Impact and acceptance of evolutionary theory
The theory of evolution makes statements about three different, though related, issues: (1) the fact of evolution; that is, that organisms are related by common descent; (2) evolutionary historythe details of when lineages split from one another and of the changes that occurred in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.
The first issue is the most fundamental and the one established with utmost certainty. Darwin gathered much evidence in its support, but the evidence has accumulated continuously ever since, derived from all biological disciplines. The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of the Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a fact; the evolutionary origin of organisms is accepted by virtually every biologist.
But the theory of evolution goes much beyond this first issue, the general affirmation that organisms evolve. The second and third issues involve seeking to ascertain the evolutionary relationships between particular organisms and the events of evolutionary history, as well as to explain how and why evolution takes place. These are matters of active scientific investigation. Some conclusions are well established; for example, that the chimpanzee and gorilla are more closely related to humans than is any of those three species to the baboon or other monkeys; or that natural selection, the process postulated by Darwin, explains the adaptive configuration of such features as the human eye and the wings of birds. Many matters are less certain, others are conjectural, and still otherssuch as the characteristics of the first living things and when they came aboutremain completely unknown.
Since Darwin, the theory of evolution has gradually extended its influence to other biological disciplines, from physiology to ecology and from biochemistry to systematics. All biological knowledge now includes the phenomenon of evolution. In the words of Dobzhansky, Nothing in biology makes sense except in the light of evolution.
The term evolution and the concept of change through time have also become incorporated into scientific language well beyond biology, and even into common language. Astrophysicists speak of the evolution of the solar system or of the universe; geologists, of the evolution of the Earth's mantle; psychologists, of the evolution of the mind; anthropologists, of the evolution of cultures; art historians, of the evolution of architectural styles; and couturiers, of the evolution of fashion. These and other disciplines share only the slightest commonality of meaning: the notion of gradual, and perhaps directional, change over the course of time.
Darwin's notion of natural selection has also been extended to other areas of human discourse, particularly in the fields of sociopolitical theory and economics. The extension can only be metaphorical, because in Darwin's intended meaning natural selection applies only to hereditary variations in entities endowed with biological reproduction, that is, to living organisms. That natural selection is anatural process in the living world has been taken by some as a justification for ruthless competition and for survival of the fittest in the struggle for economic advantage or for political hegemony. Social Darwinism was an influential social philosophy in some circles through the late 19th and early 20th centuries. At the other end of the political spectrum, Marxist theorists have resorted to evolution by natural selection as an explanation for mankind's political history.
These uses and abuses of the terms evolution and natural selection have in turn stimulated resistance against biological evolution and natural selection. In addition, the theory of evolution has been seen by some people as incompatible with religious beliefs, particularly those of Christianity. The first chapters of the book of Genesis describe God's creation of the world, the plants, the animals, and man. A literal interpretation of Genesis seems incompatible with the gradual evolution of humans and other organisms by natural processes. Independently of the biblical narrative, the Christian beliefs in the immortality of the soul and in man as created in the image of God have appeared to many as contrary to the evolutionary origin of man from nonhuman animals.
Religiously motivated attacks started during Darwin's lifetime. In 1874 Charles Hodge, an American Protestant theologian, published What Is Darwinism?, one of the most articulate assaults on evolutionism. Hodge perceived Darwin's theory as the most thoroughly naturalistic that can be imagined and far more atheistic than that of his predecessor Lamarck. He argued that the design of the human eye evinces that it has been planned by the Creator, like the design of a watch evinces a watchmaker. He concluded that the denial of design in nature is actually the denial of God.
Other Protestant theologians saw a solution to the difficulty in the idea that God operates through intermediate causes. The origin and motion of the planets could be explained by the law of gravity and other natural processes without denying God's creation and providence. Similarly, evolution could be seen as the natural process through which God brought living beings into existence and developed them according to his plan. Thus, A.H. Strong, the president of Rochester (N.Y.) Theological Seminary, wrote in his Systematic Theology (1885): We grant the principle of evolution, but we regard it as only the method of divine intelligence. The brutish ancestry of man was not incompatible with his excelling status as a creature in the image of God. Strong drew an analogy with Christ's miraculous conversion of water into wine: The wine in the miracle was not water because water had been used in the making of it, nor is man a brute because the brute has made some contributions to its creation.
Arguments for and against Darwin's theory came from Roman Catholic theologians as well. Gradually, well into the 20th century, evolution by natural selection came to be accepted by the majority of Christian writers. Pope Pius XII in his encyclical Humani Generis (1950; Of the Human Race) acknowledged that biological evolution was compatible with the Christian faith, although he argued that God's intervention was necessary for the creation of the human soul. In 1981 Pope John Paul II stated in an address to the Pontifical Academy of Sciences: