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FRONTIERS OF BIOLOGY that the genes now responsible for their synthesis derive from a common ancestral gene that, by a process of duplication (presumably independent of normal mitotic division), doubled and redoubled at some time in the past, with the progeny genes free to mutate In different directions. Thus, the answer to our question is that the increased DNA found in cells of higher organisms resulted from duplication of the old DNA, followed by an independent evolutionary pattern occurring as a result of mutation and selection. Population genetics has also demonstrated that evolution need not be observed only over geologic time. It can happen, and perhaps it is happen- ing, in current human populations. The rates of change that could arise from the crossing of human races, from increasingly high correlation in intelligence of husband and wife, from changes in the mutation rate as a result of radioactivity in the atmosphere, from a possible law requiring sterilization of persons with mental retardation, can all be predicted with reasonable accuracy on the basis of current understanding of the way in which natural selection acts on genetic variation. It also enables prediction of some of the ecological consequences of the introduction of insecticides, pesticides, or other new agricultural practices. Clearly the most striking practical achievements based on genetic under- standing have been in agriculture, particularly the creation of fertile hybrids between divergent plants, e.g., the radish-cabbage hybrid or the wheat-rye hybrid mentioned above, resistant strains of almost all primary food crops and, of course, the development of hybrid corn. The well-known success of that endeavor has led to intensive efforts to generate equally successful hybrid wheat and rice; these studies are not yet complete. In medicine, the most important practical result has been the deeper insight into disease- producing mechanisms related to genetic variability. These will be dis- cussed in greater detail in Chapter 2. THE DIVERSITY OF LIFE Systematic biology was begun by amateur students of natural history who catalogued the animal and plant life in the world about them. During the eighteenth century, Linnaeus was able to treat both animal and plant king- doms in their entirety, but he was the last person to do so. In 1818, Lamarck published an account of all invertebrate animals, and again he was the last person able to do so. By now, two centuries after Linnaeus, systema- tists have discovered, described, and taxonomically classified about 1.5 million different species of organisms; yet each year more than 10,000 133
THE LIFE SCIENCES previously unknown species are discovered, described, and catalogued. Withal, this may represent no more than one third of actually existing species, and it is clear that at least 99 percent of all species that have appeared on the face of the Earth are now totally extinct. One of the tasks of the systematic biologist, in addition to the labor of love entailed in finding, observing, describing, and cataloguing new species, is to determine how speciation actually comes about, what the essential attribute of a species really is, how current species relate to those of the past, what mechanisms account for extinction, and how this multitude of species cohabits our planet. This is by no means a purely academic task. It has been just such knowledge that has made possible many of the breed- ing programs of modern agriculture, that underlies all attempts at bio- logical control of pest species, and that must suggest future attempts at large-scale cultivation of species not currently utilized as foodstuffs for man or for the production of new drugs, fibers, fermentations, etc. Previous successes in such activities are treated in greater detail in Chapter 2. What Is a Species? With increasingly sophisticated biological understanding, with introduction of chemical techniques for examination of proteins, polysaccharides, and lipids, the description of a given set of organisms tentatively considered a "species" has become increasingly detailed and has permitted discrimina- tions impossible in an earlier era. Currently, a species of animal or plant is characterized by two major properties: a gene pool adapted to occupy a particular niche in nature and protection mechanisms that prevent mixing with other gene pools. The gene pool cannot yet be described by direct examination of the genes themselves; it is described rather by phenotypic expression in the structure, physiological function, biochemical composition, breeding and mating patterns, choice of food, annual life cycle, etc. This mode of analysis is a sophisticated extension of the endeavor, once totally sum- marized by Linnaeus. The processes by which cross-breeding with other species is prevented are called "isolating mechanisms." In a general way the protection mechanisms operate to prevent the mixing of two incom- patible genetic systems that could lead to the production of disharmonious and selectively inferior hybrids. Classically, this was exemplified by failure of the sperm of one species to fertilize the egg of another. And for dis- tantly related organisms this is indeed the case. But distantly related orga- nisms have little tendency to attempt such breeding in the first instance. The more important mechanisms are those that prevent cross-breeding between gene pools where, indeed, the biological event might be possible. Among
