EVOLUTION

theory in biology postulating that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory. (See also human evolution.) The diversity of the living world is staggering. More than 2,000,000 existing species of plants and animals have been named and described; many more remain to be discoveredfrom 10,000,000 to 30,000,000 according to some estimates. What is impressive is not just the numbers but also the incredible heterogeneity in size, shape, and way of life: from lowly bacteria, measuring less than one-thousandth of a millimetre in diameter, to the stately sequoias of California, rising 300 feet (100 metres) above the ground and weighing several thousand tons; from bacteria living in the hot springs of Yellowstone National Park at temperatures near the boiling point of water to fungi and algae thriving on the ice masses of Antarctica and in saline pools at -9 F (-23 C); and from the strange wormlike creatures discovered in dark ocean depths at thousands of feet below the surface to spiders and larkspur plants existing on Mt. Everest more than 19,868 feet above sea level. The virtually infinite variations on life are the fruit of the evolutionary process. All living creatures are related by descent from common ancestors. Humans and other mammals are descended from shrewlike creatures that lived more than 150,000,000 years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600,000,000 years ago; all plants and animals are derived from bacteria-like microorganisms that originated more than 3,000,000,000 years ago. Biological evolution is a process of descent with modification. Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time. The 19th-century English naturalist Charles Darwin argued that organisms come about by evolution, and he provided a scientific explanation, essentially correct but incomplete, of how evolution occurs and why it is that organisms have featuressuch as wings, eyes, and kidneysclearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution. Since the 1960s a related scientific discipline, molecular biology, has advanced enormously knowledge of biological evolution and has made it possible to investigate detailed problems that seemed completely out of reach a few years earlierfor example, how similar the genes of humans and chimpanzees might be (they differ in about 1 or 2 percent of the units that make up the genes). This article discusses evolution as it applies generally to living things. For a discussion of human evolution, see the article human evolution. For a more complete treatment of a discipline that has proved essential to the study of evolution, see human genetics and heredity. Specific aspects of evolution are discussed in the articles coloration and mimicry. Applications of evolutionary theory to plant and animal breeding are discussed in the articles plant breeding and animal breeding. A detailed discussion of the life and thought of Charles Darwin is found in the article Darwin, Charles. by T.H. Huxley Evolution, Huxley explained in the ninth edition (187589) of Encyclopdia Britannica, is at present employed in biology as a general name for the history of the steps by which any living being has acquired the morphological and the physiological characters which distinguish it. The ninth edition's article Evolution consists of two parts, the first titled Evolution in Biology and the second Evolution in Philosophy. Huxley wrote the first part, and extracts from its second section, which surveys the theory of the evolution of the sum of living beings, are reproduced here. In these extracts Huxley sets out eight scientific principles and discoveries that had contributed to the theory of evolution as it was understood in the late 19th century; he then argues for the validity of each discovery, one by one. In so doing he demonstrates his lucid writing style, his fearlessness in provoking the religious and academic establishment, and his unapologetic advocacy of the scientific method. clasbody, evolution Additional reading Early seminal works of evolutionary theory include Charles Darwin and Alfred Wallace, On the Tendency of Species to Form Varieties, and on the Perpetuation of Varieties and Species by Natural Means of Selection, Journal of the Proceedings of the Linnean Society, 3(9):4562 (1858); and Charles Darwin, On the Origin of the Species by Means of Natural Selection, or, The Preservation of Favoured Races in the Struggle for Life (1859), also available in many modern editions, and The Descent of Man, and Selection in Relation to Sex, 2 vol. (1871, reprinted in 1 vol., 1981). G. Mendel, Experiments in Plant Hybridisation (1965; originally published in German, 1866), provides the groundwork for all subsequent studies in heredity, including R.A. Fisher, The Genetical Theory of Natural Selection, 2nd rev. ed. (1958); and J.B.S. Haldane, The Causes of Evolution (1932, reissued 1966). Theodosius Dobzhansky, Genetics and the Origin of Species (1937, reprinted 1982), is the classic foundation of the synthetic theory of evolution; see also Julian Huxley, Evolution: The Modern Synthesis, 3rd ed. (1974).The history of evolutionary theories from Darwin to the present is traced in Ronald W. Clark, The Survival of Charles Darwin: A Biography of a Man and an Idea (1984, reissued 1986), which also presents an engaging biography of Darwin. The most authoritative historical treatise of evolutionary ideas from antiquity to the present is Ernst Mayr, The Growth of Biological Thought: Diversity, Evolution, and Inheritance (1982). Ernst Mayr and William B. Provine (eds.), The Evolutionary Synthesis: Perspectives on the Unification of Biology (1980), contains historical articles by several of the great evolutionists who formulated the synthetic theory.Modern treatments of evolutionary