Evolutionary theory is a theory that explains how organisms change over time, and it uses mechanisms such as natural selection and gene drift to illustrate the process. Modern evolutionary theory has had a profound impact on many fields beyond biology, including the humanities and social sciences, and its importance has been expanded by attempts to explain the evolution of culture and language.
People often hear the word “evolution” in broadcast media, cartoons, and movies, but they often don’t know exactly what it is. Biological evolution is the phenomenon of organisms changing over time, and the theory that explains it is evolutionary theory. Evolution has long been a core theory in biology, and now it’s becoming increasingly important as it influences not only other scientific disciplines, but also the humanities, philosophy, and more. In this article, we’ll take a look at some of the key concepts and developments in evolutionary theory and how it’s impacting other fields.
By biological definition, evolution is “the phenomenon of changes in the frequency and proportion of genes within a population of organisms over generations. Whether it’s observing how organisms have changed over time through the creation of fossils or analyzing how species with the same ancestor have changed into different forms in different environments, there is a wealth of evidence to support the existence of evolution. However, the causes and mechanisms by which evolution occurs are still widely debated, with the most widely accepted mechanisms being natural selection and gene drift.
Natural selection is the process by which individuals with traits that are better suited to a particular environment have an advantage in survival and reproduction over those that are not, and those traits gradually become dominant in a population. For natural selection to occur, the reproductive process, in which genes are passed on to offspring, is essential, as is the trigger for new traits to appear. This happens when genes are changed, usually by mutations. Mutations are usually disadvantageous to an individual, but if a favorable mutation occurs over generations, natural selection will favor individuals with that trait, leaving more offspring. In this process, individuals that are well adapted to their environment will survive and influence the next generation.
There are three main forms of natural selection in a population. The first is directional selection, where evolutionary pressure is exerted in one direction by the external environment. The second is divisive selection, in which opposing traits evolve to become increasingly polarized. The third is stability selection, in which the middle traits of a population become more prevalent and the extreme traits become less prevalent. The African cichlid fish is a classic example of directional selection, and studies analyzing their different mouth structures and feeding behaviors have confirmed the effects of directional selection.
Early evolutionary theorists, including Darwin, thought that trait selection occurred primarily during mating. Females typically spend more energy on reproduction and nurturing than males, resulting in the phenomenon of “sexual selection,” where females choose males. Initially conceived to explain the benefits of sexual reproduction over asexual reproduction, the concept of sexual selection was long dismissed in the context of women’s rights, but in modern times, various studies have been conducted. For example, the “red queen hypothesis” explains that the diversity of sexual reproduction is advantageous to adapt to changing environments and survive competitors such as parasites. This hypothesis describes a mutualistic competition between parasites and their hosts that puts each under evolutionary pressure: parasites find it difficult to adapt to the genetic diversity of their hosts, and vice versa. Another theory, the handicap hypothesis, explains why males have traits that are unnecessary for survival. For example, a peacock’s ornate tail or a stag’s large antlers don’t help them survive, but females see them as evidence of survivability and health and select for them, leading to evolutionary forces that perpetuate the handicap. Despite this explanation, however, there are limitations to the theory, including the fact that the purpose of mating is not limited to reproduction and that it has difficulty explaining homosexuality.
Another major mechanism of evolution is gene drift, which is when the frequency of an allele changes over generations. In the case of sexual reproduction, the genes of the parents are passed on to the offspring in a randomized combination, so children of the same parents can have different traits. If not influenced by the environment, this tends to accumulate like sampling error in statistics. However, when certain traits are subject to selection in a particular environment, the frequency of traits that are favorable for survival increases and the frequency of traits that are unfavorable decreases. If this process is repeated, genetic traits can gradually become fixed or disappear. In small populations, this process is particularly rapid, and is known as a genetic bottleneck. Bottlenecks can reduce genetic diversity among individuals, threatening the maintenance of the species.
The neutral theory of evolution considers gene immobility to be a major factor in evolution. According to this theory, most mutations are not selected because they do not have a direct effect on life, and the vast majority of mutations are neutral because about 90% of human DNA is filled with sequences that are irrelevant to life. This is different from natural selection, in which specific variants are selected, by explaining that neutral variants create new alleles, which then become fixed through genetic drift and evolve. Modern evolutionists believe that both gene drift and natural selection are important factors in evolution.
Evolutionary theory has evolved over the years and accumulated a lot of evidence in various fields, but there are still gaps in the evolutionary tree. These are “missing links” caused by the fact that not all fossils are preserved, and just as we can infer the overall picture of a jigsaw puzzle even if we don’t have all the pieces, so too can evolutionary theory explain overall phenomena despite the lack of evidence. Early evolutionists argued for gradualism, the idea that certain genetic traits accumulate and change over time. However, the theory was criticized for the lack of fossils of intermediate forms due to the slow rate of evolution.
Interrupted equilibrium theory, which emerged in the 1970s, argues that evolution often occurs rapidly over short periods of time, with little change during the rest of the time. According to piecemeal equilibrium theory, the reason why the missing link has not been found is likely because evolution occurs in bursts and there is no evidence of a specific time period. Richard Dawkins suggested that the two theories can coexist, arguing that the rate of evolution is explained by different perspectives, and that changes at the gene level can be gradual, but phenotypes can change intermittently.
In the modern era, advances in life science research and analytical techniques have led to an increase in the study of evolution. Scientists have discovered “horizontal gene transfer,” which is the direct transmission of genes from individual to individual or species to species, bypassing reproduction. This phenomenon occurs not only in microorganisms but also in plants and animals, and as a typical example, chloroplasts in plant cells and mitochondria in animal cells are organelles with individual DNA and double membrane structures that came into existence in cells through horizontal gene transfer. This discovery further expands the possibilities of evolution.
Epigenetics is also suggesting new evolutionary mechanisms. Epigenetics argues that the environment can influence gene expression, which partially accepts Lamarck’s insoluble theory that acquired traits can be inherited. In the case of nematodes, the environmental record of ancestors has been passed down to their descendants, and epigenetic phenomena have been identified in higher organisms.
Evolutionary theory is not limited to biology, but has influenced many other fields. The theory that culture and language evolve is often discussed in academia, and memes in particular have been recognized as an element of cultural evolution. Linda Focus has studied language from an evolutionary perspective, and in recent years, social structures and economic, political, and even religious phenomena have begun to be interpreted from an evolutionary perspective. Although there are many differences between biological and cultural evolution, the attempt to understand society through the basic concepts of evolutionary theory shows the influence of evolutionary theory.
