The Basis of Progressive Evolution

By G. Ledyard Stebbins

For Professors: Free E-Exam Copies

• Language: English
• Publisher: The University of North Carolina Press
• Release date: Jul 1, 2014
• ISBN: 9781469622859

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1: LEVELS OF ORGANIZATION AND THEIR EVOLUTION

The revolution in biology that has swept through the world of science during the past twenty years is beginning to affect our concept of the nature of the world and of life to a far greater degree than most scientists realize. Furthermore, I believe that it is destined to have even more profound effects upon our thinking during the next twenty years.

In the minds of most biologists and biochemists, the biological revolution has centered about the discovery that nucleic acids, particularly DNA, are the chemical basis of heredity and that the specificity of proteins, the building blocks of the body, is based upon the translation of a genetic code that consists of specific sequences of nucleotide bases. This is, of course, the kernel and fountainhead of the revolution. Nevertheless, its effects are spreading far beyond these particular relationships. In the present chapter, the breadth and depth of these effects will be probed, while the remaining chapters will discuss some of them in greater detail.

Life Depends upon Organization

Perhaps the most significant of these broader effects of our understanding of the genetic code is confirmation and reinforcement of a belief previously held by most biologists. The essence of life is not a particular substance or substances, but a particular kind of organization. In textbooks, this belief has been expressed by likening an organism to a candle flame. Just as in a quiet atmosphere, the flame keeps its shape in spite of the fact that new particles of wax are being continually fed into it while the remains of burned particles are rising in smoke, so the living cell keeps its shape in spite of the constant replacement by synthesis and degradation of the molecules that compose it. Life as organization is a concept that is also supported by our understanding of death. When a man, an animal, or a cell dies, it does not necessarily lose any substance in the act. Death is a loss of function rather than substance. It is based upon changed relationships between organs, tissues, or cells, or between the molecules of which the cell consists. More will be said about this topic at the end of the present chapter.

The Principle of Relational Order

The statement that life is both orderly and complex is a truism that by itself tells us little. Recent discoveries concerning the molecular structure of living matter have, however, revealed two facts about the nature of this order which are of the utmost significance. In the first place, the macromolecules of which living matter chiefly consists, the proteins and nucleic acids, owe their specificity to particular linear arrangements of the smaller molecular units that make them up. The order of living macromolecules is not that of orbits within orbits, as is that of the solar system as well as of the atoms composing it; nor is it the order of geometrical crystals. Rather, it is like the order of a printed page or a taped message, the symbols of which are like the letters of the alphabet or the dots and dashes of a telegraphic code. Secondly, the ordered sequences of protein molecules, which are the working units of living systems, are perpetuated not by any properties that are inherent in their own structure but by a complex relationship existing between this order and that found in the nucleic acids that are the vehicles of heredity. The molecules of nucleic acid are like a designer’s blueprints for the construction of a machine tool, a work bench, or the parts of a building, written in a telegraphic shorthand that consists of four symbols. Given the environment of the living cell or of certain ones of its components, particularly the enzymes and coenzymes necessary for translation, the order of units (purine and pyrimidine bases) in a molecule of nucleic acid specifies precisely the order of units (amino acids) in a molecule of protein. The two kinds of molecules bear a colinear relationship to each other. This colinear relationship is the basis of both heredity and functional order in living systems; its alteration through changes in the base sequences of nucleic acids, which we now recognize as the chemical basis of genetic mutation, provides the hereditary variability necessary for evolution.

Other relationships between macromolecules, as well as between ordered combinations of them, also play vital functional roles in living systems. They form a hierarchy of order with at least eight levels: macromolecule, macromolecular aggregate, organelle, cell, tissue, organ, system of organs, individual. At each level, precise relationships between equally complex structures of different kinds are necessary for ordered function. We are all aware of such relationships at the level of organs such as nerves and muscles and of organ systems such as the respiratory system that regulates our breathing and the circulatory system that distributes oxygen through our bodies. Functional relationships at the levels of tissue, cell, organelle, and macromolecular aggregate will be described below.

Recognition of the hierarchy of ordered structures that makes up the living organism leads us to a basic principle about the nature of life. This can be designated as the principle of relational order and defined as follows. In living organisms, the ordered arrangement of the basic parts or units of any compound structure is related to similar orders in other comparable structures of the same rank in the hierarchy, permitting the structures to cooperate in performing one or more specific functions. The functions that depend upon relational order can be as diverse as muscular contraction, the transmission of nervous stimuli, the manufacture of sugar by photosynthesis, the synthesis of the protein molecules that form the building blocks of the living cell, and the transmission of heredity. Consequently, relational order is of primary importance as a unifying principle for all of life.

