Tool Module: Darwin's Natural Selection

All scientists now agree that all the species that we see today have evolved from primitive forms of life that existed several million years ago. The existence of biological evolution is thus recognized as a fact for which there are countless pieces of historical evidence.

Biologists are less certain, however, as to what mechanisms were the source of this evolution. Darwin’s great contribution is that he not only revealed the phenomenon of evolution and supported it with many examples, but also provided a number of indications as to how it may have occurred.

According to Darwin, the driving force behind evolution is a phenomenon that he called natural selection. Today we know that natural selection is not the only mechanism of evolution, but it has certainly played a preponderant role.

The process of natural selection may be summarized as follows:

• organisms display certain variations in all aspects of their biology and behaviour;
• these variations are hereditary; individuals inherit them from their parents and pass them on to their descendants;
• these hereditary variations can have positive or negative effects on survival and reproduction, depending on the environment in which the organism lives;
• those individuals that display the variations that are the most favourable in their environment will live longer and leave more descendants who also possess these variations;
• after many generations, these favourable variations will be found in all individuals in the population.

The more limited the resources in a given environment, the more efficient this process will be, thus placing greater “evolutionary pressure” on those individuals who are less well adapted to this particular environment and favouring the reproduction of those individuals whose traits give them an advantage in it.

Giraffes, for example, did not get their long necks by stretching them to reach the leaves on trees (which is what people thought before Darwin). Instead, within the entire giraffe population, some individuals underwent mutations that gave them slightly longer necks than other giraffes. This gave them an advantage, because they could obtain food more easily. Hence they remained in better health and so produced more descendants. These descendants inherited their slightly longer necks. If food became scarce and giraffes needed to be able to reach the highest branches of trees to survive, then those giraffes with shorter necks would die, leaving the entire habitat to the giraffes with longer necks.

But if the environment in which a species lives should change (as happens all the time), then a trait that used to constitute a handicap could quite possibly become an asset for survival, and it will be the individuals with this trait who will then leave more descendants.

It is therefore important to note that mutations, which occur at random and are the origin of any given trait in an individual, are never good or bad in and of themselves, but only in relation to the environment in which this individual lives.

It should also be noted that it is the diversity of life as a whole that ensures its future evolution.

Another point of interest is that Darwin himself did not use the term “evolution”, which in his era was too closely associated with the idea of steady progress toward an ideal. On the contrary, in his view, life forms as elementary as an amoeba could be perfectly adapted to their environment. In other words, human beings were no closer to any evolutionary ideal than the other forms of life were.

Lastly, for Darwin, not only were organisms’ physical characteristics subject to natural selection, but so were their psychology and behaviour, thus explaining for the first time how all of the highly sophisticated instincts observed in the animal kingdom may have come to be.