FRONTIERS OF BIOLOGY animals, behavioral barriers that prevent mating are far more important than genetic incompatibility. Two main classes of premating isolating mechanisms are commonly employed. Signals by which individuals attract and identify other individuals as being reproductively ready members of the opposite sex are most effective because they are misunderstood only rarely by members of the same species and are likely to be ignored by members of other species. In birds, frogs, and insects, vocalizations are important signals of this type. In other kinds of animals such as mammals, salamanders, many insects, and invertebrate species, specific secretions (scents, sex-attractant chemicals, etc.) serve as primary means of com- munication among potential mates. In still other animals, such as insects, fishes, reptiles, and birds, visual signals involving color patterns and ritualized displays and postures serve as premating isolating mechanisms. Another category of such mechanisms is separation of the reproductive activities of sympatric (cohabiting) species by space or time. They may reproduce at different times e.g., one species of elm that flowers in the spring and a different species in the same area that Dowers in the late sum- mer or they may select slightly different breeding sites, as do related species of toads, one of which breeds in running water and the other in rainpools of the same small valley. Isolating mechanisms of this category are widespread in plants as well as animals. Origins of Species From examination of both living and fossil forms, it is clear that geo- graphical speciation has been the principal mechanism by which new gene pools have originated protective isolating mechanisms. This involves the spatial separation of a subpopulation from the gene pool of a parental species and the gradual building up of isolating mechanisms in the isolated population. When these have been reasonably perfected through repeated mutational changes, the external barrier can break down and the daughter species can now coexist protected by the isolating mechanisms. In a gen- eral way, small founder populations have been found to speciate far more rapidly than have large populations, particularly when the small group has either invaded a new territory or been subject to imposed climatic change. The great representatives of these generalizations are to be found in the manner in which marsupials in Australia have so adapted as to fill most of the ecological niches that, on other continents, are claimed by placental mammalia. Darwin's original example, the finches of the Galapagos, were a prime example because the descendants of the original finches now show beak forms that vary from one like that of the grosbeak to long, slender needles for feeding on nectar.
136 THE LIFE SCIENCES In general, much remains to be learned about geographical isolation mechanisms. It is unclear how these have operated in the sea, in large freshwater bodies, or in the intertidal zones of shorelines, which are remarkably rich in fauna and flora. Nor is it clear what factors determine the actual number of species found in a given locality. Why is a tropical reef ten times as rich in species as a rocky reef in cool waters? Why is a tropical forest so very rich in the number of species of trees, insects, and birds, while each species is represented by relatively few individuals? Documentation of geographical speciation is overwhelming; witness changes in the protectively colored coat of mice with changes in soil color, or seemingly identical grasses that may flower in March along the Gulf Coast but in midsummer in Kansas. No matter how well populations respond to the demands of local environments, there is a line the geographical species border which is the limit of tolerance of the genetic system of that species. A tantalizing question remains as to why the frontier populations of a species do not respond by the selection of new genotypes that would permit expanding the species border over time. Some aspects of the genetic system of the species render it geographically cohesive and place limits on expan- sion. Nor is the situation entirely clear in any instance. Patently, the enter- ing of a new niche has resulted with great frequency in a budding off of the phyletic line by the formation of new gene pools (speciating). Yet, with at least equal frequency, entering of a new niche can be effected by a minor reconstruction of the genotype; witness the diversity of niches occupied by the woodpeckers, hummingbirds, albatrosses, ducks, and swifts, despite the fact that these birds are anatomically, physiologically, and in most aspects of their life histories remarkably similar wherever they are found. In some degree, the answer is to be found in the fact that the more closely adapted a given species is to life in a restricted environmental circumstance, the less well it can radiate into other circumstances. The influence of the environment is equally apparent in the phenomenon of evolutionary convergence in which similar selective pressures drive unre- lated genotypes into similar forms. Thus, the cactus growth form has appeared in several distinct families of plants quite distantly remote from the true cacti. In Australia, where the ordinary frog is absent, a tree frog has evolved with habits, body size, shape, and appearance of the common leopard frog of North America, even though the two species are only very remotely related genetically. Origin of Higher Groups Macroevolution, the development of new broad assemblages of species such as birds, beetles, and ferns, is less understood than is speciation itself. But
