The natural selection theory of evolution

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Understanding the theory of evolution by natural selection is absolutely essential for understanding the history of living organisms and, consequently, human history.

A little history of science

The theory of evolution, which postulates that living beings evolve over time, emerged in the second half of the 19th century.

Until the 18th century, the predominant theory was that of fixism, which held that living beings had been created by the hand of God and were therefore immutable. For example, a name often associated with fixism is that of Carl von Linné (1707-1778). For him, all existing species were created during a single divine creation. Linnaeus is best known for his Systema Naturae. The most famous edition is that of 1758, in which he created our species, Homo sapiens, and classified it among the primates. However, for him, this classification of species has only one objective: to make intelligible the divine plan of Creation, of which human beings are the culmination.

Georges Cuvier (1769-1832) developed the theory of catastrophism. According to him, there is no evolution of living beings, but rather a succession of catastrophes and mass extinctions. These “catastrophes” would be followed by new divine creations. This explains the presence of fossils of now-extinct animal species found in ancient geological layers.

The first theory of evolution, in the general sense of changes in species over time, was formulated in the 18th century by Jean-Baptiste de Lamarck (1744-1829). This theory is known as transformism. According to Lamarck, species change over time. There is thus continuity between extinct fossil species and present-day species. Physical factors are thought to be at the origin of this modification of living beings, and the acquired traits are then passed on to offspring. Although this theory is erroneous, it is nevertheless the first theory in the direction of species evolution, representing an important step towards evolutionism.

Transformism according to Lamarck – the example of the giraffe’s neck

To illustrate his theory, Lamarck used the now-famous example of the evolution of giraffe neck size. In his work, Philosophie Zoologique, published in 1809, he explains:

“In terms of habits, it is curious to observe the product in the particular shape and size of the giraffe(camelo pardalis).): we know that this animal, the largest mammal, inhabits the interior of Africa, and that it lives in places where the earth, almost always arid and without grass, obliges it to graze the foliage of the trees, and to continually strive to reach them. As a result of this habit, which has been sustained for a long time in all individuals of its race, its front legs have become longer than its hind legs, and its neck has lengthened so much that the giraffe, without standing on its hind legs, raises its head to a height of six meters.

Philosophie Zoologie, book I, chapter VII.

Here, the environment determines the evolution of the giraffe’s neck size. It’s the function that shapes the organ. Because the giraffe needs to reach high up on leaves, its neck has lengthened, which over time has led to a change in the species. This acquired trait is then passed on to descendants.

Charles Darwin, Alfred Wallace & natural selection

The theory of evolution by natural selection was developed independently by Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) in the second half of the 19th century. Darwin developed this theory through his explorations aboard the Beagle, on which he spent 5 years (1831-1836). However, it wasn’t until 1859, 23 years after his round-the-world voyage, that Darwin published his famous book On the Origin of Species by Natural Selection or the Preservation of Favored Races in the Struggle for Life.

For the record, Darwin was prompted to publish his manuscript when he discovered that Wallace was about to publish a theory similar to his own. Finally, Darwin and Wallace’s work was presented on the same day to the London Linnaean Society in 1858. Nevertheless, it’s Darwin’s name that has gone down in history.

What is the natural selection theory of evolution?

Following his observations in the field, Darwin developed the following reasoning. First, he notes that there is variability among individuals of the same species. What’s more, these variations are heritable and selectable. Darwin arrives at this conclusion by observing the breeding practices and selection for reproduction that are applied to some livestock. In this way, he notes that selected traits are passed on to offspring. The question then arises: does this selection also take place in the wild, or only in a breeding context where breeders play the role of selection agents? Darwin identified that this selection agent is present in nature and corresponds to the environment in which individuals live.

There may be several variants for a single trait, and each variant is carried by several individuals. If environmental conditions favor one or more of these variants, individuals carrying them tend to leave more offspring than others. This differential reproductive success is what we call natural selection. On a population scale, this natural selection leads to transformations within species. The characteristics of advantaged individuals eventually become dominant within a population, as these individuals have a higher reproductive rate. This leads to a change in the population over time.

