This blog post examines Gould’s punctuated equilibrium theory, focusing on the ‘missing link’ problem that existing evolutionary theory has failed to explain.
Historically, newly discovered scientific facts have astonished the world and sometimes plunged it into great turmoil. Among these, Darwin’s theory of evolution stands out as the most representative. Even after more than 150 years, evolutionary theory remains a hotly debated topic among biologists, each advancing their own theories. Before delving into the main discussion, I will briefly introduce several theories.
First, orthodox Darwinian theory posits that evolution occurs gradually. Individuals more advantageous for survival and reproduction, and better adapted to their environment, are selected by natural selection. Over long periods, these favorable traits accumulate, leading to the gradual evolution of organisms. However, orthodox Darwinian theory fails to satisfactorily explain the incompleteness, or ‘missing link’, in the evolutionary process: the absence of fossil evidence for intermediate species. Neo-Darwinism emerged as a complementary theory to address this. Neo-Darwinism posits that intermediate species are unlikely to be preserved as fossils because they rarely form stable populations, making their discovery statistically improbable. Another theory, the orthodox adaptationist Darwinian theory advocated by the renowned evolutionist Richard Dawkins, seeks to understand everything in nature as a direct result of natural selection and adaptation.
Like Dawkins, the renowned evolutionist Stephen Jay Gould proposed a theory that shattered the common sense of the above theories: punctuated equilibrium. Punctuated equilibrium explains that the reason fossil forms of intermediate species are not found is precisely because intermediate species themselves do not exist. That is, evolution does not proceed gradually from one species to another, but rather species abruptly change (punctuate) into other species. When I first learned about evolution as a child, I accepted the theory of natural selection without hesitation. The theory of punctuated equilibrium was a fresh shock to me. To explore Stephen Jay Gould’s argument more deeply, I intend to read his seminal work, Full House, and delve into its contents.
People often say that humans are the most special and most advanced organisms in Earth’s history. I, too, believed this statement to be true before encountering this book. Thanks to the consciousness unique to humans, we can use language, think, form societies governed by rules, and live in a special way. But does this ‘specialness’ truly equate to being the most advanced? After reading this book, my answer was “No.” We humans are merely the latest to appear on Earth since life began—just a tiny new branch sprouting on the tree of life. Yet, because humans possess this special consciousness, they fall into arrogance. They attempt to interpret the vast Tree of Life based on that tiny branch, committing the error of interpreting biological evolution as a tendency progressing ultimately toward humans.
Stephen Jay Gould argues that evolution does not occur through progress, and that the very notion of it having any directional tendency is mistaken. In other words, evolution is unpredictable and directionless, driven by chance. As I mentioned earlier, before encountering this book, I too believed humans were the ultimate goal of evolution and the most superior organism. But now, I fully agree with Stephen Jay Gould’s argument.
Recently, fear has spread worldwide that the coronavirus might herald humanity’s end. Beyond COVID-19, diseases caused by such viruses have consistently existed. Numerous diseases exist, including the swine flu that spread a few years ago, Ebola, and SARS. While we don’t know exactly when viruses first appeared on Earth, one thing is certain: they emerged long before humans. If evolution is driven by progress, it cannot explain how genetically more advanced humans suffer and even die from viruses that are more degenerate. Furthermore, it is difficult to explain why the most advanced form of humanity is not the most abundant organism on Earth. Because these questions cannot be answered by evolutionary theory based on progress, I also believe evolution is not driven by progress. I think avoiding this error is essential to truly understanding Darwin’s theory.
However, while Stephen Jay Gould’s major theory, punctuated equilibrium, did indeed offer a new direction for interpreting evolutionary theory, I do not believe it perfectly explains evolutionary theory 100%. I will examine my thoughts on punctuated equilibrium below.
To redefine Stephen Jay Gould’s punctuated equilibrium theory in technical terms: it is an evolutionary theory stating that the evolutionary pattern of sexually reproducing species is divided into periods of stasis with little change over most of their history and periods of rapid speciation occurring in relatively short bursts. This theory directly challenges the traditional gradualist theory, which posits that evolution occurs gradually over very long periods and across many generations. Let’s illustrate this using the horse species Mesohippus and Maiohippus, examples cited by Stephen Jay Gould in his book. The traditional gradualist theory described the evolution from Mesohippus to Maiohippus as gradual and continuous. However, Stephen Jay Gould proposed the theory of punctuated equilibrium, arguing that the fossilized foot bones of these two species are clearly distinguishable. He noted that the two species coexisted for a long time, appeared suddenly in the geological record, and then persisted unchanged for extended periods. This suggests that evolutionary change occurs at specific points where branches diverge. Stephen Jay Gould argues that speciation occurs during divergent periods due to unpredictability and chance.
However, this raises a question. If evolution, as Stephen Jay Gould suggests, is not gradual but occurs suddenly due to the randomness of variation, how can we scientifically explain that randomness? To be more specific, citing Stephen Jay Gould’s example: when the evolution from Mesohippus to Maiohippus occurred, it implies that by some random mutation, a Mesohippus mother gave birth to a Maiohippus offspring. This is not scientifically explainable. To give a slightly more extreme example, the punctuated equilibrium theory essentially claims that the evolution from apes to Australopithecus, the human ancestor, did not occur gradually over time, but rather appeared suddenly one day. While punctuated equilibrium does explain the lack of intermediate fossil species that the existing gradualistic theory couldn’t account for, it raises the question of how the missing links are connected. This cannot be fully and satisfactorily explained by chance. This inevitably becomes a very good weakness for those who believe in creationism to attack evolutionary theory. While I may have raised some questions in a somewhat abrupt manner, I believe that for evolutionary theory to defeat creationism, it must be scientifically flawless, showing no weaknesses whatsoever. As someone pursuing a path in engineering, I look forward to the day when ongoing new research and investigations advance evolutionary theory beyond mere theory to the status of a law.
While the book left me with lingering questions, I largely agree with Stephen Jay Gould’s theories. I was particularly struck by the ideas that evolution is not driven by progress and that humans are not the most advanced organisms. Currently, humans occupy the terminal branch of the tree of life, but in the distant future—or perhaps a future closer than we imagine—another organism might branch out from that same terminal point. No one knows what mutations will occur or what new life forms might emerge. One thing is certain: life continues to evolve.
