Evolution and natural selection
Let's talk a little about the evolution of species by natural selection, an argument that, if for some it is a definitively established concept as a factual reality, for others, either out of ignorance, or out of fanaticism, it is a reason for conflict where it would come into question one's own religious faith of monotheistic matrix and the concept of creationism associated with it. Therefore, since here we are in Italy, a country that literally floats on holy water and that is in no less serious conditions than the USA, where confusion, religious sects in profusion and widespread ignorance are the masters, since statistics indicate that more than 50 % of Italians do not even know what Evolution is, putting aside the chills that cross my back, I will try to give my small contribution so that the light of culture is not definitively extinguished by the icy wind of obscurantism, against which we must not never let your guard down, on pain of sinking into a new Middle Ages.
First of all, a little bit of Ecology
Biodiversity is a term used to indicate the variety of animal and plant species present on Earth. Another, less simple but more complete definition is: the totality of genes, species and ecosystems of a region.
The advantage of this second definition is that it summarizes the three levels in which biodiversity is traditionally studied:
- genetic diversity - given both by genetic variability and at the level of individuals belonging to the same population, as well as between populations belonging to the same species;
- diversity of species - given by the variety of species present in a given habitat;
- ecosystem diversity - given both by the great differences that exist between the different types of ecosystems, and by the variety of natural habitats and communities that interact with each other within the same ecosystem.
As for the individual terms:
- Population: Set of individuals, belonging to the same species, present in a given area.
- Habitat: Natural or semi-natural environment where an organism grows and develops, which is distinguished by its geographic, biotic and abiotic characteristics.
- Community: Set of animal and plant organisms within a given geographical area, of variable surface or volume, subjected to certain environmental conditions.
- Ecosystem:
- (a) environment characterized by the presence of biotic (autotrophic: vegetable, and heterotrophic: animal) and abiotic (light, chemical composition of the atmosphere, climatic conditions, humidity, inorganic and organic compounds present in the soil) factors in perfect balance between They;
- (b) set of living beings, the surrounding environment and chemical-physical relationships, in a well-defined space.
Evolution and natural selection
"The preservation of favorable individual differences and variations and the destruction of harmful ones have been called by me 'natural selection' or 'survival of the fittest'.Variations which are neither useful nor harmful will not be influenced by natural selection, and will remain in the state of fluctuating elements, as can be observed in certain polymorphic conditions"
(Charles Darwin, The Origin of Species, 1859, p. 147).
It took about 3.5 billion years for the incredible number of species that populate our planet to develop from the first forms of life that appeared on Earth. If we look at the "final" result (which is not final, why evolution is always in progress) and we are naively surprised, warning "perfect" forms apparently shaped by some divine intervention, in reality the story is much more complicated and laborious than a simple magical snap of the fingers. Every year new species are discovered and, of these, many have not yet been classified (to date the identified species are about 1,770,000, but scientists assume that there are probably between 10 and 15 million different species, most of which is concentrated in the tropics or along the continental shelves). All species of animals, fungi, bacteria, algae or plants, whether known or unknown, have developed through a very slow natural process that we call evolution.
An important process in evolution is natural selection. Through natural selection, organisms that have adapted better to their environment, as a result of random changes in their genetic makeup, survive better and longer.
These organisms give life to a larger progeny than other individuals belonging to their own species and, over many generations, changes can occur in the entire population; the population changes, or even gives rise to a new species.
Natural selection, a concept introduced by Charles Darwin in 1859 in the book The Origin of Species, is the mechanism by which the evolution of species and according to which, in the context of the genetic diversity of populations, there is a progressive (and cumulative) increase in the frequency of individuals with optimal characteristics (fitness) for the living environment.
Referring to the competition between individuals, Darwin described the concept of "struggle for existence", which was based on the observation that organisms, multiplying at too high a rate, produce a progeny that is quantitatively greater than that which limited natural resources can sustain, and as a result are forced into stiff competition to reach adulthood and reproduce.
Individuals of the same species differ from each other in genetic (genotype) and phenotypic (ie morphological and functional, resulting from the interaction of the genotype with the environment) characteristics. The theory of natural selection provides that within this variability, deriving from random genetic mutations, in the course of the generations following the onset of the mutation, those mutations that lead individuals to have more advantageous characteristics in given environmental conditions are favored ("selected"), thus determining, that is, an adaptive advantage (better adaptation) in terms of survival and reproduction.