theory include G. Ledyard Stebbins, Darwin to DNA, Molecules to Humanity (1982), a readable discussion providing coverage of human evolution, both biological and cultural. A fairly comprehensive text requiring only general biology as background is Francisco J. Ayala and James W. Valentine, Evolving: The Theory and Processes of Organic Evolution (1979). A more advanced text is Theodosius Dobzhansky et al., Evolution (1977). Francisco J. Ayala, Population and Evolutionary Genetics: A Primer (1982), provides an introduction to the genetics of the evolutionary process. A more advanced and mathematically demanding work is Philip W. Hedrick, Genetics of Populations (1983, reissued 1985). The origin of species is the subject of Michael J.D. White, Modes of Speciation (1978); and of the more comprehensive Ernst Mayr, Animal Species and Evolution (1963), which is a classic work. G. Ledyard Stebbins, Flowering Plants: Evolution Above the Species Level (1974), discusses plant speciation and evolution.A good introduction to the fossil record is a collection of articles from Scientific American, edited by Lo F. Laporte, The Fossil Record and Evolution (1982). George Gaylord Simpson, The Meaning of Evolution: A Study of the History of Life and of Its Significance for Man, 2nd rev. ed. (1967, reissued 1971), is written for the general reader yet is an authoritative work dealing particularly with paleontological principles and the evolutionary process through time; somewhat more technical is his Major Features of Evolution (1953, reprinted 1969). An authoritative treatise on paleontological principles is Stephen Jay Gould, Ontogeny and Phylogeny (1977).Two good introductions to molecular evolution are Francisco J. Ayala (ed.), Molecular Evolution (1976); and Masatoshi Nei and Richard K. Koehn (eds.), Evolution of Genes and Proteins (1983). The neutrality theory is presented in full by its main theorizer in Motoo Kimura, The Neutral Theory of Molecular Evolution (1983); and the theory that evolutionary changes happen not gradually but abruptly is advanced by one of its originators in Niles Eldredge, Time Frames: The Rethinking of Darwinian Evolution and the Theory of Punctuated Equilibria (1985). Francisco Jose Ayala The process of evolution Species and speciation The concept of species Darwin sought to explain the splendid multiformity of the living world: thousands of organisms of the most diverse kinds, from lowly worms to spectacular birds of paradise, from yeasts and molds to oaks and orchids. His Origin of Species is a sustained argument showing that the diversity of organisms and their characteristics can be explained as the result of natural processes. Species come about as the result of gradual change prompted by natural selection. Environments are continuously changing in time, and they differ from place to place. Natural selection, therefore, favours different characteristics in different situations. The accumulation of differences eventually yields different species. Everyday experience teaches that there are different kinds of organisms and how to identify them. Everyone knows that people belong to the human species and are different from cats and dogs, which in turn are different from each other. There are differences among people, as well as among cats and dogs; but individuals of the same species are considerably more similar among themselves than they are to individuals of other species. External similarity is the common basis for identifying individuals as being members of the same species. But there is more to it than that; a bulldog, a terrier, and a golden retriever are very different in appearance, but they are all dogs because they can interbreed. People can also interbreed with one another, and so can cats, but people cannot interbreed with dogs or cats, nor these with each other. It is, then, clear that although species are usually identified by appearance, there is something basic, of great biological significance, behind similarity of appearance; individuals of a species are able to interbreed with one another but not with members of other species. This is expressed in the following definition: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. The ability to interbreed is of great evolutionary importance, because it determines that species are independent evolutionary units. Genetic changes originate in single individuals; they can spread by natural selection to all members of the species but not to individuals of other species. Individuals of a species share a common gene pool that is not shared by individuals of other species. Different species have independently evolving gene pools because they are reproductively isolated. Although the criterion for deciding whether individuals belong to the same species is clear, there may be ambiguity in practice for two reasons. One is lack of knowledge; it may not be known for certain whether individuals living in different sites belong to the same species, because it is not known whether they can naturally interbreed. The other reason for ambiguity is rooted in the nature of evolution as a gradual process. Two geographically separate populations that at one time were members of the same species later may have diverged into two different species. Since the process is gradual, there is not a particular point at which it is possible to say that the two populations have become two different species. A related situation pertains to organisms living at different times. There is no way to test whether or not today's humans could interbreed with those who lived thousands of years ago. It seems reasonable that living people, or living cats, would be able to interbreed with people, or cats, exactly like those that lived a few generations earlier. But what about the