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Science cannot do without chance

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We can well know the causes at the origin of the mutation of organisms but we cannot, with very few exceptions, predict its future evolution: determinism does not mean predictability

Given the interest aroused by readers of this page on the occasion of a discussion of some general aspects of biological evolution, I would like to suggest some additional considerations to illustrate a basic principle: A physical process such as that described by Darwin can be deterministic, but its outcome is unpredictable and often also very chaotic in its historical discovery. In other words, we can know very well why populations or living species change, without being able in any way, except for a few very special cases, to predict their future evolution.

This seeming inconsistency can easily be illustrated by reasoning first The general mechanism by which Darwinian evolution takes place: When a population perpetuates itself by creating copies of the individuals it comprises and when individuals have a limited existence in time, so long as the copies created are not perfect, a variety of individuals will be created. Under conditions ideally devoid of any constraint, each individual would have the same probability as everyone else of contributing to the next generation; But as far as biological organisms are concerned, the constraints are innumerable, beginning with the presence of other individuals competing for the same resources needed for replication, and it follows that the population will soon be enriched in individuals selected from among those that differ from others able to improve the chances of leaving capable offspring on reproduction.

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Now, the above statement has as general validity as the one defining the sum of the interior angles of a triangle: that is, it is a logical probabilistic derivation which cannot be contradicted in terms of its application. though, It is interesting to note that this statement contains nothing that could allow us to make detailed predictions about the course of evolution: does not specify, in particular, the criteria which determine which species of a given 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 conditions in which he finds himself living and reproducing; That is, it is a child of the phenotype (and therefore also the genotype) of the individual and of the particular environment in which he finds himself directing his existence.

Now, both the varieties which are produced in each generation of the population, and the conditions in which they will find themselves living, are strongly conditioned by issue. The shell weight in the environment can be limited in some periods, for example in stable environmental conditions in a particular geographical area, in order to generate stable selective effects, but in any case the diversity of individuals that appear in each generation is random (taking into account also That the mechanism of many spontaneous mutations is quantitative), therefore We cannot know in advance which type of individual will become more fit, even under stable environmental conditions. In these stable environmental conditions, we can at most expect that individuals endowed with certain functions will be selected, and so that the population will tend to evolve traits capable of performing these functions; However, we can’t know which traits will actually evolve, nor when, nor on a molecular level how, until much later.

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However, in fact, geologically speaking, the ecological and selective conditions are stable for long periods of time exception: the simple evolution of new species would be, and indeed is, sufficient to radically change the ecological panorama (think of the emergence of a new parasite), as well as the climate, catastrophic changes of all kinds, the indiscriminate colonization of new environments, the arrival of alien species and countless other events of a historical and therefore inconsequential nature. Formalities all contribute to the eclectic, time-shifting forces of history. Not to mention the purely random effects of sexual selection, which can even be adaptive, of the random exchange of genetic material between organisms in every living kingdom and of the thousands of other mechanisms that randomly increase variation or randomly change direction. of choice. So here’s why A deterministic law, such as the one that explains the evolution of a group of individuals with a finite life cycle based on reward selection for the best replicators, can only be used to illustrate the engine’s performance at work in nature, but not the ways it would travel to generate the incredible variety of forms that survive and survive..

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