Evolution is the process by which all living things have developed from primitive organisms through changes occurring over billions of years, a progression that includes the most advanced animals and plants. Exactly how evolution occurs is still a matter of debate, but that it occurs is a scientific fact. Biologists agree that all living things arose through a long history of changes shaped by physical and chemical processes that are still taking place. It is plausible that all organisms can be traced back to the origin of life from inanimate matter.
The most direct proof of evolution is furnished by the science of paleontology, or the study of life in the past through fossil remains or impressions, usually in rock. Additional evidence comes from comparative studies of living animals and plants, including their structure (comparative anatomy), biochemistry, embryology, and geographical distribution. Approximately 2 million different species of organisms are now living, but it is estimated that at least 99.9 percent of the species that have ever lived are now extinct and that some 2 billion species have evolved during the past 600 million years.
Changes occur in living organisms that serve to increase their adaptability, or potential for survival and reproduction, in the face of changing environments. Evolution apparently has no built-in direction or foreordained purpose. A given kind of organism may evolve only when it occurs in a variety of forms differing in hereditary characteristics, or traits, that are passed from parent to offspring. Purely by chance, some varieties prove to be ill adapted to their current environment and thus disappear, whereas others prove to be adaptive, and their numbers increase. The elimination of the unfit, or the "survival of the fittest," is known as Natural Selection because it is nature that discards or favors a particular variant. Basically, evolution takes place only when natural selection operates on a population of organisms containing diverse inheritable forms. Recently, natural selection was demonstrated for the first time outside of the laboratory when scientists observed guppies change their reproductive behavior over an 11-year period in direct response to being transferred to a new environment that had different predators.
Pierre Louis Moreau de Maupertuis (1698-1759) was perhaps the first to propose a general theory of evolution. He concluded that hereditary material, consisting of particles, was transmitted from parents to offspring. His appreciation of the part played by natural selection had little influence on other naturalists, however.
Until the mid-19th century, naturalists believed that each species was created separately, either through a supreme being or through spontaneous generation--the concept that organisms arose fully developed from soil or water. The work of the Swedish naturalist Carolus Linnaeus (in 1761 a patent of nobility was granted by the crown to Linnaeus, and he was styled Carl von Linne) in advancing the classifying of biological organisms focused attention on the close similarity between certain species. Speculation arose as to the existence of a sort of blood relationship between these species. These questions--coupled with the emerging sciences of geology and paleontology--gave rise to hypotheses that the life-forms of the day evolved from earlier forms through a process of change. Extremely important was the realization that different layers of rock represented different time periods and that each layer had a distinctive set of fossils of life-forms that had lived in the past.
Jean Baptiste Lamarck was one of several theorists who proposed an evolutionary theory based on the "use and disuse" of organs. Lamarck stated that an individual acquires traits during its lifetime and that such traits are in some way incorporated into the hereditary material and passed to the next generation. This was an attempt to explain how a species could change gradually over time. According to Lamarck, giraffes, for example, have long necks because for many generations individual giraffes stretched to reach the uppermost leaves of trees; in each generation the giraffes added some length to their necks, and they passed this on to their offspring. New organs therefore arise from new needs and develop in proportion to the extent that they are used; conversely, disuse of organs leads to their disappearance.
Later, the science of genetics disproved Lamarck's theory: it was found that acquired traits cannot be inherited. No evidence exists that acquired traits can alter genetic makeup and be passed to succeeding generations.
Thomas Robert Malthus, an English clergyman, through his work An Essay on the Principle of Population, had a great influence in directing naturalists toward a theory of natural selection. Malthus proposed that environmental factors such as famine and disease limited population growth.
Darwin and Wallace
After more than 20 years of observation and experiment, Charles Darwin presented his theory of evolution through natural selection to the Linnaean Society of London in 1858. He presented his discovery simultaneously with another English naturalist, Alfred Russel Wallace, who independently discovered natural selection at about the same time. The following year Darwin published his full theory, supported with enormous evidence, in On the Origin of Species.
Having read Malthus's essay on population, Darwin realized that similar principles of environmental selection apply to all living species. His triumph was in seeing the relationship between natural selection and heritable variations in populations, although he never understood how heritable changes occurred or resulted in variation.