ancestors removed by 1,000 or 1,000,000 generations? The ancestors of modern humans that lived 500,000 years ago (about 20,000 generations) are classified in the species Homo erectus, whereas present-day humans are classified in a different species, Homo sapiens. There is not an exact time at which Homo erectus became Homo sapiens, but it would not be appropriate to classify remote human ancestors and modern humans in the same species just because the changes from one generation to the next are small. It is useful to distinguish between the two groups by means of different species names, just as it is useful to give different names to childhood and adulthood, even though there is no one moment when one passes from one to the other. Biologists distinguish species in organisms that lived at different times by means of a commonsense morphological criterion. If two organisms differ from each other about as much as two living individuals belonging to two different species differ, they will be classified in separate species and given different names. The definition of species given above applies only to organisms able to interbreed. Bacteria and blue-green algae do not reproduce sexually, but by fission. Organisms that lack sexual reproduction are classified into different species according to criteria such as external morphology, chemical and physiological properties, and genetic constitution. The origin of species Reproductive isolation In sexual organisms individuals able to interbreed belong to the same species. The biological properties of organisms that prevent interbreeding are called reproductive isolating mechanisms (RIM's). Oaks on different islands, minnows in different rivers, or squirrels in different mountain ranges cannot interbreed because they are physically separated, but not necessarily because they are biologically incompatible. Geographic separation, therefore, is not an RIM, since it is not a biological property of organisms. There are two general categories of reproductive isolating mechanisms: prezygotic (those that take effect before fertilization) and postzygotic (those that take effect after). Prezygotic RIM's prevent the formation of hybrids between members of different populations through ecological, temporal, ethological (or behavioral), mechanical, and gametic isolation. Postzygotic RIM's reduce the viability or fertility of hybrids or their progeny. The process of evolution Evolution as a genetic function The concept of natural selection The central argument of Darwin's theory of evolution starts from the existence of hereditary variation. Experience with animal and plant breeding demonstrates that variations can be developed that are useful to man. So, reasoned Darwin, variations must occur in nature that are favourable or useful in some way to the organism itself in the struggle for existence. Favourable variations are ones that increase chances for survival and procreation. Those advantageous variations are preserved and multiplied from generation to generation at the expense of less advantageous ones. This is the process known as natural selection. The outcome of the process is an organism that is well adapted to its environment, and evolution often occurs as a consequence. Natural selection, then, can be defined as the differential reproduction of alternative hereditary variants, determined by the fact that some variants increase the likelihood that the organisms having them will survive and reproduce more successfully than will organisms carrying alternative variants. Selection may be due to differences in survival, in fertility, in rate of development, in mating success, or in any other aspect of the life cycle. All of these differences can be incorporated under the term differential reproduction because all result in natural selection to the extent that they affect the number of progeny an organism leaves. Darwin maintained that competition for limited resources results in the survival of the most effective competitors. But natural selection may occur not only as a result of competition but also as a result of some aspect of the physical environment, such as inclement weather. Moreover, natural selection would occur even if all the members of a population died at the same age, simply because some of them would have produced more offspring than others. Natural selection is quantified by a measure called Darwinian fitness, or relative fitness. Fitness in this sense is the relative probability that a hereditary characteristic will be reproduced; that is, the degree of fitness is a measure of the reproductive efficiency of the characteristic. Biological evolution is the process of change and diversification of living things over time, and it affects all aspects of their livesmorphology, physiology, behaviour, and ecology. Underlying these changes are changes in the hereditary materials. Hence, in genetic terms, evolution consists of changes in the organism's hereditary makeup. Evolution can be seen as a two-step process. First, hereditary variation takes place; second, selection is made of those genetic variants that will be passed on most effectively to the following generations. Hereditary variation also entails two mechanisms: the spontaneous mutation of one variant to another, and the sexual process that recombines those variants to form a multitude of variations. The variants that arise by mutation or recombination are not transmitted equally from one generation to another. Some may appear more frequently because they are favourable to the organism; the frequency of others may be determined by accidents of chance, called genetic drift. Genetic variation in populations The gene pool The gene pool is the sum total of all of the genes and combinations of genes that occur in a population