MECHANISM : MUTATION

Mutation: Genetic recombination produces enormous number of variability for natural selection to choose from. But when one talks of millions of years of natural selection one should look for other mechanisms which would provide totally new factors and bring about fresh variability. Mutation provides totally new factors. In a broad sense a mutation is a hereditary change not due to the simple recombination of genes. Such genotypic changes include changes in chromosomal number, gross changes in the structure of chromosomes and changes in individual genes. Many mutations involve changes in single base pairs; this is called point mutation.

These mutations can be triggered by radiations and chemicals such as 5-Bromouracil, aminopurine, ethylmethane sulfonate etc. Mutations can take place in the reproductive or the non-reproductive cells of an organism. Those in the non-reproductive cells affect the host and are not passed on to its progenies and therefore are not important in evolution. Those mutations which occur in reproductive cells are passed on to progenies and are important to evolution.

Most mutations are lethal but some are beneficial. The earliest recorded mutation in domestic animals was that observed by Seth Wright in 1791 on his farm in Massachusetts. He noticed a peculiar male lamb with unusually short legs in his flock of sheep. It occurred to him that it would be advantageous to have a whole flock of these short-legged sheep which could not jump over the low stone fences. And so he used the new short-legged ram for breeding his fifteen ewes in the next season. Two of the fifteen lambs produced had short legs. Short-legged sheep were then bred together, and a line was developed in which the new trait was expressed in all individuals.

In this case the mutation was favourable to man and it is he who did the selection. Natural selection may or may not have favoured this mutation. Mutation alone cannot account for evolution; rather it furnishes the raw materials on which other forces act to bring about evolutionary change.

MECHANISM : INHERITANCE

Inheritance: A major weakness in Darwin’s theory of evolution was his lack of knowledge about the inheritance of variations. Gregor Mendel, a contemporary of Darwin, experimented with the common garden pea and worked out with beautiful simplicity and in detail the fundamental laws governing the transmission of character from parent to offspring, giving birth to genetics.

Mendel had fertilised plants which produced round seeds with plants which produced wrinkled seeds. From this cross, all of the progeny, known as the first filial or F1 generation, were like the round parent. He called dominant those traits that were expressed in the F1 (the round trait), and recessive those traits that did not appear in the F1 (the wrinkled trait). The F1 progeny were then self-fertilised to produce the F2 generation. In the F2, a ratio of three round plants to one wrinkled was obtained. The F2 wrinkled plants all produced wrinkled plants but of the F2 round plants one-third produced round and two-third behaved like the F1 giving three round to 1 wrinkle offspring.

From these results Mendel drew certain inferences. Since the wrinkle factor was present in one of the parents but not observed in F1, it was a factor which was present but not expressed in the F1 generation. F1 carries a factor for wrinkled as well as for round and therefore is a hybrid. Since the wrinkle factor appeared unchanged in the F2, the passage of the factor through the F1 hybrid did not affect its nature or purity. These results and conclusions led to the formation of Mendel’s first law: the principle of segregation. The segregation is more easily visualised by a checkerboard:

After Mendel had established the way in which two factors of a single trait were transmitted from generation to generation he took four factors of two traits and formed his second law: the principle of independent assortment. He said that the segregation of one trait occurs independently of any other trait. He crossed yellow, wrinkled seeds with green, round ones. He got F1 as yellow and round. Self-fertilising the F1 he saw that, in the F2 generation produced, the two traits assorted independently.

The major weakness in the theory of natural selection that Darwin had faced was the lack of understanding of variation and its mode of transmission from generation to the next. Here Mendel has solved beautifully Darwin’s little problem. We can see that the number of combinations possible with n factors can be shown equal to (n2+n)/2. Hence the variability due to multiple factors is by no means trivial and increases very rapidly as the number of factors increase.

With the total amount of factors in the thousands, natural selection has an enormous amount of combinations to choose from.

MECHANISM : NATURAL SELECTION

Natural selection: The primary factor controlling the course of evolution is natural selection. Darwinian concept of natural selection took into account a population more or less numerically stable with a reproductive rate far higher than necessary to ensure the maintenance of the population size. Because there is an enormous amount of variations in the population, deaths occur more frequently among the less adapted individuals and the better adapted types survive. This is natural selection or the survival of the fittest.

The modern concept of natural selection involves a subtle change in emphasis from differential survival to differential reproduction. From the standpoint of evolution it matters little whether an individual survives to the age of 2 or 102 if he dies without offspring (his genes are lost from the population). Traits which bring about differential reproduction are the traits favoured by natural selection. Some of these traits could be survival and longevity, fertility and fecundity, competition and cooperation, disease and parasite resistance, physiological tolerance, colour patterns, behaviour patterns and so on and so forth. The favourable traits will increase in frequency while the less favourable traits will decline in frequency each generation. The net effect is the production of organisms well adapted to survive in their particular environments.

Since many selective pressures operate, the organism must make some adjustments to all of them. For example in parts of Africa people die of sickle cell anaemia and many more, otherwise healthy, die after contracting malaria. It is observed that people who are carriers for sickle cell anaemia don’t get malaria. And thus they are affected by neither of the diseases and are better off than those who are absolutely free from sickle cell anaemia. This is a fine example of two selection pressures. By all these we then see that natural selection brings about adaptations, maybe to a changing environment or improvements to a fairly stable environment. Evolution may thus be thought of as progressive adaptations.