Relational Order and the Activity of Living Systems

A basic feature of relational order is that it is not a series of static relationships, like those between the molecules of a chemical crystal or between the beams and rafters of a house. The ordered relationships among parts of all levels of the organismal hierarchy help organisms both to carry out certain chemical reactions and to bring about co-ordinated movements of parts at all levels. At the molecular level, in fact, many of the chemical reactions of metabolism are inextricably bound up with, and dependent upon, the orderly movements of macromolecules and small organelles, such as the protein fibrils found in muscle cells (Figure 1–3).

The precise relational order between the macromolecules that make up cellular organelles is also in many instances associated with ordered movements. The regular beatings of cilia that propel the cells of protozoa such as Paramecium through the water are made possible by a complex organized system of protein filaments. The cilia themselves have an elaborate, stereotyped structure of nine peripheral protein filaments that are contractile and two central filaments that probably conduct stimuli. Co-ordination is achieved by a complex network of filaments that connect cilia with each other. Even more remarkable are the regular movements of chromosomes and associated structures during mitotic cell division. Mitosis is, in fact, an ideal illustration of how orderly movements of organelles that have been made visible to us depend upon equally orderly, systematic movements of individual molecules about which we still can only make inferences. This is true in spite of the fact that the molecular activities accompanying mitosis are still very imperfectly known. The bridge between visible movement and molecular behavior is being built by three kinds of research activities: detailed motion pictures of cells and chromosomes in mitosis both under normal and abnormal conditions; electron micrographs of dividing cells, in both mitosis and meiosis; and biochemical studies of the orderly succession of different cellular environments or states that is essential for the course of mitosis.

Motion pictures, particularly those of Andrew Bajer, have shown that the separation of daughter chromosomes from each other depends upon the contraction of specific, recognizable protein fibrils. Furthermore, this contraction is like that of muscle fibrils in that it involves changes in the shape but not in the volume of the molecules concerned. Electron micrographs have revealed the geometric structure of these fibrils and have shown that other protein fibrils, much like them, exist in the cell before mitosis begins (Figure 1–1). In fact, the orientation of these fibrils, known as microtubules, may in some instances be a predetermining factor that dictates that a cell will divide in a particular direction, long before the division itself begins.

Physiological and biochemical studies have shown that the orderly movements of chromosomes and other structures during mitosis are correlated with an orderly succession of different cellular environments. Oxygen consumption increases just before mitosis begins and drops while the process is going on. Soluble sulfhydryl (SH) groups show a similar rise and decline. Mitochondria, the energy supplying organelles or engines of the cell, execute ordered movements correlated with stages of the mitotic cycle. Regular changes in the viscosity of the living matter also accompany mitosis. While the meaning of all of these changes is not yet clear, we can hardly doubt that they have a direct connection with the visible movements of organelles that make cell division possible.

Muscular Activity and the Hierarchy of Order

The hierarchy of order in structure and function is well illustrated by the activity of muscle. Starting with the

Figure 1–3. The hierarchy of complexity in muscle tissue. A, a single muscle, similar to that shown in the hand (Fig. 1-2), magnification x2. B, a group of fibers, with nerve endings attached, x100. C, portion of a single fiber, x1000. D, portion of a single fibril, showing striations, x10,000. E, the indicated portion of D, x43,500. F, the same, showing relative positions of the thick and thin filaments, which are, respectively, groups of myosin and actin molecules. G, diagram showing the arrangement of individual myosin molecules in a thick filament and portions of the rope-like structure that consists of actin molecules wound together. The heads of the myosin molecules are shown as zigzag lines pointing toward the actin filaments. Magnification x130,500. If the entire muscle shown in A were magnified to the same scale as the molecules shown in G, it would be more than 5 miles long!

From H. E. Huxley in Scientific American 199(5), 1958, and 213(6), 1965. Reproduced by permission.

organism as a whole, we all recognize that the eyes, ears, and other sense organs are continually sending messages to the brain, where they are sorted out. Impulses then go to various muscles, which respond in ordered ways. At this level, we see the brain as the ultimate center of integration and order, while all of the bodily organs can be regarded as merely tools for carrying out its directives. We can go one step down in the hierarchy by focusing our attention upon a single organ, for instance the hand. If we dissect away the skin from a human hand, we see a labyrinth of muscles and tendons (Figure 1–2). Each muscle is supplied by a nerve or nerve branch that comes from the brain. In addition, the skin is well supplied with nerve endings, which give us a sense of touch. The coordinated action of nerves, brain, and muscles enables the hand to do anything from avoiding a flame to