Early Evolutionary Studies.

The notion that populations of organisms can be transformed over generations into descendant populations of different kinds has been suggested repeatedly since the early recorded history of ideas ( see Heredity ). In various forms it was held by some schools of ancient Greek philosophy and discussed by philosophers and theologians through the succeeding ages of history, but no scientific explanations of evolutionary processes were attempted until the 18th century. The growth of natural history then led to an increasingly detailed knowledge of living and fossil organisms, and the concept of evolution attracted serious students. The most outstand ing evolutionist in the early 19th century was Jean Baptiste de Lamarck, who argued that the patterns of resemblance found in various creatures arose through evolutionary modifications of a common lineage-for example, that lions, tigers, and other animals of the cat family had all descended from a catlike ancestor. Naturalists had already established that different animals are adapted to different modes of life and environmental conditions; Lamarck believed that environmental changes evoked in individual animals direct adaptive responses that could be passed on to their offspring as inheritable traits. This generalized hypothesis of evolution by acquired characteristics was not tested scientifically during Lamarck's lifetime.

Darwinian Theory.

A successful explanation of evolutionary processes was proposed by Charles Darwin. His most famous book, On the Origin of Species by Means of Natural Selection (1859), is a landmark in human understand ing of nature. Pointing to variability within species, Darwin observed that while offspring inherit a resemblance to their parents, they are not identical to them. He further noted that some of the differences between offspring and parents were not due solely to the environment but were themselves often inheritable. Animal breeders, he observed, were often able to change the characteristics of domestic animals by selecting for reproduction those individuals with the most desirable qualities-speed in racehorses, milk production in cows, trail scenting in dogs. This is change by artificial selection. Darwin reasoned that, in nature, individuals with qualities that made them better adjusted to their environments or gave them higher reproductive capacities would tend to leave more offspring; such individuals were said to have higher fitness. Because more individuals are born than survive to breed, constant winnowing of the less fit-a natural selection-should occur, leading to a population that is well adapted to the environment it inhabits. When environmental conditions change, populations require new properties to maintain their fitness. Either the survival of a sufficient number of individuals with suitable traits leads to an eventual adaptation of the population as a whole, or the population becomes extinct. Thus, according to Darwin's theory, evolution proceeds by the natural selection of well-adapted individuals over a span of many generations.

The parts of Darwin's theory that were the most difficult to test scientifically were the infer ences about the heritability of traits, or characteristics, because heredity was not understood at that time. The basic rules of inheritance became known to science only at the turn of the century, when the earlier genetic work of Gregor Mendel came to light. Mendel had discovered that characteristics are transmitted across generations in discrete units, now known as genes ( see Genetics ) that are inherited in a statistically predictable fashion. The discovery was then made that inheritable changes in genes, termed mutations, could occur spontaneously and randomly without regard to the environment. Since mutations were seen to be the only source of genetic novelty, many geneticists believed that evolution was driven onward by the random accumulation of favorable mutational changes. Natural selection-that is, evolution directed by adaptive fitness-was reduced to a minor role by mutationists such as Hugo De Vries, Thomas Morgan, and William Bateson, whose ideas were prominent well into the 1930s.

Population Genetics.

Even while mutationism was replacing Darwinism, the leading evolutionary theory, the science of population genetics was being founded by Sewall Wright, J. B. S. Haldane, and several other geneticists, all working independently. They developed arguments to show that even when a mutation that is immediately favored appears, its subsequent spread within a population depends on such variables as the following: (1) the size of the population; (2) the length of generations; (3) the degree to which the mutation is favorable; and (4) the rate at which the same mutation reappears in descendants.