The principal contribution of genetics to the understanding of evolution has been the explanation of the inheritance of variability in individuals of the same species. Gregor Mendel discovered the basic principles of inheritance in 1865, but his work was unknown to Darwin. Mendel's work was "rediscovered" by other scientists around 1900. From that time to 1925 the science of genetics developed rapidly, and many of Darwin's ideas about the inheritance of variations were found to be incorrect. Only in the years since 1925 has natural selection again been recognized as essential in evolution. The modern, or synthetic, theory of evolution combines the findings of modern genetics with the basic framework supplied by Darwin and Wallace, creating the basic principle of Population Genetics. Modern population genetics was developed largely during the 1930s and '40s by the mathematicians J. B. S. Haldane and R. A. Fisher and by the biologists Theodosius Dobzhansky, Julian Huxley, Ernst mayr, George Gaylord simpson, Sewall Wright, Berhard Rensch, and G. Ledyard Stebbins. According to the synthetic theory, variability among individuals in a population of sexually reproducing organisms is produced by mutation and genetic recombination. The resulting genetic variability is subject to natural selection in the environment.
For the purpose of study, the unit of evolution is not the individual but the group of interbreeding individuals. The word population is used in a special sense to describe such a group. The study of single individuals provides few clues as to the possible outcomes of evolution because single individuals cannot evolve in their lifetime. An individual represents a store of genes that participates in evolution only when those genes are passed on to further generations, or populations. The gene is the basic unit in the cell for transmitting hereditary characteristics to offspring. Individuals are units upon which natural selection operates, but the trend of evolution can be traced through time only for groups of interbreeding individuals; populations can be analyzed statistically and their evolution predicted in terms of average numbers.
The Hardy-Weinberg law--which was discovered independently in 1908 by a British mathematician, Godfrey H. Hardy, and a German physician, Wilhelm Weinberg--provides a standard for quantitatively measuring the extent of evolutionary change in a population. The law states that the gene frequencies, or ratios of different genes in a population, will remain constant unless they are changed by outside forces, such as selective (differential) reproduction and mutation. This discovery reestablished natural selection as an evolutionary force because the basis of natural selection is differential reproduction by the most fit organisms. Comparing the actual gene frequencies observed in a population with the frequencies predicted, or calculated, by the Hardy-Weinberg law gives a numerical measure of how far the population deviates from a static, or nonevolving, state, called the Hardy-Weinberg equilibrium. Given a large, randomly breeding population, the Hardy-Weinberg equilibrium will hold true, because it depends on the laws of probability.
In genetics a population is defined in terms of its gene pool, the sum total of all alleles (two or more alternate forms of a gene that determine different characteristics) for all genes in the population. The frequencies of alleles vary from generation to generation, and this change is defined as evolution according to population genetics. Changes are produced in the gene pool through mutations, gene flow, genetic drift, and natural selection.
A mutation is an inheritable change in the character of a gene. Mutations most often occur spontaneously, but they may be induced by some external stimulus, such as irradiation or certain chemicals. The rate of mutation in humans is extremely low; nevertheless, the number of genes in every gamete, or sex cell, is so large that the probability is high for at least one gene to carry a mutation.
New genes can be introduced into a population through new breeding organisms or gametes from another population, as in plant pollen. Gene flow can work against the processes of natural selection.
A change in the gene pool due to chance is called genetic drift. The frequency of loss of alleles is greater the smaller the population. Thus, in small populations there is a tendency for less variation because mates are more similar genetically.
Over a period of time natural selection, or differential reproduction, will result in changes in the frequency of alleles in the gene pool, or greater deviation from the nonevolving state, represented by the Hardy-Weinberg equilibrium.
Populations can exhibit a wide variety of variations, which are promoted by sexual reproduction and by other means, such as self-incompatibility in plants that discourages self- fertilization and encourages outbreeding.
In sexual reproduction, each individual inherits equal numbers of chromosomes and, hence, genes from both parents; the combination of genes is therefore different from that of either parent. The number of genes is so large that a particular combination is probably never repeated in the history of the species. The other process is recombination, in which each pair of chromosomes inherited from the parents trades segments through physical breakage and exchange.
As a result, the original combination of inherited genes is recombined, thus amplifying the number of possible new hereditary patterns. In such asexually reproducing populations as certain bacteria, algae, and fungi, mutation is the only source of inherited variation.
