The theory of biological evolution
Man is a curious being who throughout history has questioned everything around him and has devised the most diverse ideas to explain it.
No wonder our ancestors also wondered about the animals and plants they saw around them: were they always like this or have they changed over time? And if there were differences, what are the mechanisms that have been used to carry out these modifications?
These are the main unknowns that have been tried to solve through what we know today as the theory of biological evolution, which is at the base of biology and communicates with a good part of the realm of psychology, by telling us about the origin of certain innate tendencies that could be influencing our behavior and our way of thinking. Let’s see what this is all about.
Evolution of a theory
Until the 19th century, the predominant idea about the origin of the species was creationism, according to which an all-powerful entity had created every living being in existence, and these had not changed over time. This kind of belief dates back to Ancient Greece, and although it never became hegemonic in Europe, it left its mark on the thinking of some theorists and intellectuals. But with the period of the Enlightenment, more complex theories began to emerge in Europe that were closer to reality.
The most remarkable at the beginning of the nineteenth century was proposed by Jean-Baptiste Lamarck ; this French naturalist proposed that all species had the will to change and the ability to transfer to their offspring these changes acquired through their actions, a mechanism of transmission of characteristics known as inheritance of acquired characters .
Lamarck, in opposition to the creationists, defended the idea of the evolution of species, but accepted that species were generated spontaneously and did not have a common origin. In other words, his theory talked only about the mechanism by which living things change over time, and not about how they first emerge. I won’t go on any longer, since you have a very complete article on Lamarckism here: “The Lamarck Theory and the Evolution of Species.
Charles Darwin and the theory of biological evolution
A great step had been taken in admitting the idea of biological evolution through entirely natural mechanisms, but Lamarck’s theory had many cracks. It wasn’t until 1895 that the British naturalist Charles Darwin published the book The Origin of Species, in which he proposed a new theory of evolution (which would become known as Darwinism). Little by little, this theory would take shape in his successive writings, and it would be seen that it explained biological evolution through a natural mechanism: natural selection combined with sexual selection. We will see later on what they consist of.
Together with the also British naturalist Alfred Russel Wallace, (who curiously carried out similar investigations and reached almost identical conclusions without having crossed words with him), Darwin put forward new ideas in favour of evolution; however, with great caution, because the implications of his work put the Church establishment in an uncomfortable situation, which had always attributed the existence of all forms of life to the direct intervention of God.
Natural selection
According to Darwin, all species come from a common origin, from which they have diversified thanks, in part, to natural selection . This evolutionary mechanism can be summarized in the fact that the species that are best adapted to the environment they are in, reproduce more successfully and have offspring that, in turn, have a greater chance of successfully reproducing, giving way to new generations. The English naturalist also accepted the idea of extinction, which was the other side of the coin: the species less adapted to the environment tended to reproduce less and less, in many cases even disappearing.
Thus, first populations of living beings with different characteristics appeared on the scene, and the environment exerted a pressure on them that made some of them more successful in reproduction than others, causing their characteristics to spread and making others disappear.
What characterized this process was its natural character, alien to the influence of a supernatural entity that would direct it; it occurred automatically, in the same way that a snowball becomes bigger due to the influence of the force of gravity applied to the side of a mountain.
Sexual selection
Another of the evolutionary mechanisms described by Darwin’s theory of evolution is sexual selection, which consists of the set of natural dispositions and behavior that make certain individuals seen as more desirable for having offspring with them, and others less desirable for the same.
Thus, sexual selection performs a double game . On the one hand, it is complemented by natural selection, because it provides elements that explain why certain individuals have greater reproductive success than others; but on the other hand, it plays against them, since there are traits that can be advantageous from the point of view of sexual selection, but disadvantageous from the point of view of sexual selection (that is, the result of interaction with the environment, with the exception of possible reproductive partners).
An example of the latter is the peacock’s long tail: it makes it easier to find a mate, but harder to stay out of the reach of predators.
Neo-Darwinism
Despite removing the divinity in creation and explaining a basic mechanism by which species change and diversify over time, Darwin did not know the term genetic variability, nor did he know about the existence of genes. In other words, he did not know how the variability of characteristics on which the pressure of natural selection acts appeared. For this reason, he never completely rejected the idea of inheritance of acquired characteristics proposed by Lamarck.
Unlike Darwin, Wallace never accepted this idea, and from this dispute appeared a new evolutionary theory called Neodarwinism , promoted by the naturalist George John Romanes, which in addition to rejecting in its entirety the Lamarckian ideas, believed that the only evolutionary mechanism was natural selection, something that Darwin never maintained. It was not until the beginning of the 20th century that Mendel’s laws were accepted, showing that mutations in DNA are pre-adaptive, that is, first you suffer a mutation and then you test whether the individual in whom it has occurred is better adapted to the environment or not, breaking the idea of the inheritance of acquired characters.
With this premise, the geneticists Fisher, Haldane and Wright gave a new twist to Darwinism. They integrated the theory of evolution of species through natural selection and genetic inheritance proposed by Gregor Mendel, all with a mathematical basis. And this is the birth of the theory most accepted by the scientific community today, known as the synthetic theory. This proposes that evolution is a more or less gradual and continuous change explained through genetic variability and natural selection.
The social impact of the theory of evolution
The biggest problem Darwin had was to dispense with the figure of the hand of God in his theory about what the explanatory mechanism of biological diversity could be, something that was unforgivable in times when religion and creationism were hegemonic.
