Due to the emergence of various concepts in evolutionary theory, there is an extensive discussion on cardinal problems related to the graduality or discontinuity of evolution. American paleontologists Niles Eldredge and Stephen Gould (1993) found a lack of gradual transition between successive forms. The species, which existed for millions of years without any significant changes, suddenly disappeared and replaced a new one with a completely different quantitative characteristic of the main morphological feature. The described process of alternating the stable state (stasis) of the species and its rapid replacement with a new one was called punctuated equilibrium. According to this new model, evolution occurs in rare and rapid tremors; it seems to pulse.
What are the arguments for and against punctuated equilibrium?
The adherents of the punctuated equilibrium theory are based on extensive paleontological material and have quite strong theoretical justifications. The species is a relatively stable self-supporting system. For a new system to occur, the old system must be destroyed. Some changes are needed, leading to the destruction and destabilization of internal ties. Against punctuated equilibrium, one can argue that the species’ geographical movements can explain the differences in fossils that laid the foundation for the theory. However, there is experimental evidence about a sharp change in conditions or intensive selection, such as in animal domestication experiments (Gould & Eldredge, 1993). Changes do not lead to a smooth and gradual shift in the population’s morphological characteristics, but a sharp increase in variability. Later, a new variety or species may arise from this destabilized population.
How does punctuated equilibrium compare with gradualism?
Punctuated equilibrium is traditionally opposed to the ideas of gradualism. According to studied theory, evolution occurs as a result of rare and rapid periods of formation of new species. Gradualism, in turn, claims that transformations occur gradually and over a long period. Eldredge and Gould (1993) showed the contrast between the lengths of time involved in stasis and speciation and raised the question of the structure of the evolutionary process. Thus, the establishment of the fact of stasis existence opposes the idea of the continuous evolutionary fluidity of life forms. Nevertheless, it is essential to note that the authors of punctuated equilibrium do not consider the stasis as passive stability that corresponds to the environment’s constancy. They see it as the genetically active state of the species.
Which one of the theories gradualism or punctuated equilibrium makes the most sense?
In my opinion, although at first the disputes between the representatives of gradualism and punctuated equilibrium were quite vigorous, today, these two concepts can coexist within a single developing evolutionary theory. They may not contradict but complement each other to understand progression better. Most experts recognize that evolution can sometimes follow a punctuated, sometimes a gradualistic scenario. Moreover, there is a study proving the possibility of both scenarios of evolution. Scientists concluded that 22% of DNA differences occur during brief periods of intensive speciation, and the remaining 78% gradually accumulate during long gradualistic periods (Pagel et al., 2006). The leaps of evolution are more characteristic for plants and fungi, and less characteristic for animals.
Do Gould and Eldredge make a convincing argument that punctuated equilibrium is the only process that occurs?
The authors of the theory do not argue that punctuated evolution is the only possible process. Firstly, in their article, they talk about researches that can cause concern for their approach, despite their strong faith in it (Gould & Eldredge, 1993). Secondly, each scientist, even if he or she is confident in specific ideas and concepts, should allow the possibility of its rebuttal and the appearance of alternative options. In the opposite case, the development of science will stop, and scientific discourse will be meaningless.
References
Gould, S., & Eldredge, N. (1993). Punctuated equilibrium comes of age. Nature, 366(6452), 223-227.
Pagel, M., Venditti, C., & Meade, A. (2006). Large punctuational contribution of speciation to evolutionary divergence at the molecular level. Science, 314(5796), 119-121.