Furthermore, a given gene is favorable only under certain environmental conditions. If conditions change in space, then the gene may be favored only in a localized part of the population; if conditions change over time, the gene may become generally unfavorable. Because different individuals usually have different assortments of genes-no two humans (except identical twins) are precisely alike genetically-the total number of genes available for inheritance by the next generation can be large, constituting a vast store of genetic variability. This is called the gene pool. Sexual reproduction ensures that the genes are rearranged in each generation, a process termed recombination. When a population is stable, the gene frequency-that is, the frequency of occurrence of each gene in proportion to the total number of genes in the gene pool-remains the same, even though the genes are recombined in different ways in each individual. When the gene frequencies in the pool change in a sustained manner, evolution is occurring. Mutations provide the gene pool with a continuous supply of new genes; through the process of natural selection the gene frequencies change so that advantageous genes occur in greater proportions.

Despite the mathematical support that was developed for this view of evolution, most evolutionists adhered to the theory of evolution by random mutations until the late 1930s. At that time Theodosius Dobzhansky, in Genetics and the Origin of Species, extended the mathematical arguments with a wide range of experimental and observational evidence. For example, he demonstrated adaptive genetic changes in large populations of fruit flies as a result of controlled environmental changes. Dobzhansky proved that the facts of genetics are compatible with Darwinian natural selection, which is the chief cause of sustained changes in gene frequencies and therefore of evolutionary changes in a population's characteristics. In the ensuing decades outstand ing contributions were made to the revitalized Darwinian theory of evolution from nearly all fields of biological and paleontological science.

The Synthetic Theory.

As the new evolutionary theory became enriched from such diverse sources, it became known as the synthetic theory. Three American scientists made especially important contributions. The German-born Ernst Mayr (1904- ), a zoologist, showed that new species usually arise in geographic isolation, often following a genetic "revolution" that rapidly changes the contents of their gene pools ( see Species and Speciation ). George Simpson, a paleontologist, showed from the fossil record that rates and modes of evolution are correlated: New kinds of organisms arise from the invasion of a new adaptive zone, usually evolving rapidly. G. Ledyard Stebbins (1906- ), a botanist, showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has demonstrated diverse adaptive responses to environmental pressures and opportunities. In addition, these biologists reviewed a broad range of genetic, ecological, and systematic evidence to show that the synthetic theory was strongly supported by observation and experiment. The theory has formed the basis of textbook accounts of evolution since the 1950s. It has also led to a renewed effort to classify organisms according to their evolutionary history. See Classification .

During the establishment of the synthetic theory of evolution, the science of heredity underwent another profound change in 1953, when James Watson and Francis Crick demonstrated the way genetic material is composed of two nucleic acids (q.v.) , deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acid molecules contain genetic codes that dictate the manufacture of proteins, and the latter direct the biochemical pathways of development and metabolism in an organism. Mutations are now known to be changes in the position of a gene, or in the information coded in the gene, that can affect the function of the protein for which the gene is responsible. Natural selection can then operate to favor or suppress a particular gene according to how strongly its protein product contributes to the reproductive success of the organism. These findings have made possible the study of evolution at the molecular level, tracing the history of changes in particular genes and in gene organization.

Today evolutionary studies extend into all branches of biology. The present state of all forms of life, from bacteria to humans, has been achieved by evolution, and to study the physiology of a leech, the ecology of a seashore community, or the behavior of a hummingbird is to study features that have been achieved through evolutionary change. As Dobzhansky has said, nothing in biology makes sense except in the light of evolution.

Speciation.

One of the clearest examples of natural selection at work in the modern world is afforded by gene-frequency changes in carefully studied populations of the peppered moth, a species of the genus Biston found in England. These moths, originally light grayish but with a small proportion of dark-colored individuals, are eaten by birds that locate them visually. When the British countryside near cities became blackened by smoke from industrial processes, the lighter moths, previously well disguised against light-colored tree trunks, were easily found by birds and thus became less fit. The dark moths became common because they were more difficult to discern against the darker background. A single gene, coding for the dark color rather than the light color, was spread by means of natural selection and raised to a high frequency in industrial regions. Subsequent reduction of smoke pollution resulted in a reduction of the dark moth variety.