of organisms of the same species. It can be described by citing the frequencies of the alternative genetic constitutions. Consider, for example, a particular gene (which geneticists call a locus), such as the one determining the MN blood groups in humans. One form of the gene codes for the M blood group, while the other form codes for the N blood group; different forms of the same gene are called alleles. The gene pool of a particular population is specified by giving the frequencies of the alleles M and N. Thus, in the United States the M allele in Caucasoids occurs with a frequency of 0.539 and the N allele with a frequency of 0.461. In other populations, these frequencies are different; the frequency of the M allele is 0.917 in Navajo Indians and 0.178 in Australian Aborigines. The necessity of hereditary variation for evolutionary change to occur can be understood in terms of the gene pool. Assume, for instance, that at the gene locus that codes for the MN blood groups there is no variation; only the M allele exists in all individuals. Evolution of the MN blood groups cannot take place in such a population, since the allelic frequencies have no opportunity to change from generation to generation. On the other hand, in populations in which both alleles M and N are present, evolutionary change is possible. The process of evolution Patterns and rates of species evolution Reconstruction of evolutionary history Evolution within a lineage and by lineage splitting Evolution can take place by anagenesis, in which changes occur within a lineage; or by cladogenesis, in which a lineage splits into two or more separate lines. Anagenetic evolution has, over the course of 2,000,000 years, doubled the size of the human cranium; in the lineage of the horse, it has reduced the number of toes from four to one. Cladogenetic evolution has produced the extraordinary diversity of the living world, with its more than 2,000,000 species of animals, plants, fungi, and microorganisms. The most essential cladogenetic function is speciation, the process by which one species splits into two or more species. Because species are reproductively isolated from one another, they are independent evolutionary units; that is, evolutionary changes occurring in one species are not shared with other species. Over time, species become more and more divergent from one another as a consequence of anagenetic evolution. Descendant lineages of two related species that existed millions of years ago may now be classified into quite different taxonomic categories, such as different genera or even different families. The evolution of all living organisms, or of a subset of them, can be seen as a tree, with branches that divide into two or more as time progresses. Such trees are called phylogenies. Their branches represent evolving lineages, some of which eventually die out, while others persist in themselves or in their derived lineages down to the present time. Evolutionists are interested in the history of life and hence in the topology, or configuration, of phylogenies. They are concerned as well with the nature of the anagenetic changes along lineages and with the timing of the events. Phylogenetic relationships are ascertained by means of several complementary sources of evidence. First, there are the discovered remnants of organisms that lived in the past, the fossil record, which provides definitive evidence of relationships among some groups of organisms. The fossil record, however, is far from complete and is often seriously deficient. Second, information about phylogeny comes from comparative studies of living forms. Comparative anatomy contributed the most information in the past, although additional knowledge came from comparative embryology, cytology, ethology, biogeography, and other biological disciplines. In recent years the comparative study of informational macromoleculesproteins and nucleic acidshas become a powerful tool for the study of phylogeny. Morphological similarities among organisms have probably always been recognized. In ancient times, Aristotle and later his followers and those of Plato, particularly Porphyry, classified organisms (as well as inanimate objects) on the basis of similarities. The Aristotelian system of classification was further developed by some medieval Scholastics, notably Albertus Magnus and Thomas Aquinas. The modern foundations of taxonomy, the science of classification, were laid in the 18th century by Linnaeus and by the French botanist Michel Adanson. Lamarck dedicated much of his work to the systematic classification of organisms. He proposed that their similarities were due to ancestral relationshipsin other words, to the degree of evolutionary proximity. The modern theory of evolution provides a causal explanation of the similarities among living things. Organisms evolve by a process of descent with modification. Changes, and therefore differences, gradually accumulate over the generations. The more recent the last common ancestor of a group of organisms, the less their differentiation; similarities of form and function reflect phylogenetic propinquity. Accordingly, phylogenetic affinities can be inferred on the basis of relative similarity. Convergent and parallel evolution A distinction has to be made between resemblances due to propinquity of descent and those due only to similarity of function. Correspondence of features in different organisms that is due to inheritance from a common ancestor is called homology. The forelimbs of humans, whales, dogs, and bats are homologous. The skeletons of these limbs are all constructed of bones arranged according to the same pattern because they derive from an ancestor with similarly arranged forelimbs. Correspondence of features due to similarity of function but