FRONTIERS OF BIOLOGY the first step must always be the entrance of a population of some species into a new adaptive zone. Once this step is taken, a new selection pressure arises favoring all individuals that by their genetic constitution are superior in adapting to the new niche. Presumably this is how the earliest ancestors of all terrestrial animals became amphibious and how the earliest ancestors of all flying animals began to glide. It probably is extremely rare that any particular genotype has the potential to initiate such a new major group. But, with millions of species in existence simultaneously, this must have happened many times in the course of the 3 billion years of life on earth. Even when the first adaptation was behavioral, like air-swallowing in some ancestor of the present lung fish, it set the stage for additional structural and functional adaptations that made the occupation of the new adaptive zone more effective. New organs or structures have come into existence by means of two mechanisms. Sometimes it has been merely the intensifi- cation of the function of the previously existing organ, as when lungs devel- oped as sinuses of the esophagus in air-gulping fishes living in stagnating swamps. In other instances, a pre-existing structure has taken on a second function without interference with the original function until the new function became the primary one. Thus, the initial acquisition of bird feathers almost certainly facilitated maintenance of constant body tempera- ture, and only secondarily acquired their function as enlargements of the gliding surface of the wing. What is not clear is why certain groups have remained virtually unchanged for hundreds of millions of years-indeed appear to be immortal while others simultaneously have undergone radi- cal changes in what would appear to have been the same environment. Indeed, the entire fossil record is punctuated by instances in which one or another group entered upon almost explosive diversification leading to a simultaneous invasion of all sorts of new adaptive zones, referred to as adaptive radiation. But the circumstances that prompted such events are unclear. Understanding of such events requires reconstruction of the intermediary stages that led from one kind of organism to another. Such reconstruction is immensely aided when there is available a key fossil that actually appears to be a linking organism e.g., Archaeopteryx between reptiles and birds, the mammal-like reptiles (therapsids) between reptiles and mammals, and Ichthyostega between certain fishes and amphibians. But most such transi- tions are still shrouded in complete mystery. Transitions among the plants are even less well understood. The higher plants (angiosperms), now the dominant group of plants in most of the world, have not been clearly traced to any ancestral group among the lower plants. Some 25 major phyla are recognized for all the animals, and in virtually no case is there fossil evidence to demonstrate what the common ancestor of any two phyla looked like. Nevertheless, there is a possibility 137
138 THE LIFE SCIENCES of finding these ancestors because only a tiny fraction of the fossil record of the Cambrian and Pre-Cambrian eras, when the first animals turned up in the fossil record, has been subjected to careful examination. Extinction Extinction, the termination of an evolutionary line without descendants, is even less well understood than the origins of new species. Although there has been progress in explaining the extinction of certain species owing to changes in the physical or biotic environment, explanations offered for the decline and final extinction of entire major groups, as has occurred many times in the geological past, are totally unsatisfactory. Of 60 trilobite families, the dominant group of animals at the close of the Cambrian period, 40 disappeared from the subsequent fossil record and, after flowering in the Ordovician, the entire phylum disappeared before the end of the Paleozoic. Near the end of the Permian, about 24 orders, half of the then- current phyla, became extinct; again, during the last part of the Cretaceous, one quarter of all animal families were eliminated. (See Table 1.) TABLE 1 Geologic Time Scale ERA PERIOD EPOCH (BEGAN) YEARS AGO r Holocene QUaternarY ' ~ Pleistocene Pliocene ....... ........ Miocene ..... .. Oligocene Eocene ....................................... Paleocene ........ Cenozoic . .. ~ , ~ Mesozoic .... ~ Jurassic ........ ~ Triassic .......... Tertiary . .. Ccretaceous .......................................................................... Permian .............. Carboniferous .... Devonian ... ..... Paleozoic . .. ~ I Silurian ........ | Ordovician .............. ~ Cambrian ......... ......... - 11,000 29OOO,OOO 13,000,000 25,000,000 36,000,000 58,000,000 63,000,000 13 57OOO,OOO 18O7OOO,OOO 230,000,000 280,000~000 345,000,000 400,000,000 425~000,000 500,000,000 600,000~000 P C brian over 600000 000 re- am .............................................................................. . .