We can summarize the theory of evolution by natural selection as follows: individuals carrying an advantageous mutation in an environment will leave more descendants than other individuals not carrying this mutation. If the environmental conditions favorable to this mutation persist over time, the frequency of the favored variant in a given population will eventually reach 100%. As a result, the population, if not the species, will have changed. Subsequently, new mutations will occur for the same trait, leading to a new modification of the species if the environment is favorable. It is important to note that these mutations are the result of chance. The survival of a species is thus directly linked to its ability to have many variations for a single trait.

Work in genetics in the first half of the 20th century led to the discovery that these variations and mutations are genetic in origin, and that evolution therefore results from a change in allele frequency within a population. In short, mutations, which occur by chance, are the fuel of evolution, while natural selection is its driving force. Natural selection is at the root of a population’s adaptation to an environment.

Darwin’s example of the giraffe’s neck

Darwin uses Lamarck’s example of the giraffe’s neck:

“The high stature and elongation of the neck, forelimbs, head and tongue are conditions in the giraffe that adapt its entire frame admirably to the habit of grazing on the high branches of trees. […] in that individuals with one or more parts more elongated than usual have generally been the only ones to survive. Their cross-breeding has produced descendants either inheriting the same bodily characteristics, or a tendency to vary in the same way, while individuals less favored in the same respects will have been more likely to perish.”

The Origin of Species, 6th edition, 1872.

The main difference between Lamarck and Darwin lies in the mechanism underlying the transformation of species. According to Lamarck, it is environmental conditions that induce the modification of a character. In the example of the giraffe, the need to stretch the neck to reach the leaves directly lengthens the neck. This acquired physical trait is then passed on to offspring.

For Darwin, on the other hand, the driving force behind the transformation of species is natural selection. Giraffes with a genetic variation resulting in a longer neck have an advantage in their environment, as they can access food more easily. This advantage increases their chances of survival and, consequently, their ability to reproduce. As giraffes carrying this mutation have more offspring, its frequency increases in the population, leading to a change in the population.

Figure 1: Diagrams showing Lamarck’s and Darwin’s theories of evolution.

The theory of evolution by natural selection at a glance

Here are a few points to remember if you want to fully understand the theory of evolution by natural selection:

  • Biological evolution applies not to an individual but to a population. Natural selection has an effect on individuals, but it’s a population that evolves and changes over time.
  • Evolution has no purpose; it is the result of the randomness of genetic mutations. There is no intention or will behind evolution.
  • Our species, Homo sapiens, is no more evolved than any other animal species. Homo sapiens is an animal, a mammal, a primate and an ape like any other (if you want to learn more about them, click here). We are not the end product of evolution, we are the result of chance.
  • The evolution isn’t over yet! All species are always evolving, including us! In reality, a species that doesn’t evolve is doomed to extinction. Consequently, the term “living fossils” is biologically meaningless.
  • Similarly, there are no evolutionary stages or steps in evolution, it’s a continuous process.

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The Prehistory Travel team.

Bibliography :

[1] Boyer Charles, “Le cou de la girafe : Lamarck, et puis Darwin”, L’Enseignement philosophique, 2011/2 (61e Année), pp. 48-54. DOI : 10.3917/eph.612.0048. URL:

[2] Darwin Charles, On the Origin of Species by Natural Selection or the Preservation of Favored Races in the Struggle for Life, ed. orginale 1859, 2022, éditions Flammarion.

[3] Lamarck Jean-Baptiste, Philosophie zoologique ou Exposition des considérations sur l’histoire naturelle des animaux, tome I, ed. original 1809, 2017, Hachette Livre BNF.

[4] Lecointre Guillaume (dir.), Guide critique de l’évolution, 2009, BELIN EDUCATION.

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