The resulting genetic variability is subject to natural selection. Individuals with characteristics making them more successful in using the resources of the environment are more likely to survive and reproduce, whereas the others with less favorable characteristics are less likely to reproduce. The hereditary patterns controlling the more favorable characteristics are therefore passed on in greater frequency to the next generation. The resulting change in the genetic makeup of the population in the next generation constitutes evolution.
New species may evolve either by the change of one species to another or by the splitting of one species into two or more new species. Splitting, the predominant mode of species formation, results from the geographical isolation of populations of species. Isolated populations undergo different mutations, recombinations, and selection pressures and may evolve along different lines. If the isolation is sufficient to prevent interbreeding with other populations, these differences may become extensive enough to establish a new species (a group of organisms that does not successfully interbreed with any other group). The evolutionary changes brought about by isolation include differences in the reproductive systems of the group. When a single group of organisms diversifies over time into several subgroups by expanding (radiating) into the available niches of a new environment, it is said to undergo adaptive radiation.
Darwin's Finches, in the Galapagos Islands, west of Ecuador, illustrate adaptive radiation. They were probably the first land birds to reach the islands, and, in the absence of competition, they occupied several ecological habitats and diverged along several different lines. Such patterns of divergence are reflected in the biologists' scheme of classification of organisms, which groups together animals that have common characteristics. An adaptive radiation of great importance followed the first conquest of land by vertebrates.
Natural selection can also lead populations of different species living in similar environments or having similar ways of life to evolve similar characteristics. This is called convergent evolution and reflects the similar selective pressure of similar environments. Examples of convergent evolution are the eye in cephalod mollusks, such as the octopus, and in vertebrates; wings in insects, extinct flying reptiles, birds, and bats; and the flipperlike appendages of the sea turtle (reptile), penguin (bird), and walrus (mammal).
An outpouring of new evidence supporting evolution has come in the 20th century from molecular biology, an unknown field in Darwin's day. The fundamental tenet of molecular biology is that genes are coded sequences of the DNA molecule in the chromosome (genetic code), and that a gene codes for a precise sequence of amino acids in a protein. Mutations alter DNA chemically, leading to modified or new proteins (transposon). Over evolutionary time, proteins have had histories that are as traceable as those of large-scale structures such as bones and teeth. The further in the past that some ancestral stock diverged into present-day species, the more evident are the changes in the amino-acid sequences of the proteins of the contemporary species.
Biologists believe that plants arose from the multicellular green algae (phylum Chlorophyta) that invaded the land about 1.2 billion years ago. Evidence is based on modern green algae having in common with modern plants the same photosynthetic pigments, cell walls of cellulose, and multicellular forms having a life cycle characterized by alternation of generations. Photosynthesis almost certainly developed first in bacteria. The green algae may have been preadapted to land. Adaptations present in most plants today include the cuticle, which slows water evaporation; the waxy coating; the stomata, pores that allow carbon dioxide to enter leaves and stems during photosynthesis; and the protective cells around the sex organs to prevent desiccation.
The two major groups of plants are the bryophytes and the tracheophytes; the two groups most likely diverged from one common group of plants. The bryophytes, which lack complex conducting systems, are relatively small and are found in moist areas. The tracheophytes are vascular plants with efficient conducting systems; they dominate the landscape today. The seed is the major development in tracheophytes, and it is most important for survival on land.
Fossil evidence indicates that land plants first appeared during the Silurian Period of the Paleozoic Era (425-400 million years ago) and diversified in the Devonian Period. Near the end of the Carboniferous Period, fernlike plants had seedlike structures. At the close of the Permian Period, when the land became drier and colder, seed plants gained an evolutionary advantage and became the dominant plants.
Plant leaves have a wide range of shapes and sizes, and some variations of leaves are adaptations to the environment; for example, small, leathery leaves found on plants in dry climates are able to conserve water and capture less light. Also, early angiosperms adapted to seasonal water shortages by dropping their leaves during periods of drought.
EVIDENCE FOR EVOLUTION
The fossil record furnishes important insights into the history of life. The order of fossils, starting at the bottom and rising upward in stratified rock, corresponds to their age, from oldest to youngest.
Deep Cambrian rocks, up to 570 million years old, contain the remains of various marine invertebrate animals--sponges, jellyfish, worms, shellfish, starfish, and crustaceans. These invertebrates were already so well developed that they must have become differentiated during the long period preceding the Cambrian. Some fossil-bearing rocks lying well below the oldest Cambrian strata contain imprints of jellyfish, tracks of worms, and traces of soft corals and other animals of uncertain nature (ediacaran fauna).
