This blog post systematically examines the development of evolutionary theory, from Lamarck’s Theory of Use and Disuse to Darwin’s Theory of Natural Selection and modern evolutionary theory.
Why do humans live? Where did humans come from? Everyone has pondered these questions at least once. While seemingly simple, these questions are profoundly complex, having troubled countless ancient philosophers and theologians. During the Middle Ages, theology dominated, and people sought answers through the belief that a Creator made everything, including humans, and that all species, once created, remained unchanged forever. In other words, they established a transcendent being called the Creator and delegated the resolution of complex problems to it. However, through the Renaissance and into the 17th century, attempts to explain and understand all natural phenomena through common laws gradually increased. People began to observe the living organisms around them closely and question the common notion that living things remained forever unchanged. These ideas gradually developed and gave birth to the theory of evolution. Now, I will explain how the theory of evolution has developed, how modern evolutionary theory explains how organisms change, and how evolutionary theory has been applied to other fields.
The first systematic theory expressing the idea that organisms change was Lamarck’s theory of use and disuse. Erasmus Darwin, in his ‘Zoology’ written between 1794 and 1796, argued that “all warm-blooded animals possess the power to transform parts of themselves, and these improved traits are passed on to their offspring.” Influenced by these ideas, Lamarck, in his 1809 work ‘Philosophical Zoology’, posited that traits acquired by organisms during their lifetime through adaptation to their environment are passed on to the next generation. For example, he posited that a short-necked giraffe could only eat leaves within reach, but by continually stretching its neck to reach higher leaves, its neck gradually lengthened. This acquired trait was then passed on to its offspring. This process, repeated over generations, led to the long necks seen in modern giraffes. However, this hypothesis lost its strength when it was discovered that acquired traits are not inherited through genes in germ cells.
The next theory to emerge was Darwin’s theory of natural selection. Published in 1859 in ‘On the Origin of Species’, natural selection posits that evolution occurs through the process of ‘individual variation → survival competition → natural selection’. Taking the giraffe as an example, while the use-disuse theory suggests that originally short giraffes gradually grew taller, natural selection assumes that tall and short giraffes coexisted from the beginning. Here, taller giraffes had an advantage in survival because they could reach higher leaves. Consequently, taller giraffes were selected, and this trait was passed on to their offspring, leading to the evolution of taller giraffes. The concepts of individual variation, competition for survival, and natural selection proposed by Darwin provided the most crucial ideological foundation among modern evolutionary theories and have been widely applied in various fields, including the social sciences.
However, an unresolved challenge in Darwin’s theory was how traits were transmitted to offspring. At the time, the theory of blending inheritance, which posited that parental characteristics would appear in offspring as a mixture of both parents, was dominant. Yet, if this theory were correct, even if a specific trait were passed to offspring, it would be diluted during the mating process. This would contradict Darwin’s theory of natural selection, which states that biological characteristics change gradually over long periods. To resolve this, Darwin proposed the pangenesis hypothesis. According to this hypothesis, every cell in an organism contained a self-replicating particle called a gemmule. During reproduction, these gemmules were secreted and transmitted traits to offspring. However, this theory also lost credibility because acquired traits were not inherited.
Mendel’s theory of genetics, published in 1865, completely refuted the blending theory. Through his pea experiments, Mendel proposed that an organism possesses two genetic factors determining a specific trait, each inherited from one parent. He explained that when alleles differ, only the dominant gene is expressed. These alleles separate during gamete formation, entering different gametes, and then recombine during fertilization. Subsequently, Weismann clearly distinguished germ cells from somatic cells, demonstrating that inheritance occurs solely through germ cells and proposing the germ plasm theory. Ultimately, in 1953, Watson and Crick elucidated the structure of DNA, the genetic material, clearly revealing the process by which traits are transmitted to offspring.
De Vries proposed the mutation theory through his experiments with evening primroses, explaining how individual variation arises within Darwin’s theory. Wagner and Romanes also argued that evolution occurs through isolation. By the 20th century, population genetics—which understands evolution through changes in the frequency of specific alleles within a population rather than through changes in individual traits—was introduced by Hardy, Weinberg, S. Wright, Fisher, and Haldane, completing modern evolutionary theory.
Modern evolutionary theory explains that when individuals belonging to a single population become isolated in different environments, various variations arise through mutations in germ cells and hybridization. Individuals possessing these variations then undergo natural selection, becoming reproductively isolated and diverging into different species. Notably, population genetics, developed by Hardy, Weinberg, S. Wright, Fisher, and Haldane, shifted the focus of evolutionary research from individuals to populations, introducing the concept of the ‘gene pool’. The gene pool represents the entire set of alleles present in a population at a specific time, and changes in the frequency of these alleles within the gene pool are interpreted as evolution. Four factors were identified as changing the gene pool: mutation, natural selection, genetic drift, and gene flow. Mutation, a concept borrowed from de Vries’ theory, refers to changes in genetic material that result in the emergence of new traits. Natural selection follows Darwin’s theory of natural selection, while genetic drift refers to sudden, rapid changes in the frequency of specific alleles due to random events like wildfires. Finally, gene flow describes the phenomenon where new alleles are introduced and their frequencies change due to migration of individuals from neighboring populations.
New theories on evolution continue to emerge even today, prompting reevaluations of existing theories. For instance, Lamarck’s theory of use and disuse is gaining renewed attention following claims that acquired characteristics can be inherited through the process of ‘DNA methylation’. Research indicates that exercise alters the inherent DNA methylation patterns within tissue cells, and these changes may be passed on to offspring. While still controversial, this demonstrates that evolutionary theory is an ongoing process, continually generating new perspectives.
Furthermore, evolutionary theory has transformed people’s values and profoundly influenced numerous other disciplines. In the 19th century, Herbert Spencer introduced Social Darwinism. It argued that survival competition and natural selection also applied to human life, justifying the concept of the survival of the fittest and the survival of the fittest. As a result, it sometimes led to the formation of inhumane societies where racial discrimination was justified over equality and welfare. Furthermore, some exploited this to justify imperialist policies, using it as a means to legitimize colonization. Evolutionary theory thus significantly influenced not only perceptions of the nature of human society but also ethical values and policies.
