This blog post examines whether evolutionary theory merely explains changes in organisms or if it can also encompass the evolution of culture and language.
People often encounter the word ‘evolution’ in broadcast media, comics, and movies, yet many don’t truly understand what evolution is. Biological evolution refers to the phenomenon of organisms changing over time, and the theory explaining this is evolutionary theory. Evolutionary theory has long been established as a core theory in biology, and its importance continues to grow as it now influences not only other scientific fields but also various disciplines like the humanities and philosophy. This article will examine the core concepts and development of evolutionary theory, as well as its impact on other fields.
Biologically defined, evolution signifies ‘the phenomenon where the frequency and proportion of genes change within a biological population across generations.’ Countless pieces of evidence confirm the existence of evolutionary phenomena, such as observing how organisms have changed over time through the dating of fossils, or analyzing how species sharing a common ancestor have diverged into different forms under different environmental conditions. However, research into the causes and mechanisms of evolution is still ongoing. Among these, the most widely accepted mechanisms are natural selection and genetic drift.
Natural selection refers to the phenomenon where individuals possessing traits better suited to a specific environment gain an advantage in survival and reproduction over those without such traits, leading to the gradual dominance of those traits within the population. For natural selection to occur, the reproductive process through which genes are passed to offspring is essential, and an event triggering the emergence of new traits is also necessary. Mutations, which cause changes in genes, are the primary trigger for this event. While mutations are generally disadvantageous to the individual, if advantageous mutations occur over generations, natural selection allows individuals possessing these traits to leave more descendants. In this process, individuals well-adapted to their environment survive and influence the next generation.
The patterns of natural selection within a population can be broadly categorized into three types. The first is directional selection, which occurs when evolutionary pressure from the external environment drives evolution in one specific direction. The second is divergent selection, an evolutionary pattern where opposing traits gradually become polarized. The third is stabilizing selection, which describes an evolutionary pattern where intermediate traits become dominant within a biological population, while extreme traits diminish. African cichlid fish are a prime example of directional selection; studies analyzing their diverse mouth structures and feeding methods have confirmed the influence of directional selection.
Early evolutionists, including Darwin, believed trait selection primarily occurred during mating. Generally, since females invest more energy in reproduction and rearing than males, the phenomenon of ‘sexual selection’ emerges where females choose males. Initially introduced to explain the advantages of sexual reproduction over asexual reproduction, the concept of sexual selection was long neglected due to its association with women’s rights. However, diverse research emerged in modern times. A prime example is the ‘Red Queen Hypothesis,’ which explains that the diversity of sexual reproduction provides an advantage for adapting to changing environments and surviving against competitors like parasites. This hypothesis describes a case of mutual competition between parasites and hosts, where each faces evolutionary pressure: parasites struggle to adapt to the host’s genetic diversity, while hosts, in turn, face evolutionary pressure to counter this. Another theory, the ‘Handicap Hypothesis,’ explains why males possess traits unnecessary for survival. For example, the peacock’s colorful tail or the stag’s large antlers do not aid survival, but females perceive them as evidence of survival ability and health, leading to selection and evolution in a direction that fixes these handicaps. However, despite these explanations, theoretical limitations are pointed out, such as the fact that the purpose of mating is not limited to mere reproduction and that it struggles to explain homosexuality.
Another major mechanism of evolution, genetic drift, refers to the phenomenon where the frequency of alleles changes over generations. In sexual reproduction, parental genes are randomly combined and passed to offspring, meaning even siblings from the same parents can possess different traits. If unaffected by the environment, this tends to accumulate like sampling error in statistics. However, when specific traits are selected for in particular environments, the frequency of advantageous traits increases while disadvantageous ones decrease. Repeated cycles of this process can gradually fix or eliminate genetic traits. This fixation occurs particularly rapidly in small populations, a phenomenon termed the ‘bottleneck effect’. Bottlenecks can reduce genetic diversity among individuals, potentially threatening species survival.
The ‘neutral theory of evolution’ considers genetic drift a primary factor in evolution. According to this theory, most mutations do not directly affect biological functions and thus are not selected. Since approximately 90% of human DNA consists of sequences unrelated to biological functions, the majority of mutations are neutral. This differs from natural selection, where specific mutations are selected. Instead, neutral mutations create new alleles, which become fixed through genetic drift, leading to evolution. Modern evolutionists view both genetic drift and natural selection as crucial factors in evolution.
Evolutionary theory has developed over a long period across various fields, accumulating substantial evidence. However, gaps remain in the evolutionary lineage. These ‘missing links’ arise because not all fossils can be preserved. Just as one can infer the entire picture of a jigsaw puzzle even if some pieces are missing, evolutionary theory can explain overall phenomena despite the absence of some evidence. Early evolutionists advocated gradualism, the theory that specific genetic traits gradually accumulate over time to bring about change. However, this theory was criticized because the slow pace of evolution theoretically resulted in a lack of intermediate fossil forms.
The punctuated equilibrium theory, which emerged in the 1970s, argues that evolution often occurs rapidly over short periods, with little change during the remaining time. According to punctuated equilibrium, the reason missing links remain undiscovered may be that evolution occurs discontinuously, leaving no evidence for specific periods. Richard Dawkins proposed that the differing evolutionary speeds observed across theories could be explained by the possibility that genetic-level changes occur gradually while phenotypic changes occur abruptly, suggesting the two theories could coexist.
Today, advances in life science research and analytical techniques have revitalized evolutionary studies. Scientists have discovered ‘horizontal gene transfer,’ a phenomenon where genes are directly transferred between individuals or species without undergoing reproduction. This occurs not only in microorganisms but also in plants and animals. A prime example is that plant chloroplasts and animal mitochondria, which are organelles possessing their own DNA and double-membrane structures, are believed to have entered cells via horizontal gene transfer. This discovery further broadens the possibilities of evolution.
Epigenetics also presents a new evolutionary mechanism. Epigenetics asserts that the environment can influence gene expression, partially accepting Lamarck’s theory of acquired characteristics being heritable. In the case of the nematode Caenorhabditis elegans, there are instances where environmental records from ancestors were passed on to descendants, and epigenetic phenomena have also been confirmed in higher organisms.
Evolutionary theory is not limited to biology but influences various fields. Theories that culture and language evolve are frequently discussed in academia, and memes, in particular, are gaining attention as elements of cultural evolution. Linda Fokker studied language from an evolutionary perspective, and recently, social structures, economics, politics, and even religious phenomena have begun to be interpreted through an evolutionary lens. While biological evolution and cultural evolution differ significantly, attempts to understand society through the basic concepts of evolutionary theory demonstrate the far-reaching influence of evolutionary thought.