Paleozoic waters were dominated by bizarre arthropods called trilobites and large scorpionlike forms called eurypterids. Common in all Paleozoic periods (570-230 million years ago) were the nautiloids, which are related to the modern nautilus, and the brachiopods, or lampshells. The odd graptolites, colonial animals whose carbonaceous remains resemble pencil marks, attained the peak of their development in the Ordovician Period (500-430 million years ago) and then abruptly declined. In the mid-1980s researchers found fossil animal burrows in rocks of the Ordovician Period; these trace fossils indicate that terrestrial ecosystems may have evolved sooner than was once thought.
Many of the prominent Paleozoic marine invertebrate groups either became extinct or declined sharply in numbers before the Mesozoic Era (230-65 million years ago). During the Mesozoic, shelled ammonoids flourished in the seas, and insects and reptiles were the predominant land animals. At the close of the Mesozoic the once-successful marine ammonoids perished and the reptilian dynasty collapsed, giving way to birds and mammals. Insects have continued to thrive and have differentiated into a staggering number of species.
During the course of evolution plant and animal groups have interacted to one another's advantage. For example, as flowering plants have become less dependent on wind for pollination, a great variety of insects have emerged as specialists in transporting pollen. The colors and fragrances of flowers have evolved as adaptations to attract insects. Birds, which feed on seeds, fruits, and buds, have evolved rapidly in intimate association with the flowering plants. The emergence of herbivorous mammals has coincided with the widespread distribution of nutritious grasses, and the herbivorous mammals in turn have contributed to the evolution of carnivorous mammals.
Fish and Amphibians
During the Devonian Period (390-340 million years ago) the vast land areas of the Earth were largely barren of animal life, save for rare creatures like scorpions and millipedes. The seas, however, were crowded with a variety of invertebrate animals. The fresh and salt waters also contained a highly diversified and abundant assemblage of cartilaginous and bony fish. From one of the many groups of fish inhabiting pools and swamps emerged the first land vertebrates, starting the vertebrates on their conquest of all available terrestrial habitats.
Prominent among the numerous Devonian aquatic forms were the Crossopterygii, lobe-finned fish that possessed the ability to gulp air when they rose to the surface. These ancient air- breathing fish represent the stock from which the first land vertebrates, the amphibians, were derived. Scientists continue to speculate about what led the crossopterygians to venture onto land. The crossopterygians that migrated onto land were only crudely adapted for terrestrial existence, but because they did not encounter competitors, they survived.
Lobe-finned fish did, however, possess certain characteristics that served them well in their new environment, including primitive membranous lungs and internal nostrils, both of which are essential for atmospheric breathing.
Such characteristics, called preadaptations, did not develop because the crossopterygians were preparing to migrate to the land; they were already present by accident and became selected traits only when they imparted an advantage to the fish on land.
The early land-dwelling amphibians were slim-bodied with fishlike tails, but they had limbs capable of locomotion on land. These limbs probably developed from the crossopterygians' lateral fins, which contained fleshy lobes that in turn contained bony elements.
The ancient amphibians never became completely adapted for existence on land, however. They spent much of their lives in the water, and their modern descendants--the salamanders, newts, frogs, and toads--still must return to water to deposit their eggs. The elimination of a water-dwelling stage, which was achieved by the reptiles, represented a major evolutionary advance.
The Reptilian Dynasty
Perhaps the most important factor contributing to the emergence of reptiles from the amphibians was the development of a shell- covered amniotic egg that could be laid on land. This development enabled the reptiles to spread throughout the Earth's landmasses in one of the most spectacular adaptive radiations in biological history.
Like the eggs of birds, which developed later, reptile eggs contain a complex series of membranes that protect and nourish the embryo and help it breathe. The space between the embryo and the amnion (a thin membrane loosely enclosing the embryo) is filled with an amniotic fluid that resembles seawater; a similar fluid is found in the fetuses of mammals, including humans. This fact has been interpreted as an indication that life originated in the sea and that the balance of salts in various body fluids did not change very much in subsequent evolution. It is also significant that the membranes found in the human embryo are essentially similar to those in reptile and bird eggs. The human yolk sac remains small and functionless, and the allantois exhibits no elaborate development in the human embryo. Nevertheless, the presence of a yolk sac and allantois in the human embryo is one of the strongest pieces of evidence documenting the evolutionary relationships among the widely differing kinds of vertebrates. This suggests that mammals, including humans, are descended from animals that reproduced by means of externally laid eggs that were rich in yolk.