Individuals better adapted to a certain habitat will find food more easily and mate more easily than other individuals of the same species that do not have these characteristics. In other words, it is the environment that selects the mutations according to the criterion of advantage described above: the genes harbingers of adaptive advantage can thus be transmitted, through reproduction, to subsequent generations and with the succession of generations, a progressive affirmation can be obtained. good genes at the expense of useless or harmful genes. The species will be able to progressively evolve thanks to the development of characteristics that will make it better adapted to the environment, up to a situation of equilibrium between the environment and the population that will persist until an environmental change triggers a new evolutionary phenomenon.
A typical example is the evolution of the neck of giraffes. Over the course of millions of years, the genetic mutations that led some individuals to have (increasingly) longer necks proved advantageous: these individuals could more easily reach the leaves of tall trees, which, in conditions of food scarcity, resulted in a better adaptation to the environment than individuals with shorter necks: better ability to obtain food, therefore greater probability of surviving, reaching the age of reproduction and reproducing, therefore, greater probability of transmitting one's genetic heritage (and therefore neck length) to subsequent generations.
The three types of natural selection
According to the frequency with which the different phenotypes occur in a population, the following are distinguished: a directional selection, which acts in favor of the phenotypes corresponding to one of the ends of the interval of the phenotypic variation curve of the population itself; a divergent or disruptive selection, if individuals near both extremes are favored; and a stabilizer selection, when the favored individuals are those who have a phenotype that concerns the average of the population.
Sexual and artificial selection
Sexual selection occurs when the sn concerns the choice of a reproductive partner, made on the basis of the suitability of secondary sexual characteristics; similarly, artificial selection occurs when the selective couplings are made by man in animal or vegetable species, in order to obtain varieties that have particular characteristics, essentially for a better economic and productive use, or for aesthetic purposes.
Consequences of natural selection: adaptation and speciation.
Natural selection is at the basis of the processes of adaptation and speciation, and therefore of the evolution of species.
Adaptation is the set of characteristics, both structural and behavioral, which have been
favored by natural selection because they increase the chances of survival and reproduction of an organism in its natural habitat. Adaptation is the consequence of the changes in the gene pool that occur within populations following the selective pressures of the environment, which favor individuals with higher fitness. Variations in the gene pool can result not only from natural selection, but also from genetic drift. However, it must be pointed out that the concept of adaptation is relative, because what may be suitable in one environment may not be suitable in another and, following environmental variations, advantageous characteristics can suddenly become disadvantageous and / or vice versa. Furthermore, in nature adaptation is never perfect, but results as a compromise between the adaptive needs of the different characteristics of an organism. To say therefore that a species is perfectly adapted or to the vertex of evolution, is to assert nonsense, and this also applies to the human species, which in the common anthropocentric language is often mistakenly considered to have reached some non-existent "evolutionary goal".
A classic example of adaptation, described by Darwin, is represented by the beak of the woodpecker, perfectly adapted to extract insects from the bark of trees. Another typical example is cryptic mimicry, with which the color and shape of some animals adapts perfectly to the environment in which they live, protecting them from predators.
Typical examples of adaptive evolution can easily be observed in populations of microorganisms, such as bacteria or viruses, thanks to their very short generation times. For example, in a population of bacteria there are individuals who contain genes that confer resistance to certain drugs, such as antibiotics. In the absence of this drug, all individuals have the same probability of survival, but if exposed to the specific antibiotic, sensitive bacteria will be inhibited, while those that contain the resistance gene will be able to multiply undisturbed and, after a few generations, the population will be formed almost exclusively of resistant bacteria; the population has therefore adapted. Some viruses, such as the influenza virus and the acquired immunodeficiency virus, undergo a continuous evolution, due to the presence of strains resistant to therapies and reactions host immune systems. If, on the one hand, it is relatively simple to explain the adaptive evolution of simple phenotypic characteristics, which derive from one or a few genes, such as mimicry, drug resistance, etc., it is more complicated to follow the evolution of a complex organ, for example the vertebrate eye. According to evolutionary theory, these complex traits, which derive from multiple genes, have undergone successive adaptations, occurring in many stages, each of which involved mutations in different genes that conferred an additional advantage on the individuals carrying them. A slow and progressive evolution has therefore taken place which has led to the development of organs specialized in certain functions.
Since adaptation represents a response of organisms to the selective pressures of the
environment, which occurs with the development of organs that have a certain function, it also manages to explain other evolutionary phenomena, such as evolutionary convergence, on the basis of which different species living in similar environments and therefore subjected to the same type of selection, they developed similar organs or functions.