The light and dark moth varieties belong to the same species and interbreed freely. If pollution had continued, however, the rural moth population would have become entirely light and the industrial entirely dark. Then each population would be subject to somewhat different selective pressures because the two environments vary. In time, the dark and light populations would differ by groups of genes, with each group advantageous locally. The moth populations might eventually become incapable of interbreeding. At this point, natural selection would have caused them to diverge from a common ancestor into two species.

Populations of various species have easily become isolated in different habitats on various islands and have differentiated into new species. Island chains provide a particularly well-studied context of speciation. Darwin himself was struck by the finches of the Galapagos Islands, which radiated into 14 species, each with distinctive form and habits, following an invasion of the islands by a single species from the American mainland. In the Hawaiian Islands some 500 species of fruit flies have descended from one or two found ing species in less than 10 million years; they have had a complex history of island hopping and rehopping, in the process forming isolated populations that were the start of new species.

Natural selection is not the only source of genetic change in the evolution of species. Gene frequencies may also change by the chance fail ure of progeny to reproduce the exact gene proportions of their parents. This is termed genetic drift and is most important in small populations, where genes may be lost from the original gene pool simply by not being represented in successive matings. Also, failure to carry the full range of genes in the parent population occurs when a few individuals migrate and found a new, isolated population, which is thus different from the very beginning; this is called the founder effect. Mutations can, of course, also change gene frequencies, but such changes occur at low rates relative to the changes brought about by the recombination of genes in offspring.

Because all the established genes in a population have been monitored for fitness by selection, newly arisen mutations are unlikely to enhance fitness unless the environment changes so as to favor the new gene activity, as in the gene for dark color in the peppered moth. Novel genes that cause large changes rarely promote fitness and are usually lethal. The genes already established by selection are carefully adjusted to one another so their biochemical effects are coordinated; a new gene with a major effect is comparable to the insertion of a chance word or rearrangement of words into a precise set of instructions. Mutations with small effects provide the basis for the genetic changes that are seen to promote fitness in experimental laboratory environments. Indeed, natural selection by a series of mutations appears to be the chief agent of evolution.

Transspecific Evolution.

Organisms are classified in a hierarchical scheme that attempts to reflect their evolutionary relationships. Closely related species are grouped into genera, closely related genera into families, with orders, classes, phyla, and kingdoms representing the higher categories of classification. The origin of the groups of organisms in the higher categories is called transspecific evolution. These groups differ from one another in many ways besides being reproductively isolated; phyla even differ from one another in the basic architectural plans of their bodies. At one time some evolutionists believed that the processes responsible for speciation were not adequate to account for these great differences, but today speciation and transspecific evolution are generally considered not fundamentally different.

Because the biological environment is patchy and variable, different modes of life are required in different habitats. Seafloor mud, open ocean water, land, and air present different problems for successful adaptation; burrowing, swimming, walking, and flying are among the locomotory solutions to life in these environments. Such widely varying solutions demand the emergence of novel body structures. The key to the development of these structures is that the necessaryinnovations must first have appeared as adaptations to some aspect of the ancestral mode of life and then proved useful in the new mode as well. Thus, large modifications evolve over time by a series of preadaptations. For example, the lungs of air-breathing creatures evolved from an early lunglike structure developed in certain fish for gulping oxygen from the surfaces of oxygen-deprived waters. Limbs for terrestrial life evolved from the stiffened fins of fish that gained further adaptive advantages from crawling up on shore. Similarly, limb modifications that evolved to prolong gliding jumps between trees later proved advantageous in the development of wings for flight. Successful pioneering lineages radiate into a number of descendant lineages with an array of well-adapted forms, thus representing distinctive new groups of organisms. The separate vertebrate classes of amphibians, reptiles, birds, and mammals can thus be traced to evolutionary adaptations of their early ancestors at the species level.

Current Evolutionary Debate.