The reptiles, and in particular the dinosaurs, endured as the dominant land animals of the Earth for well over 100 million years. The Mesozoic Era, during which the reptiles thrived, is often referred to as the Age of Reptiles.
In terms of evolutionary success, the larger the animal, the greater the likelihood that the animal will maintain a constant body temperature independent of the environmental temperature. Birds and mammals, for example, produce and control their own body heat through internal metabolic activities (a state known as endothermy, or warm-bloodedness), whereas today's reptiles are thermally unstable (cold-blooded), regulating their body temperatures by behavioral activities (the phenomenon of ectothermy). Most scientists regard dinosaurs as lumbering, oversized, cold-blooded lizards, rather than large, lively, endothermic animals with fast metabolic rates; some biologists, however--notably Robert T. Bakker of The Johns Hopkins University--assert that a huge dinosaur could not possibly have warmed up every morning on a sunny rock and must have relied on internal heat production.
The reptilian dynasty collapsed before the close of the Mesozoic Era. Relatively few of the myriad Mesozoic reptiles have survived to modern times; those remaining include the crocodile, lizard, snake, and turtle. The cause of the decline and death of the large array of reptiles is obscure, but their demise is usually attributed to some radical change in environmental conditions (extinction).
Like the giant reptiles, most lineages of organisms have eventually become extinct, although some have not changed appreciably in millions of years. The opossum, for example, has survived almost unchanged since the late Cretaceous Period (more than 65 million years ago), and the horseshoe crab, Limulus, is not very different from fossils 500 million years old. We have no adequate explanation for the unexpected stability of such organisms; perhaps they have achieved an almost perfect adjustment to a relatively unchanging environment. Such stable forms, however, are not at all dominant in the world today. The human species, one of the dominant modern life forms, has evolved rapidly in a relatively short time.
The Rise of Mammals
The decline of the reptiles provided evolutionary opportunities for birds and mammals. Small and inconspicuous during the Mesozoic Era, mammals rose to unquestionable dominance during the Cenozoic Era (beginning 65 million years ago).
The mammals diversified into marine forms, such as the whale, dolphin, seal, and walrus; fossorial (adapted to digging) forms living underground, such as the mole; flying and gliding animals, such as the bat and flying squirrel; and cursorial animals (adapted for running), such as the horse. These various mammalian groups are well adapted to their different modes of life, especially by their appendages, which developed from common ancestors to become specialized for swimming, flight, and movement on land.
Although there is little superficial resemblance among the arm of a person, the flipper of a whale, and the wing of a bat, a closer comparison of their skeletal elements shows that, bone for bone, they are structurally similar. Biologists regard such structural similarities, or homologies, as evidence of evolutionary relationships. The homologous limb bones of all four-legged vertebrates, for example, are assumed to be derived from the limb bones of a common ancestor. Biologists are careful to distinguish such homologous features from what they call analogous features, which perform similar functions but are structurally different. For example, the wing of a bird and the wing of a butterfly are analogous; both are used for flight, but they are entirely different structurally. Analogous structures do not indicate evolutionary relationships.
Closely related fossils preserved in continuous successions of rock strata have allowed evolutionists to trace in detail the evolution of many species as it has occurred over several million years. The ancestry of the horse can be traced through thousands of fossil remains to a small terrier-sized animal with four toes on the front feet and three toes on the hind feet. This ancestor lived in the Eocene Epoch, about 54 million years ago. From fossils in the higher layers of stratified rock, the horse is found to have gradually acquired its modern form by eventually evolving to a one-toed horse almost like modern horses and finally to the modern horse, which dates back about 1 million years.
Primates, the order of mammals to which humans belong, underwent adaptive radiation in Cenozoic times. Except for humans, who are fully adapted for life on the ground, primates are primarily tree dwellers. Many primate characteristics evolved as adaptations to arboreal life.
There is almost universal agreement that the apes are the closest living relatives of humans and that the line that ultimately led to the human form diverged from the ape line during the Tertiary Period (65-2 million years ago). Several humanlike fossils dating from this period have been proposed as possible human ancestors (prehistoric humans). Comparisons of genetic material suggest a divergence only about 5 million years ago, an estimate at variance with prevailing interpretations of the fossil record. The resolution of this matter awaits the unearthing and analysis of further remains from the time period in question.
