Natural selection is the system which enables the forms of life to adapt to changes in their environment. It is a self-regulating system. Alfred Russel Wallace, the co-discoverer of natural selection who precipitated the writing of the Origin of Species, took as an analogy one of the first self-regulating systems devised by man - the centrifugal governor which Watt fitted to his steam engine:
The action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident; and in like manner no unbalanced deficiency in the animal kingdom can ever reach any conspicuous magnitude, because it would make itself felt at the very first step, by rendering existence difficult and extinction almost sure soon to follow.
Wallace 1858: On the Tendency of Varieties to Depart indefinitely from the Original Type
Just as the thermostatic system of the human body regulates the blood temperature to a constant level, despite wide variations in external temperatures, so natural selection adjusts a species to keep it in the best possible harmony with the environment in which it is currently living. Animals typically produce far more young than can conceivably survive and each generation produces many variations. Only those best adapted to current conditions live to reproduce and to pass on to their offspring the variations which have enabled them to survive.
Thus a gradually changing environment can induce progressive adaptation to the new conditions which it imposes. Natural selection ensures the survival of the fittest - those most fit to survive and flourish in a particular environment at a particular time. But it is increasingly realised that natural selection is blind. It cannot predict the perhaps dramatic changes of the future. Greater fitness at a particular time, in a particular environment, frequently means specialisation - and therefore less fitness for other environments. Evolutionary lines which specialise too much are often dead ends. In the words of the evolutionist Theodosius Dobzhansky:
It may seem strange that evolution controlled by natural selection so often leads to overspecialization and consequent extinction. But this is only a consequence of the fact repeatedly emphasised above that natural selection is opportunistic and like any natural process other than the human mind, lacks foresight. Selection perpetuates what is advantageous here and now, and fails to perpetuate what may be beneficial in the future unless it is immediately useful.
Dobzhansky: Evolution, Genetics and Man P369.
Successful populations, like those of the ocean, are steady, unchanging populations. In genetic terms, they remain constant, with little genetic drift. Natural selection may enable them to gradually get just that little bit better at doing what they do, but they do not modify their limbs for new purposes, or start to breath through lungs instead of gills. They just carry on as they always so successfully have done, until something happens which breaks all the previous rules: and this typically results in their extinction. They are then replaced by new populations which are the progeny of previously less successful animals, but which have somehow acquired a new and decisive advantage over them.
In the short term, survival is best ensured by maximum adaptation to the environment in which an animal finds itself. This is the business of natural selection. But in the long term, the animal lines which prosper are those which (whilst adapting enough to ensure survival) retain sufficient flexibility to adapt to totally changed conditions when this becomes necessary. And of course this adaptability may make future success possible but cannot guarantee it - it all depends on how the circumstances and opportunities of the future pan out.
A simple example is the forelimb. In most mammals, this has been adapted for efficient quadrupedal locomotion, with some allowance often also made for weaponry (claws or hooves) and perhaps digging. Most four-footed mammals became less good at gripping branches and holding objects. But primates retained the original five digits. Despite the general evolutionary advantages of paws and hooves, they retained their digital flexibility.
It happened subsequently that primates became highly intelligent and that a particular group of primates became bipedal. The combination of high intelligence with forelimbs able to manipulate objects proved to offer completely new opportunities which nobody could have predicted.
Natural selection provides an excellent answer to the question "Why are life-forms so well adapted to their environment?" - the question which the pre-Darwinian naturalists could only answer by the argument of divine design. But to be confident that natural selection is enough to explain evolution requires faith rather than just rational deduction. Science requires more evidence.
There has never really been any question of evidence or proof. Darwinian materialism is based, like Christianity, or like Marxism, on belief. It is an answer to the nineteenth century question "If I cannot any longer believe in the book of Genesis, what can I believe in?" For those of us who grew up in the twentieth century and have never dreamed of believing in the book of Genesis, or any other simple religious package, this is an unnecessary question.
History and Prehistory
Evolution is the study of the pre-human past. Like the study of the human past which we call history, it is a fascinating story, with much to teach us. But perhaps we should approach it with open minds, in the spirit of historians rather than theologians.
Historians first try to find out what happened. Then they seek to explain how and why particular events happened. They do so not in terms of abstract theory or dogmatic belief but by studying the people, background and circumstances of the period leading up to the events in which they are interested. Only a few historians have claimed that the past can be explained in terms of a consistent long-term theoretical pattern. Marx did: he claimed to have done for history what he thought Darwin had done for evolution. For a considerable time, many people in large areas of the world believed that Marx had indeed done so. Very few people still retain that belief.
Evolution is extraordinary and cannot be easily explained. Our first feeling is one of wonder. How could it possibly have happened that, even over hundreds of millions of years, primitive and relatively simple animals - to leave out other types of life - could have evolved to produce the extraordinary wild fauna of a hundred thousand years ago? And who could have predicted that in only a short stretch of geological time, almost all the great mammals of that period would have disappeared, in a world totally dominated on land and sea by - of all things - a primate!
And is it not even more extraordinary that this primate has developed ways of researching the past and (within limits) understanding what has happened? Let us not stretch probability even further by claiming that the primate has also discovered exactly why it happened.
Professor Thomas Alerstam ends his fascinating book Bird Migration with these words:
We are compelled to resign ourselves to continuing uncertainty and confusion over how birds find the right migration route...
So, this book ends with an unsolved mystery. Actually I do not think that this is any major shortcoming. It might be hoped that we humans would gain a greater degree of inspiration from unsolved mysteries than from what we believe we know - in great things as in small. May the birds continue to fly over the earth, and may mankind wonder and investigate.
Perhaps there will always be "unsolved mysteries" and the true scientist, like a good historian, recognises them and does not pretend to an understanding which he does not have.
Speciation: the Origin of Species