Although the fact of evolution is scientifically accepted as underlying modern biology, theories that concern themselves with the processes of evolution continue to be debated and refined. Much of this work involves highly sophisticated mathematical studies, as required by the complex interactions of the various elements of the modern synthesis, from gene mutations to population genetics to large-scale ecological interactions over geological time. Because understand ing of the actual evolutionary events that took place over earth's long history depends largely on interpretations of an incomplete fossil record, much latitude remains for differences in such interpretations. One of the issues that is currently being debated among theorists derives from a notable fact observed in the fossil record. That is, when a new species appears in the record it usually does so abruptly and then apparently remains stable for as long as the record of that species lasts. The fossils do not seem to exhibit the slow and gradual changes that might be expected according to the modern synthesis. For this reason, in part, a number of evolutionists-most notably Stephen Jay Gould (1941- ) of Harvard University and Niles Eldredge (1943- ) of the American Museum of Natural History-have proposed a variant concept of "punctuated equilibria" for species evolution. According to this concept, species do in fact tend to remain stable for long periods of time and then to change relatively abruptly-or rather, to be replaced suddenly by newer and more successful forms. These sudden changes are the "punctuations" in the state of equilibrium that give this concept its name.

Although these proposed periods of rapid change would be abrupt only in terms of the geological time scale and would actually occur over periods of thousands of years, most evolutionists tend to consider the punctuated-equilibrium concept only another possible mode of evolutionary change that could take place along with the processes described by the modern synthesis, rather than as a supplanting model for evolution theory. The very incompleteness of the fossil record does not permit any such clear choice to be made, because the record of almost any species is highly selective over geological time. In addition, the small changes that would make up gradual evolutionary development according to the modern synthesis are themselves not necessarily of a nature that would be apparent in the fossil history of a species, however complete it might be over a given stretch of time. Fossils primarily show gross morphological changes, whereas changes taking place in genetic makeup could be extensive even though overall body structures do not reveal these shifts in populations of species. Arguments from the known nature of small-scale evolutionary change do not, in fact, necessarily establish long-term evolutionary events, as following either the model proposed by the modern synthesis or the one proposed by punctuated equilibrium. Evolution may just as well have proceeded along both routes.

Steps in Evolution.

Life originated more than 3.4 billion years ago, when the earth's environment was much different than that of today. Especially important was the lack of significant amounts of free oxygen in the atmosphere. Experiments have shown that rather complicated organic molecules, including amino acids (q.v.) , can arise spontaneously under conditions that are believed to simulate the earth's primitive environment. Concentration of such molecules evidently led to the synthesis of active chemical groupings of molecules, such as proteins, and eventually to interactions among chemical compounds. A rudimentary genetic system eventually arose and was elaborated by natural selection into the complicated mechanisms of inheritance known today. The earliest organisms must have fed on nonliving organic compounds, but chemical and solar energy sources were soon tapped. Photosynthesis freed organisms from their dependence on organic compounds and also released oxygen so the atmosphere and oceans gradually became more hospitable to advanced life forms.

The earliest organisms of which remains exist were already cells, resembling modern bacteria ( see Cell ). These simple unicellullar forms (procaryotes) were at first anaerobic (living without oxygen), but they diversified into an array of adaptive types from which blue-green algae descended, including aerobic photosynthesizers. Advanced cells (eucaryotes) may have evolved through the amalgamation of a number of distinct simple cell types. A large ingesting cell may have incorporated as symbionts ( see Symbiosis ) some small blue-green algal cells that evolved into chloroplasts (cell bodies that photosynthesize) and some tiny aerobic bacteria that evolved into mitochondria (cell bodies that release energy during respiration). Other features of ad vanced cells, such as their large DNA contents, may also have arisen from procaryotic symbionts. Single-celled eucaryotes then developed complex modes of living and advanced types of reproduction that led to the appearance of multicellular plants and animals. The latter are first known from about 700 million years ago, and their appearance implies that at least moderate levels of free atmospheric oxygen and a relatively predictable supply of food plants had been achieved. Between about 700 and 570 million years ago the basic body plans of modern animals were developed during a remarkable burst of evolutionary diversification. The earliest body fossils consist chiefly of impressions belonging to jellyfish and their allies, a rudimentary group. At about the same time, however, fossil burrows appeared, signaling the evolution of burrowing worms with considerably more advanced body structures. Then, beginning just before 570 million years ago, skeletons developed independently in a number of animal lineages. One wormlike lineage that pursued a swimming mode of life evolved a stiff dorsal cord and eventually an articulated internal skeleton that supported the body to improve swimming efficiency; thus, fish arose from the early invertebrates.