Relatively few anatomical and physiological differences exist between modern humans and the living great apes. Their close relationship can be clearly demonstrated by comparing the amino -acid sequences in their fibrinopeptide molecules, which in different mammalian species have undergone rapid and numerous changes. The differences between humans and other primates can be counted as follows: chimpanzees, none; orangutans, 2; gibbons, 5; Old World monkeys, 5; New World monkeys, 9 to 10; and prosimians such as the slow loris, 18. Humans and African apes are in fact more closely related to each other than either is to the Asian orangutans.
The most pronounced differences between human beings and apes have to do with locomotor habits and brain growth: humans have a fully upright posture and gait and a much larger brain. The cranial capacity of a modern ape rarely exceeds 600 cu cm (37 cu in), while the average human being has a cranial capacity of 1,350 cu cm (82 cu in). The superior intelligence of humans, gradually acquired through evolution, has helped them master a wide variety of different environments.
Many species of higher organisms, particularly birds and mammals, exhibit highly structured social interactions. Since cooperative social behavior helps members of the group to survive, it is reasonable to assume that behavioral dispositions have evolved just as morphological and physiological traits have evolved. While the evolutionary process generally tends to select organisms that are optimally designed for individual reproduction, however, in many species social behavior has evolved that promotes the reproductive advantage of certain members of the population but is generally detrimental to individual reproduction. In social insects, for example, a female worker bee refrains from breeding and devotes her entire life to assisting the queen bee. Evolutionists do not yet fully understand how it is possible to select for behavior disposition that is beneficial to the species as a whole but that is costly to the individual.
Because evolutionary events in the past are not amenable to direct observation or experimental verification, the processes of evolution over the course of Earth's history must be inferred. Most students of evolution hold the view that evolution in the past was guided by the same evolutionary forces being witnessed in operation today, and that small genetic variations have gradually accumulated in evolving lineages over periods of millions of years. Were a complete set of fossil specimens of a lineage somehow to be recovered, it would be expected to exhibit a graded series of forms changing continuously from the antecedent to the descendant species. In this view, a lack of transitional fossil forms between some given ancestral and descendant populations represents merely the imperfect nature of the fossil record, comparable to a book with random pages missing.
In recent years, Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History have challenged this conventional view. They argue that the fossil record shows that most lineages do not change much for long intervals of geologic time; they can be said to remain in stasis, or "equilibrium." Significant evolutionary changes instead are concentrated into geologically brief periods, or "punctuations," during which the lineages actually split or branch. Under this hypothesis, known as "punctuated equilibria," the apparent gaps in the fossil record are actually a faithful rendering of the evolutionary process. Perhaps the best documented illustration of the concept is the study made in 1981 of fossil snails in the Lake Turkana region of Africa. The thick fossil beds there contain lineages of at least 19 snail species, several of which remained exceptionally stable for 3 to 5 million years. When morphological changes in shell shape did occur, they were concentrated in brief periods of 5,000 to 50,000 years; these newly evolved populations of snails thereafter persisted relatively unchanged until they became extinct.
The punctuated-equilibria hypothesis has been criticized by other scientists. In the matter of the Lake Turkana snails, for example, the observation has been made that while changes over a period of 50,000 years might appear instantaneous to a paleontologist, a geneticist would probably view 50,000 years as a quite sufficient period of time for morphological changes in snail populations to accrue gradually rather than dramatically. Evolutionists in general are doubtful that punctuational changes dominated the history of life, although recognizing a role for sudden transitions.
A more recent controversy over theory has arisen in the field of molecular evolution. When amino acids of proteins were first compared from an evolutionary viewpoint, it was assumed that differences between two species were preserved by natural selection because each form of a protein was best in some way for its own species. Further study, however, showed that some amino-acid substitutions were not likely to produce either an evolutionary advantage or disadvantage. The mutational changes responsible for such unimportant substitutions might then be viewed as neutral and preserved by sheer chance rather than by natural selection. The major advocate of this view is Japanese geneticist Motoo Kimura, who holds that different species may have different amino-acid residues at a particular protein region not because it matters to the operation of the molecule but precisely because it does not matter. This position remains unacceptable to so-called selectionists, who ascribe all amino- acid substitutions to natural selection.
E. Peter Volpe