurred. The first scientific theory concerning this came from Lamarck. His ideas, written in his book Philosophy Zoologies (meaning ‘philosophical zoology’), published in 1809, and are known as Lamarckism.
Lamarck observed the changes and adaptations in certain organs in animals. He suggested that favourable changes appear due to the use or disuse of organs over a long period of time.
For example, some organs develop in size if they are in continuous use, while their disuse has an opposite effect. He concluded that such characters acquired by an organism during its lifetime are transmitted to the next generation. This inheritance of acquired characters results in the evolution of one or more new species.
However, most scientists disagree with this, as it has not been supported by experiments. For example, the offspring of a couple of mice whose tails have been cut off are not born tailless. This was demonstrated by an experiment performed by August Friedrich Leopold Weismann (1834-1914), a, German biologist.
In sexually reproducing organisms, germ cells are produced in the reproductive organs, while the rest of the body has somatic cells. Changes in somatic cells due to environmental factors are not transmitted to the offspring. This is because a change in a somatic organ caused by a physiological response by the body does not bring about a corresponding change in reproductive organs.
For example, in the earlier illustration, if beetles starve, their size will get reduced. But if they reproduce, their progeny may not have reduced body size if they get enough food. This means that starvation of the parent beetles does not alter the DNA sequence of their germ cells so as to bring about a variation in the next generation.
Even if the reproductive cells suffer from starvation, this does not lead to any change in the DNA. The son of a wrestler is, therefore, not born muscular. Similarly, cutting off the tails of mice do not change the genes of their germ cells.
Darwinism is the first modem theory that attempted to explain the origin of new species. Charles Darwin made an extensive study of the flora and fauna of the Galapagos Islands in South America. He came to certain conclusions, which he explained in his book The Origin of Species, published in 1859.
Darwin proposed that new species arise by the slow accumulation of advantageous variations over a period of time. Though he did not say how these variations arise, he said that variations are so common in nature that no two individuals are alike.
Darwin’s second observation was that although the power of reproduction of organisms is enormous, the population size of any species always remains within a limit. He explained it by saying that overpopulation results in a competition for food and shelter, ultimately leading to a struggle for existence among the members of a species.
In such a struggle, those that survive must have some favourable qualities that enable them to overcome the difficult situation. These qualities are advantageous variations. The surviving organisms repeat the process of reproduction. Biologically, a species that can reproduce and leave a large number of offspring is considered successful. When the new generation with advantageous characters begins to reproduce, the situation of overproduction and inevitable struggle is repeated.
The survivors will have more advantageous characters that help them to compete and survive. All these new features might make them considerably different from the original forms. These differences, or variations, when accumulated over a long period of time lead to the origin of new species. Thus, selection of advantageous variations by nature leads to the origin of new species.
Charles Darwin explained the mechanism of origin of new species by natural selection. But his theory fell short of explaining the mechanism or the source of heritable variations. This was explained by Hugo de Vries (1848-1935), a Dutch botanist. According to him, heritable variations arise when there is a change in the genes of the germplasm (protoplasm of a germ cell). He called it mutation.
The manner in which heritable variations are passed on to the succeeding generations was explained by Gregor Mendel after he performed his pea-plant experiments.
If a particular trait spreads in the population, it means that it is favoured by natural selection. On the other hand, an acquired trait is not transmitted to the offspring. Those animals that do not show enough variations are likely to be wiped out, as they cannot cope with changing circumstances. Genetic variability gives an ability to adapt and adds to the chances of survival of a species.
A small population of any species would have fewer mutations, resulting in lesser variability and diminished ability to adapt. For example, the small numbers of tigers surviving in the world do not have enough variations to adapt well to changes in the environment and hence may become extinct.