In this blog post, we explore how living organisms evolve across generations and how evolutionary theory explains this process.
All living organisms gradually change their characteristics over multiple generations. Through these changes, organisms adapt to their environment and evolve. This process and its outcome are called ‘evolution,’ and the various theories and academic fields related to it are collectively referred to as ‘the theory of evolution.’ The theory of evolution is divided into several theories based on how they explain the causes and processes of evolution, presenting diverse perspectives on how organisms arrived at their current forms. Evolutionary theory has developed based on ‘evolutionary biology,’ which studies the phenomenon of biological evolution and identifies its causes. This discipline forms a crucial pillar of modern biology as part of humanity’s ongoing effort to understand the origin and diversity of life.
Evolutionary biology began with theories about species change proposed from 19th-century studies of fossils and biological diversity. In 1858, Charles Darwin and Alfred Russel Wallace laid the foundation for evolutionary theory by introducing the concept of natural selection. During this period, attempts to explain biological change primarily involved analyzing the morphological similarities between fossils and living organisms, which provided a crucial foundation for understanding the mechanisms of evolution. Early evolutionary theory focused on simple morphological changes, but it evolved into modern evolutionary theory through its integration with genetics. Particularly, the rediscovery of Mendel’s laws of inheritance in the early 20th century enabled evolutionary biology to explain the mechanisms of evolution more precisely through the clear evidence of genes.
Modern evolutionary theory defines evolution as changes in gene frequencies within populations across generations, citing natural selection, genetic drift, and mutation as its driving forces. The theory of natural selection, which posits that individuals possessing traits suited to their environment gain an advantage in survival and reproduction, forms the core of evolutionary mechanisms. While Darwin’s early theory argued that traits advantageous for survival were passed on to subsequent generations, the subsequent discovery that individual variations are inherited further refined the concept of natural selection. A prime example is the 19th-century British peppered moth. In environments darkened by industrialization, the proportion of gray moths increased as they gained a survival advantage, while in less polluted areas, white moths showed higher survival rates. This phenomenon became a crucial empirical case supporting natural selection theory.
Genetic drift, alongside natural selection, is a crucial concept in modern evolutionary theory and essential for understanding evolution. It refers to changes in the frequency of alleles within a population during reproduction, occurring independently of fitness. This concept is particularly potent in small populations and plays a vital role in explaining how gene pools change. Kimura Motoo’s neutral theory of evolution, which proposed genetic drift as a major factor in evolution, argued that the majority of mutations are neutral and that evolution progresses as these mutations are randomly passed on to subsequent generations. This theory provided a new perspective for understanding the complexity of evolution and is now recognized alongside natural selection as a primary mechanism of evolution.
Another theory gaining prominence in modern evolutionary theory is Richard Dawkins’ ‘The Selfish Gene’. Dawkins argued that genes are the agents of evolution, with organisms merely serving as vehicles to aid gene replication. Building upon Darwin’s theory of natural selection, this theory seeks to explain that all behaviors of organisms, including altruistic actions, ultimately serve the survival and replication of genes. By reinterpreting the behavior of living organisms from the perspective of genes, this theory contributed to expanding evolutionary theory into philosophical discourse.
Another point of contention in evolutionary theory concerns the pace of evolution. The two main positions are gradualism, which holds that the evolution of living organisms occurs slowly, and punctuated equilibrium, which posits that evolution involves periods of stasis and periods of rapid change. Gradualism points to the incompleteness of the fossil record, arguing that evolution likely occurred slowly. Conversely, punctuated equilibrium theory, based on the discontinuous nature of the fossil record, explains that periods of rapid evolution alternated with relatively stable periods. This debate remains unresolved and requires a deeper understanding of the mechanisms of evolution.
Beyond disciplines focused on the principles of evolution, various research fields derived from evolutionary theory exist. For example, phylogenetic systematics aims to determine the relationships between organisms and understand their evolutionary history. This discipline utilizes the fossil record, anatomical similarities, developmental evidence, and molecular biological data to construct phylogenetic trees of life, focusing on revealing key events in the evolutionary process. Phylogenetics serves as a practical application of evolutionary biology, playing a crucial role in understanding the origin and diversification of various species.
Through such research, biologists seek to understand the origin of life and explore how each species has diverged and adapted. For example, the three-domain, six-kingdom classification system divides life into three domains: bacteria, archaea, and eukaryotes, further subdividing eukaryotes into plants, animals, fungi, protists, and others. This classification serves as a vital tool for systematically understanding the evolutionary history of life and forms the foundation of biological research.
Meanwhile, evolutionary theory extends beyond biological evolution, challenging questions about the origin of life itself. One field studying life’s origins is chemical evolution. Chemical evolution investigates how the first life forms emerged from the inorganic environment of the primordial Earth. Experiments like Miller’s and Fox’s provided clues about the origin of life by demonstrating how inorganic substances could form organic compounds, and further, how complex organic molecules like microspheres capable of self-replication could emerge.
Although evolutionary theory remains incomplete, the phenomenon of evolution is supported by vast evidence and is now widely accepted as established fact. This research now extends beyond biology into diverse fields such as evolutionary psychology and cultural evolution. Evolutionary theory plays a crucial role in understanding the complexity and diversity of life, establishing itself as a central pillar of modern science. Its development continues to be studied as an essential tool for understanding the past and present of living organisms, and for predicting the future.
