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. 

EVIDENCE : GENETICS

Genetic Evidence: A mule is a species hybrid, the offspring of a donkey and a horse. It is a cross between genetically dissimilar individuals. By its very existence it poses two questions. How can two clearly distinct species hybridize? Since they can form viable, vigorous offspring, why are these offspring sterile? The answers to the enigma of the mule are wrapped up in the theory of evolution. The hereditary material of the two species is quite evidently sufficiently similar for fertilisation to occur and for the normal development to proceed under the joint control of the genes from both species. The formation of normal gametes (sperm / eggs) requires however the pairing of similar or homologous chromosomes. Since the chromosomes of these two species differ both in number and composition, normal pairing or synapsis cannot take place. From that point onward normal gamete formation is disrupted. The interpretation is that these species trace back to a common ancestor in the not to distant past, and that their genetic materials are still sufficiently similar to permit normal fertilisation and development. However, during the course of evolution their chromosomes and genes have diverged to such a degree that they are no longer similar enough to form normal gametes.

The study of the chemical nature of the chromosomes from species ranging from viruses and bacteria to higher plants and animals has shown that they are DNA or deoxyribonucleic acid and RNA or ribonucleic acid. 

DNA

Chemically very similar, both have a backbone of a long chain of alternate sugar and phosphate molecules with the purine and the pyrimidine bases attached to the sugars as side groups. The differences lie in the sugar, deoxyribose in DNA and ribose in RNA, and in one of the four bases. Both have the purines, adenine and guanine, and the pyrimidine, cytosine, in common, but in DNA the other pyrimidine base is thymine, while in RNA it is uracil. In all but a few cases DNA is the hereditary material while the RNA ordinarily seems to mediate protein synthesis. The simple fact that the ultimate genetic material in nearly all species can be represented by a single type of compound, the DNA, points up to the fundamental similarity among all living things. The problem eventually will be to discover how DNA patterns have changed in the course of time to give rise to the great diversity of living species.

EVIDENCE : COMPARATIVE BIOCHEMISTRY

Comparative Biochemistry: Some biochemical traits are so fundamental that they are universally present in living things; others are widespread, characterising large groups of animals or plants; still other biochemical properties are species specific or may even be unique to a given individual. The term homology is usually associated with morphological characteristics, but biochemical homologies can be recognised. Common ancestry may be indicated just as clearly by homologous biochemical compounds. Since biochemical traits seem to change more gradually than morphological traits, the conclusions drawn from biochemical evidences are apt to be more soundly based. In some cases, biochemical evidence has made it possible to trace relationships where previously no reliable conclusions could be drawn from morphology.

Shaded boxes represent the regions with high degree of amino acid sequence homology, with their amino acid sequences given below.

Though different species may differ radically in their gross morphology, nearly all of them are formed from similar compounds, which are used metabolically in similar ways. In the digestion of carbohydrates in animals, the complex polysaccharides are hydrolysed and broken down into their constituent simple sugars or monosaccharides, of which the most important is glucose. The glucose molecules, after absorption from intestines, become the building blokes for the formation of the animal’s carbohydrates such as glycogen or, by stepwise oxidation, they become the major source of energy for the variety of purposes going on in the cell. Similarly proteins are broken down into amino acids, and fats into fatty acids and glycerol, which then, after absorption, enter into the metabolism of the animal. And thus many of the amino acids, fatty acids, and simple sugars are identical in both plants and animals. The metabolic pathways that they follow are also similar. For example the Krebs cycle, the cytochrome system, the metabolism of aromatic amino acids, glycolysis, the roles of adenosine triphosphate, and many other metabolic sequences have been identified in a wide variety of species.

The conclusion seems inescapable that the existence of these fundamental similarities must be regarded as evidence for an underlying kinship among all living things. Information about evolution can be derived from a consideration of various plant pigments. Chlorophyll is present in all photosynthetic organisms. Several types of chlorophylls have been identified and all have the same basic porphyrin or tetrapyrrole structure with magnesium attached to the ends of the pyrroles. Chlorophyll a occurs in almost all types of photosynthetic organisms, but the other kinds of chlorophyll have a more limited distribution:

The chlorophylls are bound to proteins in the chloroplasts and differ from each other only by the side chains attached to the outer ends of the tetrapyrrol nucleus. Descent with modification from a common ancestry seems clearly indicated for these photosynthetic species.

EVIDENCE : COMPARATIVE ANATOMY

Comparative Anatomy: The similarity between different species was one of the fundamental reasons for the development of the theory of evolution. In a sense comparative embryology and comparative anatomy are one and the same study, differing only with respect to the stages at which the organisms are studied. There are apparently two major reasons for similarities between species – heritage and habit. Heritage refers to a common ancestry, with similar genetic systems responsible for the resemblances. However, species with similar modes of life, with similar habits are often very much alike even though not closely related. Structures that are similar because of similar function or habits are said to be analogous and homology rests on a similar developmental origin and hereditary basis.

Characteristically there are seven cervical vertebrae in the mammalian neck. A mouse, an elephant and even a giraffe has the same number of cervical vertebrae. To the obvious question as to why animals differing so greatly in size, in structure, and in mode of life should have the same number of vertebrae in their necks, the theory of evolution presents a simple, plausible answer. All these varied forms, and the many other mammals, are descended, with modifications, from an ancestral mammalian stock that was characterized by seven cervical vertebrae. 

Morphological homologies are actually based on homologies in the hereditary materials, genetic homologies, of which they are the most obvious manifestations. The genetic homologies have been based not only in the similarities of phenotypes but on the locations of these genes in the homologous regions of the chromosomes. The existence of many organs diverse in function and yet clearly similar in structure – for example, the human hand, the seal’s flipper, and a bat’s wing – is a difficulty best explained by evolution. The list of morphological homologies can be almost endlessly extended, but the interpretation remains the same: descent with modification.

EVIDENCES : COMPARATIVE EMBRYOLOGY

Comparative Embryology: It is observed that in the development of an embryo, general traits appear before the more specialized, that the embryos of different species are more alike than the adults and depart progressively from each other during ontogeny.

Vertebrate embryos show many similarities, for which the most reasonable explanation is their common ancestry. The gill arches and the gill slits in the mammalian embryo are similar to those of a fish embryo at a comparable stage of development. They then differentiate into structures quite different from those in the fish. All the gill slits close and disappear except the one that forms the Eustachian tube, which connects the pharynx at the back of the mouth to the middle ear. The gill arches themselves have a variety of fates. The obvious question is why should there be a stage in the mammalian embryo where gills and gill arches, which never function as such, are nevertheless present, even though they differentiate into quite different adult structure. The obvious answer is that the mammals are descended from fishlike ancestors and that in the course of evolution modifications have occurred in its development.

The similarities which still persist in the ontogeny of fish and mammals are indicative of a fundamental similarity in their genotypes due to their common ancestry. The evidence indicates that evolution must operate within the framework and limitations imposed by existing patterns of development. Embryos still carry the clues to their common ancestry.