not related to common descent is termed analogy. The wings of birds and of flies are analogous. Their wings are not modified versions of a structure present in a common ancestor but rather have developed independently as adaptations to a common function, flying. The similarities between the wings of bats and birds are partially homologous and partially analogous. The skeletal structure is homologous, owing to common descent from the forelimb of a reptilian ancestor; but the modifications for flying are different and independently evolved, and in this respect they are analogous. Features that become more rather than less similar through independent evolution are said to be convergent. Convergence is often associated with similarity of function, as in the evolution of wings in birds, bats, and flies. The shark (a fish) and the dolphin (a mammal) are much alike in external morphology; their similarities are due to convergence, since they have evolved independently as adaptations to aquatic life. Taxonomists also speak of parallel evolution. Parallelism and convergence are not always clearly distinguishable. Strictly speaking, convergent evolution occurs when descendants resemble each other more than their ancestors did with respect to some feature. Parallel evolution implies that two or more lineages have changed in similar ways, so that the evolved descendants are as similar to each other as their ancestors were. The evolution of marsupials in Australia paralleled the evolution of placental mammals in other parts of the world. There are Australian marsupials resembling true wolves, cats, mice, squirrels, moles, groundhogs, and anteaters. These placental mammals and the corresponding Australian marsupials evolved independently but in parallel lines by reason of their adaptation to similar ways of life. Some resemblances between a true anteater ( Myrmecophaga) and a marsupial anteater (Myrmecobius) are due to homologyboth are mammals. Others are due to analogyboth feed on ants. Parallel and convergent evolution are also common in plants. New World cacti and African euphorbias are alike in overall appearance although they belong to separate families. Both are succulent, spiny, water-storing plants adapted to the arid conditions of the desert. Their corresponding morphologies have evolved independently in response to similar environmental challenges. Homology can be recognized not only between different organisms but also between repetitive structures of the same organism. This has been called serial homology. There is serial homology, for example, between the arms and legs of humans, among the seven cervical vertebrae of mammals, and among the branches or leaves of a tree. The jointed appendages of arthropods are elaborate examples of serial homology. Crayfish have 19 pairs of appendages, all built according to the same basic pattern but serving diverse functionssensing, chewing, food handling, walking, mating, egg carrying, and swimming. Serial homologies are not useful in reconstructing the phylogenetic relationships of organisms, but they are an important dimension of the evolutionary process. Relationships in some sense akin to those between serial homologs exist at the molecular level between genes and proteins derived from ancestral gene duplications. The genes coding for the various hemoglobin chains are an example. About 500,000,000 years ago a chromosome segment carrying the gene coding for hemoglobin became duplicated, so that the genes in the different segments thereafter evolved in somewhat different ways, one eventually giving rise to the modern gene coding for a hemoglobin, the other for b hemoglobin. The b hemoglobin gene became duplicated again about 200,000,000 years ago, giving rise to the g (fetal) hemoglobin. The a, b, g, and other hemoglobin genes are homologous; similarities in their nucleotide sequences occur because they are modified descendants of a single ancestral sequence. There are two ways of comparing homology between hemoglobins. One is to compare the same hemoglobinfor instance, the a chainin different species of animals. The degree of divergence between the a chains reflects the degree of the evolutionary relationship among the organisms, because the hemoglobin chains have evolved independently of one another since the time of divergence of the lineages leading to the present-day organisms. A second way is to make comparisons between, say, the a and b hemoglobins of a single species. The degree of divergence between the different globin chains reflects the degree of relationship among the genes coding for them. The different globins have evolved independently of each other since the time of duplication of their ancestral genes. Comparisons between homologous genes or proteins within a given organism provide information about the phylogenetic history of the genes and, hence, about the historical sequence of the gene duplication events. Whether similar features in different organisms are homologous or analogousor simply accidentalcannot always be decided unambiguously, but the distinction must be made in order to determine phylogenetic relationships. Moreover, the degrees of homology must be quantified in some way so as to determine the propinquity of common descent among species. Difficulties arise here as well. In the case of forelimbs, it is not clear whether the homologies are greater between man and bird than between man and reptile, or between man and reptile than between man and bat. The fossil record sometimes provides the appropriate information, even though the record is deficient. Fossil evidence must be examined together with the evidence from comparative studies of living forms and with the quantitative estimates provided by comparative studies of proteins and nucleic acids.