Science cannot do without chance

Given the interest aroused among the readers of this page on the occasion of the discussion of some general aspect of biological evolution, I would like to propose some further considerations to illustrate a basic principle: a physical process such as the one described by Darwin can be deterministic, but not predictable in its outcomes and often also very messy in its historical unfolding. In other words, we can know very well why populations or living species change, without however being able in any way, except perhaps in a few very particular cases, to predict their future evolution.

This apparent paradox can easily be clarified by considering first the general mechanism in which Darwinian evolution consists: when a population perpetuates itself by generating copies of the individuals of which it is composed and when the individuals have a limited existence in time, as long as the generated copies are not perfect, a certain variety of individuals will be generated. Under conditions ideally free from any constraints, each individual will have the same probability as any other of contributing to the next generation; but since as far as biological organisms are concerned the constraints are innumerable, starting with the presence of other individuals who compete for the same resources necessary for replication, it follows that soon the population will be enriched in individuals selected among those whose differences from the others are able to improve the chances of leaving offspring capable of reproducing.

Now, the above statement has as general a validity as the one that fixes the sum of the internal angles of a triangle: that is, it is a logical-probabilistic derivation which, in its conditions of application, cannot be contradicted. Despite this, however, it is interesting to note that this statement contains nothing that could allow us to make detailed predictions about an evolutionary trajectory: does not establish, in particular, the criteria that establish which varieties of a certain species are capable of leaving a greater number of fertile descendants. This characteristic is determined by the interaction between the characteristics of each individual and the circumstances in which he finds himself living and reproducing; that is, it is the daughter of the phenotype (and therefore also of the genotype) of an individual and of the particular environment in which it finds itself conducting its existence.

Now, both the varieties that are produced in each generation in a population, and the conditions in which they will find themselves living, are strongly conditioned by the case. The weight of chance in the environment can be limited in some periods, for example in stable ecological conditions in a certain geographical area, so as to generate constant selective effects, but in any case the variety of individuals that appear in each generation is random (also considering that the mechanism of many spontaneous mutations is quantum), therefore we cannot know in advance what kind of individuals will turn out to be the most suitable, even under stable environmental conditions. In these stable environmental conditions, we can at most predict that individuals endowed with certain functions will be selected, and therefore that a population will tend to evolve traits capable of performing these functions; however, we cannot know which traits will actually be evolved, nor when, nor, at the molecular level, how, except in retrospect.

In fact, however, geologically speaking, environmental and selective conditions are stable for long periods of time an exception: the simple evolution of new species would be and is already sufficient to drastically change the ecological panorama (think of the appearance of a new parasite), and in addition climate, catastrophic changes of all kinds, random colonization of new environments, arrival of alien species and a myriad of other events of a historical and therefore casual nature all contribute to the selective forces of history varying over time. This is not to mention the purely stochastic effects of sexual selection, which can even be counter-adaptive, of the random exchange of genetic material between organisms of every living kingdom and of a thousand other mechanisms that randomly increase variability or change direction equally randomly. of the selection. So here’s why a deterministic law, such as the one that explains the evolution of a population of individuals with a limited life cycle on the basis of the reward selection of the best reproducers, can only be used to illustrate the functioning of the engine at work in nature, but not the roads it will travel in generating the incredible variety of forms that live and are lived.