IN AN artide about the Darwin Wallace theory of evolution in this column last month, I declared that "evolution is a fact". A number of letters to The Irish Times took me to task for making this statement. These letters raise an important general matter: what is meant by the term "scientific theory", and does it differ from the general use of the word theory general use of the term theory and the scientific use of the term are quite different, and this often causes confusion. When someone says in general conversation, "I have a theory about that", what is usually meant is that he/she has a hunch as to the true explanation of the phenomenon. Theory in this sense may be informed by some objective evidence, maybe not. But, if so, there will usually be little supporting evidence.
A generally accepted scientific theory is a much grander and more formal matter. Scientific theories do not arise casually. An explanation of a phenomenon in science is elevated to the status of a general theory only after exhaustive study of objective data and after much testing of the idea.
The theory is intimately tied to the objective evidence and becomes generally accepted only because of the weight of the underpinning evidence. Furthermore, a theory, in addition to explaining the observable behaviour of a phenomenon, must also have predictive powers, i.e. it should be able to predict aspects of the phenomenon that have yet to be observed.
An established scientific theory, therefore, cannot be easily dismissed. It is possible to overturn an established scientific theory, of course, but this can only be done on the basis of sufficient reproducible and natural contrary evidence. This cannot be easily done, in view of the way scientific theories are constructed in the first place.
Although established scientific theories will not be readily overturned, they are in fact formulated in such a manner as to be refutable on the basis of appropriate evidence. A defining difference between a religious dogma (e.g. the Assumption) and a scientific theory is that the dogma is not refutable. It is an assertion that brooks no challenge.
A scientific theory must constantly pass the test of objective challenge. Such testing refines the theory; sometimes it modifies it; and occasionally it overturns it. In all cases the natural objective evidence decides. This is how science progresses.
There is a device used in science that more closely approximates to the general use of the term theory. This is the hypothesis. By and large, scientists do not indiscriminately observe nature and note objective data. Usually the data is collected in a discriminating and systematic way, by performing experiments based on hypotheses, and noting the results.
A scientific hypothesis is an "educated hunch" as to an explanation of a phenomenon. It is "educated" because it is based on a knowledge of past scientific observations in the area. It is a "hunch" because it is a creative act (two scientists with equal knowledge of an area will not necessarily formulate identical hypotheses).
Now, to come back to evolution. I want to differentiate between the phenomenon of evolution and the theory of evolution. In my article of April 1st, I said evolution is a fact. I did not say the theory of evolution is a fact - it cannot be declared a fact because then it would not be refutable and would not qualify as a scientific theory.
It is, of course, a cornerstone theory in biology, it explains a multitude of objective observations and it has powerful predictive force. Without the unifying theory of evolution the biological sciences would become an incoherent jumble.
The main tenet of the theory of evolution is that the species of life that now populate the Earth arose from simpler forms and, going back far enough, life first began as a single simple form.
The mechanism underlying evolution is natural selection whereby organisms that are better adapted to the environment thrive and reproduce more successfully than other organisms that are less well adapted.
New variety is introduced by random changes (mutations) in genetic material, some of which confer new benefits on an organism that can then be nurtured by natural selection. The molecular nature of the genetic material (DNA) is known, and the nature of the genetic code and of mutation is well established.
The theory of evolution is based on a vast catalogue of objective evidence: the fossil record, genetics, molecular biology, field observations of natural species changes etc. Results from one field of study support results from other fields, e.g. species that are placed closer together on the "evolutionary tree", based on structural and other evidence, are seen by molecular biology to be more closely related on the molecular level than species that are farther apart on the tree.
There have been many field observations of natural selection at work. In England, the peppered moth exists in a light and dark variety. In Darwin's time the light variety predominated because it was camouflaged against the light lichen covered tree barks, and so hidden from predators.
Later, when pollution from the Industrial Revolution darkened the trees, the dark variety became predominant. In 1952 laws were passed to clean the air, the trees became less dark and the lighter coloured moth population increased once more.
Field observations also show that one species can change into another. The herring gull (Larus argentatus) is common in Britain. Travel west to America and you also see the species, but now it is slightly different. Go west again to Siberia and the herring gull is now starting to resemble the lesser black backed gull.
Continue moving west and eventually the lesser black backed gull becomes a separate species, Larus fuscus, different from the herring gull, with which it will not inter breed. However, throughout most of the ring around the world local gulls are able to breed with gulls adjacent to them.
The theory of evolution is a grand and imposing structure. It cannot be dismissed by sporadic, observations and counter arguments. If it is ever to fall apart it will only be as a result of an enormous scientific assault, and its fall would probably be the single biggest revolution ever in the history of science.
So what do I mean by the statement "Evolution is a fact". Here I am talking about the general phenomenon of evolution, i.e. that life historically has proceeded from simpler to more complex forms and that species are not fixed and immutable.
The theory of evolution goes much further by explaining the mechanism whereby evolution happens and by tracing successive specific forms through the fossil records. I have never known a scientist who did not accept the general phenomenon of evolution, but I have met some who find serious fault with the Darwin Wallace theory, e.g. Sir Fred Hoyle.
The evidence that life has evolved, in the general sense, is so undeniable and overwhelming that it could only be denied by ignoring objective evidence. Evolution is a fact. The theory of evolution is a theory, but not "only a theory".