The theory of evolution by natural selection is the central unifying theory in biology. Many people do not believe in evolution, however, particularly in the United States. Some US school boards have ruled that students must be taught that evolution is just one of a number of theories to explain the diversity of life on earth.
Biologists are concerned about this public challenge to science and are mounting an offensive against it. As most objections to evolution come from creationists, last year John Rennie, editor in chief of Scientific American, answered their commonest arguments.
The accepted theory of evolution holds that the environment naturally selects those living varieties that are best suited to survive and procreate. It holds that the species of life now found on earth are the modified descendants of very many previous types that have inhabited the earth since life began, probably as a simple single variety, about 3.8 billion years ago. I have written about evolution before and outlined some of the mass of evidence that has been accumulated from palaeontology, genetics, ecology, molecular biology and other areas to establish evolution beyond any reasonable doubt. I won't rehearse this evidence again.
The creationists literally interpret the Genesis account of creation in the Bible and believe that God created all living species that now inhabit the earth between 7,000 and 10,000 years ago. This literal approach is not shared by mainstream Christianity, which accepts the theory of evolution.
The US experience is worrying. A Gallup poll from 1999 and a National Science Board poll from 2000 both showed that about half of Americans reject evolution. Although they were more likely to accept evolution the better educated they were, the absolute figures were also worrying. Forty per cent of US college graduates and almost 30 per cent of postgraduates and professionals agreed that "God created humans in their present form within the past 10,000 years or so".
One of the arguments used by the creationists to minimise the perceived importance of evolution is to point out that evolution is neither scientific fact nor law but only theory. Rennie answered this objection very nicely in his article.
The common understanding of theory is that it falls halfway between hypothesis and law. As Rennie points out, however, science does not use the concept in this manner. The National Academy of Sciences defines a scientific theory as "a well- substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences and tested hypotheses".
Scientific laws, such as Newton's laws of motion, are descriptive generalisations about nature. So a theory is an explanation and a law is a generalised observation, and no amount of validation will turn a theory into a law. But this does not mean that science has reservations about the truth of its many theories, such as the atomic theory and the theory of relativity.
An essential part of the power of science is that its conclusions are not dogmas: scientific theories are open to modification in the light of compelling new evidence. This, of course, applies to scientific laws as well. Newton's laws of motion, for example, were thought to be the last word on the subject until Einstein showed that they are inaccurate at speeds approaching that of light, when they must be modified.
The National Academy of Sciences defines a fact as "an observation that has been repeatedly confirmed and that for all practical purposes is accepted as true". There is a mountain of evidence that organisms have evolved through time. Nobody observed these changes happening and, in that sense, the evidence for evolution is indirect. It is nevertheless clear and compelling.
All the sciences frequently rely on indirect evidence. Atoms were not seen in a microscope until the early 1970s, for example. Until then, atomic theory had relied on indirect evidence of their existence. But the absence of direct observation of atoms before the 1970s did not make conclusions about their existence less certain. In the same way, the indirect evidence for evolution is so compelling that we are justified in treating evolution as fact.
The creationists have a two-fold fear. First, they are worried that if the literal world of the Bible is shown to be incorrect in any part, then the whole authority of the Bible will unravel. Second, they fear that science has an agenda to supplant Christianity with materialistic atheism. They are wrong on both counts.
Some parts of the Bible must be interpreted as conveying a moral rather than a literal message. Christians are in debt to science for identifying some of these parts and thereby facilitating their proper understanding.
The function of science is to provide, in so far as possible, a natural understanding of the natural world. Although science rejects supernatural explanations that conflict with its natural explanations, it has no brief to go beyond this and to deny the supernatural as such.
Some scientists, such as Richard Dawkins, may well be motivated to replace religion with materialism, but in this they are marching to the beat of a personal drum, not to the drum of science.
• William Reville is associate professor of biochemistry and director of microscopy at UCC