Fields of Study

Ecology, embryology, evolutionary science, genetics, paleontology, population biology, zoology

Summary

Evolution is the process of change in biological populations. Historically, it is also the theory that biological species undergo sufficient change with time to give rise to new species.

Principal Terms

Basic Principles

Evolution is the theory that biological species undergo sufficient change with time to give rise to new species. The concept of evolution has ancient roots. Anaximander suggested in the sixth century bce that life had originated in the seas and that humans had evolved from fish. Empedocles (fifth century bce) and Lucretius (first century bce), in a sense, grasped the concepts of adaptation and natural selection. They taught that bodies had originally formed from the random combination of parts, but that only harmoniously functioning combinations could survive and reproduce. Lucretius even said that the mythical centaur, half horse and half human, could never have existed because the human teeth and stomach would be incapable of chewing and digesting the kind of grassy food needed to nourish the horse’s body.

For two thousand years, however, evolution was considered an impossibility. Plato’s theory of forms (also called his “theory of ideas”) gave rise to the notion that each species had an unchanging “essence” incapable of evolutionary change. As a result, most scientists from Aristotle to Carolus Linnaeus in the eighteenth century insisted upon the immutability of species. Many of these scientists tried to arrange all species in a single linear sequence known as the scale of being (also called the chain of being and the scala naturae), a concept supported well into the nineteenth century by many philosophers and theologians as well. The sequence in this scale of being was usually interpreted as a static “ladder of perfection” in God’s creation, arranged from higher to lower forms.

The scale had to be continuous, for any gap would detract from the perfection of God’s creation. Much exploration was devoted to searching for “missing links” in the chain, but it was generally agreed that the entire system was static and incapable of evolutionary change. Pierre-Louis Moreau de Maupertuis, in the eighteenth century, and Jean-Baptiste Lamarck were among the scientists who tried to reinterpret the scale of being as an evolutionary sequence, but this single-sequence idea was later replaced by Charles Darwin’s concept of branching evolution. Georges Cuvier finally showed that the major groups of animals had such strikingly different anatomical structures that no possible scale of being could connect them all; the idea of a scale of being lost most of its scientific support as a result.

The Struggle to Conceptualize Evolution

The theory that new biological species could arise from changes in existing species was not readily accepted at first. Linnaeus and other classical biologists emphasized the immutability of species under the Platonic-Aristotelian concept of essentialism. Those who believed in the concept of evolution realized that no such idea could gain acceptance until a suitable mechanism of evolution could be found. Many possible mechanisms were therefore proposed. Étienne Geoffroy Saint-Hilaire proposed that the environment directly induced physiological changes, which he thought would be inherited, a theory now known as Geoffroyism. Lamarck proposed that there was an overall linear ascent of the scale of being but that organisms could also adapt to local environments by voluntary exercise, which would strengthen the organs used; unused organs would deteriorate. He thought that the characteristics acquired by use and disuse would be passed on to later generations, but the inheritance of acquired characteristics was later disproved. Central to both these explanations was the concept of adaptation, or the possession by organisms of characteristics that suit them to their environments or to their ways of life. In eighteenth century England, the Reverend William Paley and his numerous scientific supporters believed that such adaptations could be explained only by the action of an omnipotent, benevolent God. In criticizing Lamarck, the supporters of Paley pointed out that birds migrated toward warmer climates before winter set in and that the heart of the human fetus had features that anticipated the changes of function that take place at birth. No amount of use and disuse could explain these cases of anticipation, they claimed; only an omniscient God who could foretell future events could have designed things with their future utility in mind.

The nineteenth century witnessed a number of books asserting that living species had evolved from earlier ones. Before 1859, these works were often more geological than biological in content. Most successful among them was the anonymously published Vestiges of the Natural History of Creation (1844), written by Robert Chambers. Books of this genre sold well but contained many flaws. They proposed no mechanism to account for evolutionary change. They supported the outmoded concept of a scale of being, often as a single sequence of evolutionary “progress.” In geology, they supported the outmoded theory of catastrophism, an idea that the history of the earth had been characterized by great cataclysmic upheavals. From 1830 on, however, that theory was being replaced by the modern theory of uniformitarianism, championed by Charles Lyell. Charles Darwin read these books and knew their faults, especially their lack of a mechanism that was compatible with Lyell’s geology. In his own work, Darwin carefully tried to avoid the shortcomings of these books.

Darwin’s Revolution in Biological Thought

Darwin brought about the greatest revolution in biological thought by proposing not only a theory of branching evolution but also a mechanism of natural selection to explain how it occurred. Much of Darwin’s evidence was gathered during his voyage around the world aboard HMS Beagle. Darwin’s stop in the Galápagos Islands and his study of tortoises and finchlike birds on these islands are usually credited with convincing him that evolution was a branching process and that adaptation to local environments was an essential part of the evolutionary process. Adaptation, he later concluded, came about through natural selection, a process that killed the maladapted variations and allowed only the well-adapted ones to survive and pass on their hereditary traits. After returning to England from his voyage, Darwin raised pigeons, consulted with various animal breeders about changes in domestic breeds, and investigated other phenomena that later enabled him to demonstrate natural selection and its power to produce evolutionary change. Darwin’s greatest contribution was that he proposed a suitable mechanism by which permanent organic change could take place. All living species, he said, were quite variable, and much of this variation was heritable. Also, most organisms produce far more eggs, sperm, seeds, or offspring than can possibly survive, and the vast majority of them die. In this process, some variations face certain death while others survive in greater or lesser proportion. Darwin called the result of this process “natural selection,” the capacity of some hereditary variations (now called genotypes) to leave more viable offspring than others, with many leaving none at all. Darwin used this theory of natural selection to explain the form of branching evolution that has become generally accepted among scientists.

