Darwin and the Evolution of Species: A Comprehensive Look

Evolutionary Theories

From Fixism to Darwinism

Understanding the origin and evolution of species is essential to understanding their characteristics and behavior. Historically, two main theories have attempted to explain the diversity of life: fixism and evolutionism.

Fixism

Fixism, proposed by Georges Cuvier (1769-1832), posits that all species are independent and have remained unchanged since their creation.

Evolutionism

Evolutionism, in contrast, suggests that the universe and life, in all their manifestations, are the product of development. The diversity of species is the result of change and adaptation over time. The concept of evolutionism began to take shape in the 19th century, with its roots in the 18th-century theory of transformism, timidly defended by Leclerc, Count of Buffon (1707-1788), and more decisively by Moreau de Maupertuis (1698-1759). Transformism proposed that new species arise from the transformation of primitive ones.

Lamarckism

Among the early evolutionary theories, Lamarckism stands out as the first comprehensive theory of biological evolution. It was presented in Philosophie Zoologique, a work published in 1809 by the French naturalist Jean-Baptiste de Monet, Chevalier de Lamarck (1744-1829). Lamarckism is based on the following principles:

• There is a gradual progression in organisms, from the simplest to the most complex.
• Environmental changes cause organisms to adapt through the use or disuse of certain organs.
• The use of an organ leads to its development and perfection. This is summarized in the famous phrase: "The function creates the organ."
• Acquired characteristics are inherited, allowing organisms to adapt to their environment.

Although Lamarck did not provide convincing evidence that acquired characteristics are inherited, his theory was influential and sparked interest in evolutionary thought.

Darwinism

In 1858, after a voyage to the Galapagos Islands and South America, Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) presented a new theory of evolution based on their observations. According to Darwin, there is a constant struggle for survival due to limited resources, which leads to a process of natural selection. In this process, the fittest individuals, those with characteristics that best adapt them to their environment, survive and reproduce, passing on their advantageous traits to their offspring. Although Darwin's theory of natural selection was initially met with resistance, it provided a more comprehensive explanation for the mechanism of evolution. However, it lacked an adequate explanation for how variations are transmitted through heredity. This explanation would come from the field of genetics, pioneered by Gregor Mendel (1822-1884) through his experiments with peas in the garden of his monastery.

Mutationism

Mendel's research in genetics became part of biology in the early 20th century and led to a new theory called mutationism. This theory proposed that evolutionary progress depends on mutations. Around 1901, Hugo de Vries (1848-1935) distinguished two types of variations: modifications, caused by environmental changes and not heritable; and mutations, which are alterations in the genes of organisms that are transmitted by heredity. According to this theory, mutations occur randomly and are not necessarily beneficial.

Neo-Darwinism or Synthetic Theory

The integration of Darwin's theory of natural selection with the principles of genetics and the concept of mutation led to the synthetic theory of evolution, also known as neo-Darwinism. This theory was developed around 1937 by Theodosius Dobzhansky and others. It combines the principles of natural selection with the principles of genetics to explain evolution. According to this theory, mutations explain the variations in organisms that are inherited, and natural selection directs the course of evolution by eliminating less-adapted variations and perpetuating the individuals best adapted to their environment.