The concept of evolution, which is now basic to the life sciences, has provided new and in some ways revolutionary answers to questions men have been asking for centuries. The two most important of these are: “Why am I here, what is the purpose of human existence?”, and “What is the nature of the world of life that surrounds us?” Evolution tells us that we are here because of a long series of past events that do not differ in kind from the events and processes that produced the millions of different organisms that surround us. The most important processes are: (1) interactions between organisms and their environment that are highly diverse both historically and geographically; (2) the continuity of heredity and cultural tradition; and (3) the occasional disturbance of these regularities by chance.
The effects of this revolution in thinking are just beginning to be realized. If man has arrived at his present state as a result of natural processes rather than a supernatural will, he can learn to control these processes. He can improve his condition by analyses of natural phenomena, syntheses of ideas, and action to realize well-defined objectives. The limitations we must overcome are chiefly our own difficulty in understanding the vast complexity of the world that surrounds us, as well as the weight of tradition, the conservatism of cultural heredity, and the egotism of individuals which restricts our ability to act on the basis of principles that underlie natural processes. In order to solve our present problems, therefore, we must first learn as much as we can about the nature and origin of living organisms, including our own complex societies; and second, we must devise sound and reliable methods of translating this knowledge into constructive action. The purpose of this book is to present a summary of our current knowledge about the origin and evolution of the world of life.
THE COMPLEXITY OF INTERACTIONS BETWEEN POPULATION AND ENVIRONMENT
In a world that may contain four to five million different kinds of organ- isms (Dobzhansky, 1970), exploiting in various ways a large number of habitats, the variety of possible interactions between populations and their environments is enormous. Equally vast is the scope of these interactions. At the bottom of the ocean fish depend for their existence on organisms that fall to these depths after having completed their life on the surface of the water, 6,000 meters above. On the slopes of Mount Everest, 6,500 meters above sea level, a few seed plants (Delphinium) grow in the shelter of lichen-covered rocks, providing food for a few insects, which in turn support a small number of spiders. In deserts such as those along the coast of Peru and Chile and the Sahara of Africa, plant seeds and highly resistant eggs of small animals can endure rainless periods lasting for many years, and then suddenly, when the rains finally arrive, produce a flush of growth and activity (Walter, 1971).
The complexity of ecosystems is based upon thousands of different ways of exploiting the same habitat. Consider, for instance, a forest of redwood or Sequoia in northwestern California. Organisms there range in size from trees 110 meters tall to bacteria only 0.000001 meters long. The lush undergrowth supports (or did before human interference) many kinds of animals that feed upon each other according to several food chains. One chain starts with shrubs that are browsed by deer or elk, which in turn are the prey of pumas. Another starts with the sap exuded from plants, which supports yeasts, which in turn are eaten by flies such as Drosophila; these provide food for bats or passerine birds, which in turn are the prey of hawks. Still another food chain starts with decaying leaves and their associated microorganisms, on which subter- ranean insects and other arthropods subsist, these form the prey of amphibians, which are preyed upon by snakes, which often become the food of owls. In tropical rain forests food chains are far more complex. Complex food chains exist in other rich biota, such as those of tide pools along the seashore, the plankton-rich surface of the ocean, and even habitats within individual organ- isms, such as the interior of the bovine rumen (Hungate, 1966).
This hierarchy of population-environment interactions is by no means constant, but has varied greatly over both short and long periods of time. In temperate woodlands and fields the seasonal succession of biotas is one of the most obvious facts of life. Larger, long-lived organisms react to the climatic cycle either by altering their phenotypes, becoming dormant during unfa- vorable seasons, or by regular migrations. On the other hand, populations of some short-lived species having rapid reproductive cycles, such as rodents (Gershenson. 1945) and flies (Dobzhansky, 1951), alter their genetic compo- sition in response to these changes. During recorded human history greater and more permanent changes of the environment have taken place, accompanied by corresponding changes in populations. Examples are well documented of alterations in the genetic composition of populations in response to these changes, the best known of these being industrial melanism (Ford, 1971). Dur- ing the entire period of the earth’s history since the major phyla of organisms appeared, all of the regions of the earth have undergone complete alterations of their biota, and many of them have experienced several “ecological revolutions.”
THE REACTION OF ORGANISMS TO ENVIRONMENTAL COMPLEXITY
The way in which populations of organisms have reacted and are reacting to environmental changes depends upon the genetic characteristics of individuals as well as the amount and kind of genetic variability in the population. With respect to individuals, the most important characteristics are the continuity of heredity, which depends upon the self-replication of chromosomal DNA; the capacity for change by mutation, and the harmonious integration of the geno- type. With respect to populations, the most important characteristics are their unlimited capacity for increase, which requires the destruction of individuals in order to keep population numbers constant, their great store of genetic variability, which is discussed in later chapters, and the limitations of this variability in any particular population. The latter characteristic determines whether, in response to a particular environmental change, a population will adapt to the new environment or become extinct. As Simpson (1953) has pointed out, species become extinct not because some unexplained “evolutionary urge” has forced nonadaptive phenotypes upon them, or because they have lost essential genetic variability. If adaptive change or evolution takes place, the method of adaptation will depend on the kind of genetic variation available.
Given a new ecological niche to which it can become adapted, a population can achieve this adaptation in any one of several different ways. Although in many instances the particular adaptation that is adopted may depend entirely upon the chance appearance of certain favorable mutations or gene combina- tions, generally the direction taken depends chiefly upon preexisting capacities already present in the gene pools. If, for instance, a plant community becomes exposed to increasing aridity, those populations that already have somewhat succulent leaves or stems are likely to become more succulent; those whose leaf surfaces are protected from evaporation by thick, hard cuticles may evolve thicker, more complex cuticles, and those that already evade dry spells by losing their leaves and becoming dormant may evolve longer periods of dor- mancy and more rapid growth during the favorable season. If a community of small animals becomes exposed to a more efficient predator, those that have escaped previous predators by speed and agility may become faster and more agile, those that escape by burrowing in the ground may become more strongly fossorial; and those that because of a bad odor or other noxious properties are
avoided by many predators may become even more unpleasant and noxious.