Two of the most spectacular phenomena one can find in nature are the extraordinary way in which species adapt to their environment and the remarkable diversity of species. Adaptation is the consequence of natural selection, a mechanism that is both simple and unrelenting. Selection has only one criterion for evaluation: the reproductive success of the individual. In order to meet the needs of selection, the individual has to perform well in every way, never slowing down, starting as of the moment of birth or even before. There are many who fall by the wayside. The rare winners have the great privilege of becoming ancestors. In order for it to work, selection requires a wide range of individuals within a single species. The principal role of sexual reproduction is to provide nature with such diversity. However, because it makes its selection so rigorously, the selection system itself can destroy the diversity on which it flourishes. This is the price to be paid when, for example, you wish to develop and improve the bat’s flight and sonar or the mosquito’s skin piercing mouthpart or the eagle’s keen sight.

Natural selection, thus, provides a large part of the explanation as to how species adapt to their environment and why all species, except when their environment is suddenly disturbed, are well adapted to their respective milieus. However, selection has nothing to do with the fact that there are so many different species. Why should we have rainbow trout as well as brook trout, or bobcats as well as Canada lynx, two species that are almost identical, but nevertheless distinct? Why are all these living organisms not still bacteria? After all, over the first 80% of the story of life on earth from between - 3,800 to - 800 million years, there was nothing but bacteria. What was it that triggered off the arrival of organisms other than bacteria 800 million years ago? Or what caused the polar bear to emerge from a species similar to the brown bear only some tens of thousand years ago?

Natural selection has got nothing to do with the triggering of the arrival of a species, nor with its diversity. The trigger effect is due to two phenomena. First of all, life is always difficult for all species. Indeed, since individuals need to put as much energy as possible into reproduction in order to meet the needs of selection, they will always use the least possible amount of resources to ensure survival. In the second place, since individuals are always at the very edges of survival, they are constantly open to passing opportunities as that might help their life. Natural economics are so rigorous that it is as profitable to be an opportunist as to be well-adapted.

It is probable that in most cases, the organism is not equipped to profit from a casual opportunity (a herbivore cannot take advantage of the arrival on the scene of a new insect). Nevertheless in some rare cases, the organism happens to be at the right place at the right moment and to have made the adaptations that are needed to benefit from a change in its environment. This accidental combination of favourable conditions may thus trigger off the emergence of a new way of life. As soon as the organism begins to benefit from these new circumstances, natural selection can start its work and improve the ability of the species to live in these different conditions, to the point where it might even become a new species. What matters here is that selection has got nothing to do with the triggering of the process. What triggers the emerging of a new species is circumstantial chance. The species arrives at point P in its history and is equipped with the necessary means to adapt to its environment, skills that have been acquired over thousands of generations of rigorous selection. When it comes to point P, there is suddenly a change in its environment. The combination of this change, on the one hand, and the fitness of the species, on the other, is purely random or accidental. The trigger for the origin of the species is thus a mixture of history and chance. That is what we call contingency and it is what mainly explains the great diversity of species that can be observed. The extraordinary adaptation made by the polar bear is the result of natural selection, but the existence of the bear itself is purely accidental. Which gives us another reason to be happy.

Dr. Cyrille Barrette
Full professor
Biology Department
Laval University

May 2003