On the "fragmented equilibrium" theory - the great contribution of paleontologist Steven J. Gold, who died about two weeks ago, to the study of evolution
Ofer Mokdi
Even as a young child Stephen Jay Gould knew exactly what he wanted to do when he grew up. After a visit to the Museum of Natural History in New York, all of which was enthralled by a dinosaur skeleton about 6 meters tall that he saw there, the young Gould announced that he would be a paleontologist (researcher of fossils). 20 years later, in 1967, Gold did receive a doctorate in paleontology from Columbia University. From then until his death, on May 20, 2002, he researched and lectured in the fields of geology and paleontology and served as director of the paleontological collections at Harvard University. Gold died of lung cancer, a disease he had already been diagnosed with in 1982. He was only 60 years old.
Gold is best known for his contribution to the study of evolution and imparting the ideas of evolutionary thought to the general public. During his years of activity he published more than 20 books and hundreds of newspaper columns, in which he discussed evolutionary ideas and concepts from unconventional angles.
Gould knew how to explain the most complicated evolutionary phenomena in a clear and understandable language. In his books, the reader can make a thorough acquaintance with concepts such as evolutionary adjustment (adaptation) and differentiation (creation of species), and with the main driving forces in evolution - natural selection and genetic drift. Some of his books have been translated into Hebrew, including "Since Darwin" (published by "Maariv Library", 1983, translated from English by Naomi Carmel) and "Bohan the Panda" (published by "Dvir", 1990, translated from English by Amos Carmel).
The creation of species - two theories
Along with his work in bringing evolution to the general public, Gould's professional lectures were a magnet for Harvard students. The halls where he gave his lectures were full of listeners who sought to learn from him in the fields of geology, biology, evolution and even the philosophy and history of science. Among his research colleagues, he was accepted as a thinker with a critical, original and profound thought, who challenged some of the accepted concepts of evolution.
Bursts of evolution
Gold's most important scientific contribution is probably the theory known as punctuated equilibrium. This theory, which he practiced and published jointly with his friend and colleague Niles Eldredge in 1972, deals with the rate of evolutionary changes. The theory holds that far-reaching evolutionary changes, such as lead to the creation of new species, occurred in relatively short periods of time (in this context it is important to remember that thousands and even a few million years are considered "in the blink of an eye" on an evolutionary scale). These "evolutionary bursts" occurred mainly when new species were created, and most of the time no evolutionary changes occurred at all, that is, evolution occurred at a rate that changed drastically, from very fast to almost zero. In addition, most of the processes that led to changes and the creation of new species occurred in isolated populations, which were exposed to different conditions than the rest of their species. This theory has some very important implications for the study of evolution and evolutionary thought in general. Among other things, it was sometimes perceived by the scientific world, and also by Gold himself, as contradicting the description of evolution according to Darwin. Darwin's approach according to this concept deserves the nickname "Phyletic gradualism" (in free translation: gradual creation of species), and describes the course of evolution as a sequence of changes during which new species were sometimes created.
The beauty of the "fragmented equilibrium" lies, among other things, in its ability to explain problems that have plagued evolution researchers since Darwin presented his theory in 1859, chief among them the incompleteness of fossil collections. The meaning is mainly that in almost all cases large differences are evident between fossils discovered in adjacent rock layers, and it is impossible to find intermediate forms whose existence is required from the line of thought of "gradual creation". The lack of continuity in the fossil finds was traditionally explained by weathering processes that may have consumed some of the fossils, and mainly by our inability to search well, and was a constant source of general discomfort among researchers; After all, considering the great effort invested in the search for fossils, we would nevertheless expect more cases of fossil continuity, if that were the course of things.
According to Gould and Eldredge, discontinuity in fossil finds is predictable. Since the evolutionary process during which the supposed intermediate forms existed was extremely rapid and occurred at the edge of the range of any original species, there is no reason to expect to find the intermediate forms in the same lineage as the original species, or at all. Only after the formation of new species, and when they expanded their area of distribution and perhaps even pushed the feet of the original species in the center of its area of distribution, did they begin to leave fossils there. However, by then their form was already significantly different from that of the original species, as revealed in paleontological research.
An apparent contradiction
In light of the apparent contradiction between the theories of Gold and Eldredge and that of Darwin, it is no wonder that many researchers tried to bring evidence from the world of contemporary nature and the world of fossils to try to decide between the theories. In 1995, 58 works were summarized in an article, which was their goal. The authors' unsurprising conclusion was that the findings "unequivocally support that the creation of species is... sometimes gradual and sometimes in the form of a fragmented equilibrium".
Since this is so, it is appropriate to try and reconcile the differences, and see if they are not as extreme as it seemed. The best place to start is perhaps with a quote from Darwin's writings, where it is written (in a free translation) that "...many species do not undergo any change after their formation... and the periods in which changes took place, even if they lasted for many years, were certainly relatively short compared to the periods during which species preserved their form... These things indicate that Darwin was not a follower of gradualism in the sense of the rate of evolution, and it was certainly correct to accept a variable rate of evolutionary changes. The term "graduality" in Darwin's language should only be attributed to the meaning of a large number of very small steps of change. This term does not have any reference to the speed of change, or in other words, to the question of how long the many small steps of change were carried out.
In Gould's many writings it is easy to see that in his opinion there is no question about the size of an evolutionary step - through images from everyday life, referring to the animals and plants around us, and in a picturesque and fluid language, Gould sought to instill in the general public the principle originally formulated by Darwin, according to which every evolutionary innovation Occurs in the form of a series of small change steps. Gold paid no less attention to the explanation of another principle formulated by Darwin, according to which a trend of evolutionary change can change if environmental conditions change. According to this principle, it is quite possible that over long periods of evolutionary time there will be no change in the shape of a species if the trend of change has reversed time and time again (for example, an increase and decrease in the dimensions of a certain organ). This is how fossil findings can be understood, according to which over a long period of time, over geological layers covering long periods, there are species that do not show any changes.
The contribution of the "fragmented equilibrium" theory is very great, therefore, in regards to the real rate at which evolutionary changes occurred, and in removing stumbling blocks (only some of which have been reviewed in this list) from the path of evolutionary research. However, contrary to what has been written more than once, it does not contain any real contradiction to Darwin's teachings, but only an important addition.
Dr. Mokdi is a researcher and lecturer at the Nature Conservation Research Institute, Faculty of Life Sciences, Tel Aviv University
