Natural Selection and the Formation of Adaptation

Section 2: Natural Selection and the Formation of Adaptation

When the dead leaf butterfly is at rest, its wings resemble a dead leaf—this is an example of the butterfly’s adaptation to its environment. Like the dead leaf butterfly, all living organisms possess features that allow them to adapt to their environments. How do these adaptations form? This is one of the core questions in the study of biological evolution.

Universality and Relativity of Adaptation

Adaptation, as a biological term, includes two aspects: first, it refers to the morphological structure of organisms that is suited to perform certain functions; second, it refers to the suitability of an organism's morphology, structure, and functions to survive and reproduce in a given environment.

Adaptation is universally present, and you can likely think of many examples. However, are these adaptations absolute and complete?

Adaptation as a Result of Natural Selection

How do adaptive traits in various organisms form? The theory of the fixity of species posits that all organisms have always been the way they are now. French naturalist Jean-Baptiste Lamarck (1744–1829) completely rejected the theory of the fixity of species, proposing that all current organisms evolved from more ancient ones. According to Lamarck, the adaptive traits of organisms were not inherent but gradually formed during the process of evolution. However, he believed that the formation of adaptations was due to the use and disuse of organs and the inheritance of acquired characteristics. For example, he suggested that the anteater’s long tongue developed as a result of its constant licking of ants (Figure 6-5). Conversely, organs that are no longer used would degenerate, such as the mole's eyes, which have atrophied due to living underground where vision is unnecessary. Lamarck proposed that these traits acquired through use or disuse could be passed on to offspring.

Lamarck's theory of evolution was progressive for its time, but his explanation of the formation of adaptations was superficial and was not widely accepted.

Charles Darwin’s theory of natural selection provided a more reasonable explanation for biological evolution and the formation of adaptations. He proposed that adaptations arise from heritable variations and are the result of natural selection. The following is a model explaining Darwin's theory of natural selection (Figure 6-6).

Regarding the formation of adaptations, Darwin believed that under the selective pressure of a given environment, heritable beneficial variations would confer survival and reproductive advantages on certain individuals. Over generations, such individuals would become increasingly common in the population. Beneficial variations would accumulate through successive generations, becoming prominent adaptive traits, and eventually leading to the emergence of new types of organisms. Therefore, the appearance of heritable beneficial variations in a population and the directional selection exerted by the environment are necessary conditions for the formation of adaptations.

Darwin’s theory of biological evolution, with natural selection at its core, led to the understanding that the vast array of living organisms in the natural world were not preordained by a divine being and are not unchanging. Instead, they continually evolve under the influence of natural selection. This perspective freed biology from the constraints of theology for the first time and placed it on a scientific footing. It revealed that the unity of the biological world is due to a common ancestry shared by all living organisms, and that the diversity and adaptability of life are the results of evolution.

Darwin’s theory of evolution spread worldwide through its struggle against creationism and the theory of the fixity of species. Various translations of his book "On the Origin of Species" were published around the world. Karl Marx and Friedrich Engels highly praised Darwin’s theory, considering it a revolution in the history of science that greatly advanced the development of natural sciences in the 19th century. Marx dedicated the first volume of his book "Das Kapital" to Darwin, writing on the title page: “To Charles Darwin, from a sincere admirer, Karl Marx.” Engels hailed Darwin's theory of biological evolution as one of the three major discoveries of 19th-century natural science.