The vestigial organs, in turn, are structures representing a legacy of the past, which were selected for a specific function and which, due to environmental and / or genetic changes that occurred later, have lost their meaning.
Natural selection is the basis of speciation, the evolutionary process that leads to the formation of new species.
Speciation generally occurs when populations of the same species are separated by geographical or behavioral barriers and are therefore subjected to different selective pressures, which lead to the divergence of their anatomical structures, until the accumulated differences produce populations of clearly distinct individuals unable to mate.
There are four types of speciation: allopatric, peripatric, parapatric and sympatric.
A classic example of allopatric speciation is that proposed by Darwin, who found numerous species of finches in different islands of the Galapagos, all deriving from a single parental species. It is from the beaks of these birds, therefore, that took flight, if you forgive me the pun, the theory of evolution.
The fact that the Galapagos Islands, more than 1000 kilometers away from the west coast of South America, presenting a wide variety of climates and habitats, has favored phenomena of speciation leading to the evolution of several endemic both animal and plant species.
Through natural selection they have thus diversified, from an ancestor common dating back to between one and five million years ago, as many as 14 different species of finches. These birds have mostly dark plumage and similar proportions, but vary in length (7 to 12 cm) and beak shape, depending on whether they feed on seeds, insects or cactus flowers and fruits.
The new species "are born" then when a population changes so profoundly that it can no longer reproduce with other populations of the original species. This happens when plants and animals colonize new habitats, when environmental conditions change drastically for some populations, or when populations are separated geographically. During the last glaciations, for example, massive glaciers formed which divided the habitat of many species. The populations, having lost contact with each other, developed differently giving rise to new species. A similar situation was created about 200 million years ago when, due to tectonic movements, the primordial continent (known as Pangea) began to dismember itself, and islands and continents began to move away with their load of plants and animals which, in the over time, they diversified more and more.
The Extinction of Species
Far from taking into consideration that the species on Earth are always the same from the instant of a mythological "creation" (established without any criteria 6000 years ago ... yes, you read that right: someone still does not know that this universe has existed for 13 , 7 billion years), just as God dished them from the earthly paradise and then from Noah's ark, we remember that the extinction of species is a natural phenomenon that is part of the evolutionary process, but it is nevertheless a rare event. It occurs when a habitat changes and existing species find themselves living in disadvantageous conditions (even simply by competing with other species). The change can be caused by climatic variations, by major geological events or by natural disasters.
Frequently extinct species are replaced by other species more suitable for living in the new environmental conditions but, sometimes, it takes millions of years for this to happen.
The "natural" extinction of species is a recurrent, very slow phenomenon, occasionally punctuated by episodes of mass extinctions.
Under normal conditions, 1 to 10 species disappear every year, but during the last century the rate of extinction has accelerated dramatically (the figures speak of the disappearance of at least 1,000 species per year). According to some scientists, we are on the verge of another mass extinction, the sixth.
Mass extinctions have occurred five times in the history of our planet. The most striking occurred 65 million years ago (when a cataclysm triggered by a meteorite caused, in a world obscured by dust, the disappearance of 75% of living species including dinosaurs) and about 250 million years ago (when, according to some , due to a sharp decrease in the level of oxygen and a simultaneous increase in the production of hydrogen sulphate and carbon dioxide by the oceans, probably due to volcanism phenomena, according to others due to the climatic changes caused by the fall of a celestial body, 95% of the animal species that populated the marine waters, 70% of the terrestrial ones, became extinct within 8000 years an unknown number of plants).
The sixth mass extinction could be unprecedented in both magnitude and velocity. Scientists estimate that over the next few decades we could lose 60% of living species. The difference between this extinction and the previous ones lies in the "cause": this time it is not asteroids or glaciations, but it is a single species that threatens all the others: "man". The destruction of natural habitats, the introduction of invasive species, climate change due to the greenhouse effect, pollution and the excessive exploitation of natural resources are consequences of human activity, and it will be man who will have to act promptly - always if one's DNA of a devastating species allows it - to try to avert, or at least reduce, the extent of this catastrophe, in the not so falsely altruistic interest of the fate of other living beings, but of one's own survival. Here it will only be seen if we are truly adapted to the environment that we ourselves are modifying, or if we will die out prematurely (compared to the average lifespan of many species), as seems inevitable.