In order for complex animal communities to develop, plants must first become established to support herbivore populations, which in turn may support predators and scavengers. Land plants appeared about 400 million years ago, spreading from lowland swamps as expand ing greenbelts. Arthropods (some evolving into insects) and other invertebrate groups followed them onto land, and finally land vertebrates (amphibians at first) rose from freshwater fish nearly 360 million years ago. In general, the subsequent radiations of land vertebrates made them increasingly independent of water and increasingly active. Dinosaurs and mammals shared the terrestrial environment for 135 million years; dinosaurs may well have been more active, and certainly were larger, than their mammalian contemporaries, which were small and possibly nocturnal. The mammals, however, survived a wave of extinction that eliminated dinosaurs about 65 million years ago, and subsequently diversified into many of the habitats and modes of life that formerly had been dinosaurian.

Mammals among vertebrate animals and insects among invertebrates dominate the terrestrial faunas today. See also Animal Distribution ; Plant Distribution .

Human Evolution.

Humans belong to an order of mammals, the primates, which existed before the dinosaurs became extinct. Early primates seem to have been tree dwelling and may have resembled squirrels in their habits. Many of the primate attributes-the short face, overlapping visual fields, grasping hands, large brains, and perhaps even alertness and curiosity-must have been acquired as arboreal adaptations. Descent from tree habitats to forest floors and eventually to more open country, however, was associated with the development of many of the unique features of the human primate, including erect posture and reduced canine teeth, which suggest new habits of feeding.

A shift to cooperative hunting and gathering, with concomitant requirements for a high level of intelligence and social organization, accompanied the rise of the modern human species within the last 2 million years or so. See Human Evolution .

Evolutionary Patterns.

The history of life as inferred from the fossil record displays a wide variety of trends and patterns. Lineages may evolve slowly at one time and rapidly at another time; they may follow one pathway of change for some time only to switch to another pathway; and they may diversify rapidly at one time and then shrink under wide-spread extinctions.

The key to many of these patterns is the rate and nature of environmental change. Species become adapted to the environmental conditions that exist at a given time, and when change leads to new conditions, they must evolve new adaptations or become extinct ( see Endangered Species ). When the environment undergoes a particularly rapid or extensive change, waves of extinction occur; these are followed by waves of development of new species. The times of mass extinction are not yet well understood. Although the most famous one is that of the dinosaurs, about 65 million years ago, such events appear in the fossil record as far back as Precambrian time, when life first arose. In fact, five mass extinctions on the scale of that at the end of the age of dinosaurs are known over the past 600 million years. Some scientists also claim to have demonstrated a definite periodicity to smaller periods of mass extinction, and in particular a 26-million-year cycle of eight extinctions over the past 250 million years.

Controversy has arisen over the proposal made by some geologists that mass extinctions are related to periodic catastrophes such as the striking of the earth's surface by a large asteroid or comet. Many paleontologists and evolutionary theorists reject such hypotheses as unjustified; they feel that periods of mass extinctions can be accounted for by less spectacular evolutionary processes and by more earthbound events such as cycles of climatic change and volcanic activity.

Species that have adapted to a changeable environment have broad tolerances that may enable them to survive extensive changes. Human beings are uniquely adapted in that they make and use tools and devices and invent and propagate procedures that give them extended control over their environment. Humans are significantly changing the environment, however, and the likelihood is that evolutionary patterns in the future will reflect this influence.