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.