Reading Practice 32

[1] According to simple definitions, biological evolution is the process of change over time. However, mountains change over time, and biological evolution surely doesn’t apply to them. So, more aptly, biological evolution can be described in the words of Charles Darwin himself: “descent with modification.” Key to this definition is “descent;” within a species, individuals pass on genetic information through reproduction. Some genetic traits become favored; that is, they are advantages in the battle for survival. They, therefore, become passed on more frequently than those traits which are not favored. And so we have the “modification” part of the equation. Although nobody now disagrees about the basic mechanism by which evolution occurs, there has been considerable disagreement – ever since the theory of evolution was first propounded – about the rate of change and species adaptation.

 

[2] Charles Darwin’s original concept of evolution included the idea of gradualism, the idea that small changes happen gradually over time. Onlookers are unable to see great change in a short period because great changes are simply the culmination of the small changes that happen over long periods. This follows the prominent geological theories of the time put forward by James Hutton and Charles Lyell, who demonstrated that changes to the earth happen slowly and gradually.

 

[3] But even if the world came to accept evolution as a biological fact, how could a concept like gradualism be proven? To answer questions about evolution, we normally look to the fossil record for answers. By examining what is written through fossils in the different layers of the earth, we can observe how species changed over time. Darwin claimed that proof of gradualism should be able to be found in “intermediate forms” in the fossil record. That is, modifications within a species should appear in small incremental stages. There is, however, an unavoidable complicating factor: the fossil record is incomplete. We do not have a comprehensive catalog of fossils at our disposal. Instead, there are gaps in the record, gaps which proponents of gradualism say account for the lack of extant transitional forms.

 

[4] Of course, perhaps the lack of intermediate forms (in truth, the record does include some examples of transitional modifications) in the fossil record indicates a lack of transitional forms throughout history. In other words, perhaps gradualism is incorrect. So thought Stephen Jay Gould and Niles Eldredge when, in 1972, they first proposed the mode they called “punctuated equilibrium.” According to this hypothesis, adaptations within species occur quickly, in short bursts, after long periods of stasis. During such periods of stasis – the “equilibrium” of the name – little change occurs. Then, we see a rapid and dramatic variation, leading to a new species. Their hypothesis included the idea that such sudden changes typically occurred in relatively small and isolated populations within a given species, living on the periphery. These “geographical isolates,” as they put it, had advantageous morphological variations, and therefore greater success at reproduction. Subsequently, they spread through the range of the species from which they stemmed, replacing them.

 

[5] It would appear that the fossil record supports this claim of bursts in evolution, as the records show little growth or evidence of continuous changes. Instead, we see sudden major changes, sometimes resulting in a whole new species. If fossil records were less scant, scientists who support gradualism might anticipate that fossils would show traces of adaptation each year. Yet, the imperfection of fossil records lends credence to punctuated equilibrium. But what do we observe in fossil records that are more reliable, in which we have many more representative examples throughout history?

 

[6] Consider the series of trilobites and their segments that have been studied by Dr. Peter Sheldon of Cambridge University. The fossil records for these trilobites were constant, as a series of rock layers rich with fossils provide remains of several species of this extinct segmented marine animal. Sheldon’s study reviewed eight species of trilobites, finding that each species experienced a gradual increase in the number of segments. Over a span of three million years in evolution, Sheldon found that there were no bursts of change in the species. This would appear to give lie to punctuated equilibrium; however, Sheldon has admitted that the environmental conditions in which the trilobites lived remained relatively constant, which may account for the slow rate of change.