Darwin delayed the publication of his book for seventeen years after he wrote his first manuscript version. He might have waited even longer, except that his hand was forced. From the East Indies, another British scientist, Alfred Russel Wallace, had written a description of the very same theory and submitted it to Darwin for his comments. Darwin showed Wallace’s letter to Lyell, who urged that both Darwin’s andWallace’s contributions be published, along with documented evidence showing that both had arrived at the same ideas independently. Darwin’s great book, On the Origin of Species by Means of Natural Selection, was published in 1859, and it quickly wonmost of the scientific community to a support of the concept of branching evolution. In his later years, Darwin also published The Descent of Man and Selection in Relation to Sex (1871), in which he outlined his theory of sexual selection. According to this theory, the agent that determines the composition of the next generationmayoften be the opposite sex. An organism may be well adapted to live, but unless it can mate and leave offspring, it will not contribute to the next or to future generations.

Acceptance of Darwinism in the Twentieth Century

In the early 1900’s, the rise of Mendelian genetics (named for botanist Gregor Mendel) initially resulted in challenges to Darwinism. Hugo de Vries proposed that evolution occurred by random mutations, which were not necessarily adaptive. This idea was subsequently rejected, and Mendelian genetics was reconciled with Darwinism during the period from 1930 to 1942. According to this modern synthetic theory of evolution, mutations initially occur at random, but natural selection eliminates most of them and alters the proportions among those that survive. Over many generations, the accumulation of heritable traits produces the kind of adaptive change that Darwin and others had described. The process of branching evolution through speciation is also an important part of the modern synthesis.

The branching of the evolutionary tree has resulted in the proliferation of species from the common ancestor of each group, a process called adaptive radiation. Ultimately, all species are believed to have descended from a single common ancestor. Because of the branching nature of the evolutionary process, no one evolutionary sequence can be singled out as representing any overall trend; rather, there have been different trends in different groups. Evolution is also an opportunistic process, in the sense that it follows the path of least resistance in each case. Instead of moving in straight lines toward a predetermined goal, evolving lineages often trace meandering or circuitous paths in which each change represents a momentary increase in adaptation. Species that cannot adapt to changing conditions die out and become extinct.

Studying Evolution

Evolution is studied by a variety of methods. The ongoing process of evolution is studied in the field by ecologists, who examine various adaptations, including behavior and physiology as well as anatomy. These adaptations are also studied by botanists, who examine plants; zoologists, who examine animals; and various specialists, who work on particular kinds of animals or plants (for example, entomologists, who study insects). Some investigators capture specimens in the field, then bring back samples to the laboratory in order to examine chromosomes or analyze proteins using electrophoresis. Through these methods, scientists learn how the ongoing process of evolutionary change is working today within species or at the species level on time scales of only one or a few generations.

The long-term results of evolutionary processes are studied among living species by comparative anatomists and embryologists. Extinct organisms are studied by paleontologists, scientists who examine fossils. Biogeographers study past and present geographic distributions. All these types of scientists make comparisonsamong species in order to determine the sequence of events that took place in the evolutionary past. One method of reconstructing the branching sequences of evolution is to find homologies, deepseated resemblances that reflect common ancestry. Once the sequences are established, functional analysis can be used to suggest possible adaptive reasons for any changes that took place. The sequences of evolutionary events reconstructed by these scientists represent the history of life on the earth. This history spans many species, families, and whole orders and classes, and it covers great intervals of past geologic time, measured in many millions of years.

The Historical Context of Evolutionary Theory

The historical development of evolutionary theory should be viewed in two contexts: that of biological science and that of cultural history. The concept of evolution had been talked about for many years before 1859 and was usually rejected because no suitable mechanism had gained widespread acceptance. The fact that the phenomenon of natural selection was independently discovered by two Englishmen shows both that the time was ripe for the discovery and that the circumstances were right in late nineteenth century England.

Evolutionary biology is itself the context into which all the other biological sciences fit. Other biologists, including physiologists and molecular biologists, study how certain processes work, but it is evolutionists who study the reasons why these processes came to work in one way and not another. Organisms and their cells are built one wayand not another because their structures have evolved in a particular direction and can only be explained as the result of an evolutionary process. Not only does each biological system need to function properly, but it also must have been able to achieve its present method of functioning as the result of a long, historical, evolutionary process in which a previous method of functioning changed into the present one. If there were two or more ways of accomplishing the same result, a particular species used one of them because found it easier to evolve one method rather than another. Everything in biology is thus a detail in the ongoing history of life on the earth, because every living system evolves. Living organisms and the processes that make them function are all products of the evolutionary process and can be understood only in that context. As biologist Theodosius Dobzhansky once said, “Nothing in biology makes sense, except in the light of evolution.”

—Eli C. Minkoff

See also: Apes to hominids; Gene flow; Genetics; Hardy- Weinberg law of genetic equilibrium; Homo sapiens and human diversification; Human evolution analysis; Natural selection; Sex differences: Evolutionary origins.

Further Reading