FRONTIERS OF BIOLOGY Such periods of extinction have been equally devastating for dominant groups in the oceans, e.g., the ammonites, and on land, e.g., the dinosaurs. None of the theories advanced to explain these catastrophes is convincing. By contrast, plant extinctions have been far more gradual, and the important floristic changes have not coincided with the major extinction of animal groups. Neither changes in climate nor competition from evolutionary newcomers can explain the disappearance of these large, dominant, wide- soread groups. With so many species in existence, one would have expected at least some of them to shift into new adaptive zones and thus escape the fate of their relatives. -rig ~ ~ It may well be that the reasons were subtle, as in the rise of the angio- sperm plants in the Cretaceous, which undoubtedly had an adverse effect on the ruling herbivorous reptiles, this, in turn, bringing about the decline of the carnivorous types. The important point is that the entire biota at any one time has been interrelated and interdependent in an extremely complicated way. Whatever factor affects the primary producers in such a system will have a profound and selective effect on the primary and secondary consumers, which may lead to partial or complete extinction and to the formation of new ecosystems. No species has so profoundly affected the distribution of others as man. His appearance in the Pleistocene coin- cides with the disappearance of a great variety of large mammals; some were hunted, and others lost in the competition for common food sources. And this competition between man and the other species continues even in the modern era. Diversity and the Conceptual Framework of Biology The study of diversity, with its emphasis on evolution, the creativeness of the selective process, the history of genetic programs, the uniqueness of individuals in species, and the statistical properties of populations, places emphasis on the most strictly "biological" of all phenomena. The evolu- tionist is constantly aware that organisms are the products of individual genetic programs carefully adjusted by hundreds of millions of years of natural selection. The remarkable successes of the reductionist approach of the biochemist have permitted understanding of life at its essence. But understanding of DNA, of enzyme mechanisms, of membranes, and of nervous conduction could not have permitted prediction that there would be butterflies, orchids, or porpoises much less man. Evolutionary biology has produced fundamental generalizations of con- cern to every thinking human being. Perhaps the most important contribu- tion after the general theory of organic evolution itself is the development 139
40 THE LIFE SCIENCES of "population thinking." This view stresses that all classes of objects and phenomena in the living world are composed of uniquely different individuals and that all statements concerning such populations of indi- viduals must be taken in a statistical sense. One cannot understand the working of natural selection or the phenomena of race or fitness unless one appreciates the populational nature of biological phenomena. The gradual replacement during the past century of the ideology of essentialist philos- ophy, dominant from Plato and Aristotle to Kant, by population thinking has been one of the most important, although scarcely noticed, conceptual revolutions. Although man is unique, he is part of the evolutionary stream and can- not be understood as an isolated phenomenon. He must be viewed within the context of the remainder of the organic world, a comparison that reveals respects in which man resembles other organisms as a consequence of his evolutionary heritage and in which he is indeed unique. Evolutionary biology has succeeded in dealing with man objectively and scientifically, rather than as an object of ideology or dogma. The evolutionary process applies to man as to all other organisms. Every problem faced by man, whether it be disease prevention, the life-span, population control, ethical decisions, or other, will be better understood as our comprehension of the evolutionary process by which man evolved and acquired his present char- acteristics increases. Man's knowledge of his long history gives him a different perspective about his future. He has changed greatly in the past, and it is in his nature to continue to change. He understands his history and the origin of human diversities and similarities. These diversities are not unnatural and thus must be seen as part of a continuing process. It is perhaps humbling to realize that man is in this sense only a part of nature. But evolutionary biology has also shown us the central role that man is destined to play in evolution from now on, unless, of course, he engineers his own extinction. Although man arose through an evolutionary process that he did not under- stand and over which he had no control, he must now realize that he is unique in the living world and that the responsibility for continuance of the evolutionary process is his. The future evolution of the orangutan and the whooping crane and of most other species will be determined by human decisions and hardly at all by anything done by those species themselves. Thus the evolutionary view gives man not only a sense of humility but also a sense of responsibility. The question is not whether man is to influence evolution; he is already doing so, and indeed is changing things so that evolution is taking place more rapidly than at any time in recent history. He now has not only the opportunity to influence the other species as he has done in the past with domestic plants and animals, but also the oppor
FRONTIERS OF BIOLOGY tunity-perhaps the obligation- to influence his own future evolution. That he will continue to evolve is the inevitable consequence of genetic variability and differential reproduction. The capacity of biologists to develop ways by which man can determine his future evolution is undoubted. The more difficult questions are whether man will choose to take full ad- vantage of that capacity and with what wisdom.