in biology, the complex of processes by which living organisms originated on
earth and have been diversified and modified through sustained changes in form
and function. The earliest known fossil organisms are single-celled forms
resembling modern bacteria; they date from about 3.4 billion years ago.
Evolution has resulted in successive radiations of new types of organisms, many
of which have become extinct, but some of which have developed into the present
fauna and flora of the world. Extinction and diversification continue today.
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 outstanding contributions were made to the revitalized Darwinian theory of evolution from nearly all fields of biological and paleontological science.
V 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, 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, 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.
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.
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 Galápagos 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 founding 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 failure 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.
VIII 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 understanding 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 of Harvard University and Niles Eldredge 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.
Widely accepted evidence suggests that the first organisms were archaebacteria, primitive cells without nuclei. These cells may have evolved in waters with extremely high temperatures and no oxygen. Cyanobacteria, more advanced organisms, evolved from 2.5 to 3.4 billion years ago. Cyanobacteria carried out photosynthesis, a process that traps the energy of the sun and uses it to build glucose, an organic molecule, from water and carbon dioxide. Photosynthesis freed organisms from their dependence on organic molecules dissolved in water, and also released oxygen so that the atmosphere and oceans gradually became more hospitable to more advanced life forms.
Advanced cells (eukaryotes) 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 advanced cells, such as their large DNA contents, may also have arisen from prokaryotic symbionts. Single-celled eukaryotes 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. Fish arose from a wormlike lineage of early invertebrates that evolved a stiff dorsal cord and, eventually, an articulated internal skeleton that improved swimming efficiency.
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 expanding 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.
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. See Human Evolution.
XI EVOLUTIONARY PATTERNS
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. Whatever proposals may eventually prove true, however, it seems fairly certain that periodic waves of mass extinction do occur.
Species adapted to live in environments that are changeable in the short term have broad tolerances, which may better 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 environments. Humans are significantly changing the environment itself, however. The effects are most complex and cannot be predicted, and yet the likelihood is that evolutionary patterns in the future will reflect the influence of the human species. See Adaptation; Natural Selection; Sexual Selection.
For a theological interpretation of the origin of life, see Creation.
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