However, the theoretical legacy of Charles Darwin was robust, and over the years the appearance of new fossils gave good empirical support to his theory … which did not make his contribution to science look any better from religious quarters. Even today, environments closely linked to tradition and religion deny the theory of evolution, or consider it “simply a theory,” implying that creationism enjoys the same scientific endorsement. This is a mistake.
Evolution is a fact
Although we speak as the theory of evolution, is actually a fact, and there is evidence not to doubt its existence . What is being discussed is how the scientific theory that explains the evolution of the species for which there is evidence should look, but this process itself is not being questioned.
Below you can find some of the evidence that proves the existence of biological evolution.
1. Fossil record
Paleontology, the discipline that studies fossils, has shown that geological phenomena need a lot of time to be completed, such as fossilization. Many fossils are very different from today’s species, but at the same time, they have a certain resemblance. It sounds strange but with an example it will be easier to understand.
The Glyptodon was a Pleistocene mammal that bears a remarkable resemblance to a modern-day armadillo but in a giant version : it is a trace of the evolutionary tree that leads to today’s armadillos . The same fossils are also proof of extinction, since they show that in the past there were organisms that are no longer among us today. The most emblematic example is the dinosaurs.
Imperfect traces and designs
Some living beings have designs that we could say are imperfect. For example, penguins and ostriches have hollow wings and bones, but they cannot fly. The same is true of whales and snakes, which have pelvises and femurs, but cannot walk. It is he organs are known as vestiges, organs that were useful to an ancestor but now have no use .
This is further evidence of evolution which, moreover, reveals that this process is opportunistic, since it takes advantage of what is at hand to organize a new organism. The species of life is not the result of intelligent and well-planned design, but is based on functional “bottlenecks” that are perfected (or not) over generations.
3. Homologies and Analogies
When comparing the anatomy between different organisms, we can find cases that, once again, are proof of evolution . Some of them consist of homologies, in which two or more species present a similar structure in some parts of their anatomy, but are to perform different functions, which is explained because they come from the same ancestor. An example are the extremities of the tetrapods, since all of them present a similar structural disposition despite the fact that their extremities have different functions (walking, flying, swimming, jumping, etc.).
The other case is the analogies, organs of different species that do not have the same anatomy but share function. A clear example is the wings of birds, insects and flying mammals. They have been developed by different paths to reach the same function, that of flying.
4. DNA sequencing
Finally, the genetic code, with a few exceptions, is universal, i.e. all organisms use it. If it were not, it would not be possible for the E.coli bacterium to produce human insulin by introducing into it the gene (of human origin) responsible for generating this substance, as we do today. Furthermore, transgenics are another evidence that the genetic material of all forms of life has the same nature. OR tra evidence that all species have a common origin and proof of evolution .
Evolutionary mechanisms
Although we have talked about natural selection as a mechanism that uses evolution to advance, it is not the only one known. Here we will see the different types of selection that influence evolution .
1. Natural and sexual selection
In the theory of biological evolution born with Darwin, this naturalist originated the idea of natural selection from his observations in the voyage of the Beagle during its journey through the Galapagos Islands. In them, he was struck by the fact that each island had its own species of finch, but all of them had a resemblance between them and those found in the neighboring continent, South America.
The conclusion reached is that the finches of the islands originally came from the continent, and that on arriving at each island they suffered an “adaptive radiation”, in this case from the food, thus generating a range of variants from the same group of ancestors; therefore, these birds have very different beaks, having adapted to the ecosystem of each island separately .
Today we can better clarify the functioning of natural selection. The environment is not stable and changes over time. Species undergo mutations in their genome at random, and these cause them to change their characteristics. This change may favour their survival or, on the contrary, make their life difficult and cause them to die without offspring.
2. Artificial selection
It is not properly an evolutionary mechanism, but a variety of natural selection . It is said to be artificial, since it is the human being who directs evolution for his own interests. We are talking about a practice that has occurred in agriculture and livestock farming for millennia, choosing and crossing plants and animals to obtain greater productivity and yield. It also applies to domestic animals, such as dogs, where other characteristics were sought, such as more strength or more beauty.
3. Genetic drift
Before we talk about this mechanism, we must know the concept of the allele. An allele consists of all mutational forms of a particular gene. To give an example, the different eye color genes in man. Genetic drift is defined as a random change of the allele frequency from one generation to another, i.e. the environment does not act. This effect is better appreciated when the population is small, as in the case of inbreeding , where genetic variability is reduced.
This mechanism can remove or set characteristics at random, without the environment acting on your selection. And so, in small populations, it is easier for a quality to be lost or gained by chance.
Controversy related to evolution
As we have seen, the most widely accepted theory of evolution today is the synthetic theory (also known as modern synthesis), although there are alternatives that are against it because it is considered to contain certain deficiencies or concepts that are not explained or are not included.
1. Neutralism
Until not long ago, it was thought that only harmful mutations (negative selection) and beneficial mutations (positive selection) existed. But Japanese biologist Motoo Kimura said that at the molecular level there are many mutations that are neutral, that are not subject to any selection and whose dynamics depend on the mutation rate and the genetic drift that eliminates them, creating an equilibrium.
From this idea was born an idea opposed to that proposed by the synthetic theory, where beneficial mutations are common. This idea is neutralism . This branch proposes that neutral mutations are common, and beneficial ones are the minority.
2. Neolamarckism
Neolamarckism is the part of the scientific community that still holds that one cannot discard Lamarck’s theory and its inheritance of acquired characters. From there it tries to reconcile this idea with genetics, affirming that the mutations are not random but that it is the consequence of the “effort” of the species to adapt to the environment. However, its empirical basis cannot be compared to that of the synthetic theory .