Evolution

views updated

Evolution

Evolution is an all-pervading concept in modern science applied to physical as well as biological processes. The phrase theory of evolution, however, is most commonly associated with organic evolution, the origin and evolution of the enormous diversity of life on this planet.

The idea of organic evolution arose from attempts to explain the immense diversity of living organisms and dates from the dawn of culture. Attempts to formulate a naturalistic explanation of the phenomena of evolution in one form or another had been proposed, and by the end of the eighteenth century a number of naturalists from Carolus Linnaeus to Jean-Baptiste Lamarck had questioned the prevailing doctrine of fixity of species . Lamarck came the closest to proposing a complete theory of evolution, but for several reasons his theory fell short and was not widely accepted.

Two English naturalists conceived the first truly complete theory of evolution. In a most remarkable coincidence, working quite independently, Charles Darwin and Alfred Russell Wallace arrived at essentially the same thesis, and their ideas were initially publicized jointly in 1858. However, it is to Charles Darwin that the recognition of the founder of modern evolution is attributed. His continued efforts toward a detailed development of the evidence and the explication of a complete, convincing theory of organic evolution resulted in the publication in 1859 of his book On the Origin of Species by Natural Selection. With the publication of this volume, widely regarded as one of the most influential books ever written, regardless of subject matter, the science of biology would never be the same.

Darwin was the first evolutionist whose theories carried conviction to a majority of his contemporaries. He set forth a formidable mass of supporting evidence in the form of direct observations from nature , coupled with a comprehensive and convincing synthesis of current knowledge. Most significantly, he proposed a rational, plausible instrumentality for evolutionary change: natural selection.

The theory Darwin presented is remarkably simple and rational. In brief, the theoretical framework of evolution presented by Darwin contained three basic elements. First is the existence of variation in natural populations. In nature there are no individuals who are exactly alike, therefore natural populations always consist of members who all differ from one another to some degree. Many of these differences are heritable and are passed on from generation to generation. Second, some of these varieties are better adapted to their environment than others. Third, the reproductive potential of populations is unlimited. All populations have the capacity to overproduce, but populations in nature are limited by high mortality . Thus, if all offspring cannot survive each generation, the better-adapted members have a greater probability of surviving each generation than those less adapted.

Thomas Henry Huxley, when first presented with Darwin's views, was supposed to have exclaimed: "How extremely stupid not to have thought of that." This apocryphal statement expresses succinctly the immediate intelligibility of this momentous discovery.

Darwin's theory as presented was incomplete, as he was keenly aware, but the essential parts were adequate at the time to demonstrate that the material causes of evolution are possible and can be investigated scientifically.

One of the most critical difficulties with Darwin's theory was the inability to explain the source of hereditary variations observed in populations and, in particular, the source of hereditary innovations which, through the agency of natural selection, could eventually bring about the transformation of a species. The lack of existing knowledge of the mechanism of heredity in the mid-1800s precluded a scientifically sound solution to this problem.

The re-discovery in 1900 of Gregor Mendel's experiments demonstrating the underlying process of inheritance did not at first provide a solution to the problem facing Darwinian evolutionary theory. In fact, in one of the most extraordinary paradoxes in the history of science, during the first decades of Mendelian genetics, a temporary decline occurred in the support of Darwinian selectionism. This was primarily because of the direction and level of research as much as the lack of knowledge in further details of the underlying mechanisms of inheritance.

It was not until the publication of Theodosius Dobzhansky's Genetics and the Origin of Species (1937) that the modern synthetic theory of evolution began to take form, renewing confidence in selectionism and the Darwinian theory. In his book, Dobzhansky, for the first time, integrated the newly developed mathematical models of population genetics with the fats of chromosomal theory and mutation together with observations of variation in natural populations.

Dobzhansky also stimulated a series of books by several major evolutionists which provided the impetus and direction of research over the next decade, resulting in a reformulation of Darwin's theory. This became known as the synthetic theory of evolution. Synthetic is used here to indicate that there has been a convergence and synthesis of knowledge from many disciplines of biology and chemistry.

The birth of molecular genetics in 1950 and the enormous expansion of knowledge in this area have affected the further development and refinement of the synthetic theory markedly. But it has not altered the fundamental nature of the theory. Darwin's initial concept in this expanded, modernized form has taken on such stature as to be generally considered as the most encompassing theory in biology. To the majority of biologists, the synthetic theory of evolution is a grand theory that forms the core of modern biology. It also provides the theoretical foundation upon which most all other theories find support and comprehension. Or, as the distinguished evolutionary biologist Dobzhansky so succinctly put it: "Nothing in life makes sense except in the light of evolution."

If the three main elements of the Darwinian theory were to be restated in terms of the modern synthetic theory, they would be as follows. First, genetic systems governing heredity in each organism are composed of genes and chromosomes which are discrete but tightly interacting units of different levels of complexity. The genes, their organized associations in chromosomes, and whole sets of chromosomes, have a large degree of stability as units. But these units are shuffled and combined in various ways by sexual processes of reproduction in most organisms. The result of this process, called recombination, maintains a considerable amount of variation in any population in nature without introducing any new hereditary factors. Genetic experimentation, is, in fact, occurring all the time in natural populations. Each generation, as a result of recombination, is made up of members who carry different hereditary combinations drawn from the common pool of the population. Each member generation in turn interacts within the same environmental matrix which, in effect, "tests" the fitness of these hereditary combinations. The sole source of hereditary innovation from within the population, other than recombination, would come from mutation affecting individual genes, chromosomes or sets of chromosomes, which are then fed into the process of recombination.

Second, populations of similar animals—members of the same species—usually interbreed among themselves making up a common pool of genes. The gene pools of established populations in nature tend to remain at equilibrium from generation to generation. An abundance of evidence has established the fact, as Darwin and others before him had observed, that populations have greater reproductive capacity than they can sustain in nature.

Third, change, over time, in the makeup of the gene pool , may occur by one of four known processes: mutation, fluctuation in gene frequencies—known as "sampling errors," or "genetic drift," which is generally effective only in small, isolated populations—introduction of genes from other populations, and differential reproduction.

The first three of these processes do not produce adaptive trends. These are essentially random in their effects, usually nonadaptive and only rarely and coincidentally produce adaptation . Of the four processes, differential reproduction results in adaptive trends. Differential reproduction describes the consistent production of more offspring, on an average, by individuals with certain genetic characteristics than those without those characteristics. This is the modern understanding of natural selection, with emphasis upon success in reproduction. The Darwinian and Neo-Darwinian concepts, on the other hand, placed greater emphasis upon "survival of the fittest" in terms of mortality and survival. Natural selection, in the Darwinian sense, was non-random and produced trends that were adaptive. Differential reproduction produces the same result. In what may appear to be a paradox, differential selection also acts in stabilizing the gene pool by reducing the reproductive success of those members whose hereditary variation result in non-adaptive traits.

As Darwin first recognized, the link between variability or constancy of the environment and evolutionary change or stability is provided by natural selection. The interaction between organisms and their environment will inevitably affect the genetic composition of each new generation. As a result of differential selection, the most fit individuals, in terms of reproductive capacity, contribute the largest proportion of genes or alleles to the next generation. The direction in which natural selection guides the population depends upon both the nature of environmental change or stability and the content of the gene pool of the population.

From the very beginning, evolutionists have recognized the significance of the environment/evolution linkage. This linkage always involves the interactions of a great many individuals both within populations of a single species (intraspecific) and among members of different species (interspecific linkage). These are inextricably linked with changes in the physical environment.

Dobzhansky estimated that the world may contain four to five million different kinds of organisms, each exploiting in various ways a large number of habitats. The complexity of ecosystems is based upon thousands of different ways of exploiting the same habitats in which each species is adapted to its specifically defined niche within the total habitat . The diversity of these population-environment interactions is responsible for the fact that natural selection can be conservative—(normalizing selection)—and can promote constancy in the gene pool. It directs continuous change (directional selection), or promotes diversification into new species (diversifying selection).

Recent trends in research have been of special significance in understanding the dynamics of these processes. They have greatly increased the body of information related to the ecological dimension of population biology . These studies have developed methods of analysis and sophisticated models which have made major advances in the study and understanding of the nature of the selective forces that direct evolutionary change. It is increasingly clear that a reciprocal imperative exists between ecological and environmental studies to further our understanding of the role of evolutionary and counterrevolutionary processes affecting stability in biological systems over time. The urgency of the need for an expanded synthesis of ecological and evolutionary studies is driven by the accelerating and expanding counterrevolutionary changes in the world environment resulting from human actions.

Evolutionary processes are tremendously more complex in detail than this brief outline suggests. This spare outline, however, should be sufficient to show that here is a mechanism, involving materials and processes known beyond doubt to occur in nature, capable of shaping and changing species in response to the continuously dynamic universe in which they exist.

[Donald A. Villeneuve ]