CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
14. The Formation of Species and Evolutionary Change
14.5. Rates of Evolution
Although it is commonly thought that evolutionary change takes long periods of time, rates of evolution can vary greatly. Remember that natural selection is driven by the environment. If the environment is changing rapidly, changes in organisms should be rapid. Periods of rapid environmental change also result in extensive episodes of extinction. During some periods in the history of the Earth when little environmental change was taking place, the rate of evolutionary change was probably slow. Nevertheless, when we talk about evolutionary time, we are generally thinking in thousands or millions of years. Although both of these time periods are long compared with the human life span, the difference between thousands of years and millions of years in the evolutionary time scale is still significant.
The fossil record shows many examples of gradual changes in the physical features of organisms over time. For example, the extinct humanoid fossil Homo erectus shows a gradual increase in the size of the skull, a reduction in the size of the jaw, and the development of a chin over about a million years. The accumulation of these changes could result in such extensive change from the original species that we would consider the current organism to be a different species from its ancestor. (Many believe that Homo erectus became modern humans, Homo sapiens.) This is such a common feature of the evolutionary record that biologists refer to this kind of evolutionary change as gradualism (figure 14.11a).
FIGURE 14.11. Gradualism vs. Punctuated Equilibrium
(a) Gradualism is the evolution of new species from the accumulation of a series of small changes over a long period of time. (b) Punctuated equilibrium is the evolution of new species from a large number of changes in a short period of time.
Gradualism is a model for evolutionary change that evolution occurred slowly by accumulating small changes over a long period of time. Charles Darwin’s view of evolution was based on gradual changes in the features of specific species he observed in his studies of geology and natural history. However, as early as the 1940s, some biologists began to challenge gradualism as the only model for evolutionary change. They pointed out that the fossils of some species were virtually unchanged over millions of years. If gradualism were the only explanation for how species evolved, then gradual changes in the fossil record of a species would always be found. Furthermore, some organisms appear suddenly in the fossil record and show rapid change from the time they first appeared. There are many modern examples of rapid evolutionary change; the development of pesticide resistance in insects and antibiotic resistance in various bacteria has occurred recently.
In 1972, two biologists, Niles Eldredge of the American Museum of Natural History and Stephen Jay Gould of Harvard University, proposed a very different idea. Punctuated equilibrium is their hypothesis that evolution occurs in spurts of rapid change, followed by long periods with little evolutionary change (figure 14.11b). The punctuated equilibrium concept is a companion hypothesis to gradualism and suggests a different way of achieving evolutionary change. Punctuated equilibrium proposes that, rather than one species slowly accumulating changes to become a different descendant species, there is a rapid evolution of several closely related species from isolated populations. This would produce a number of species that would compete with one another as the environment changed. Many of these species would become extinct and the fossil record would show change.
Another way to look at gradualism and punctuated equilibrium is to assume that both occur. It is clear from the fossil record that there were periods in the past when there was very rapid evolutionary change, compared with other times. Also, some environments, such as the ocean, have been relatively stable, whereas others, such as the terrestrial environment, have changed significantly. Many marine organisms have remained unchanged for hundreds of millions of years, but there has been great change in the kinds of terrestrial organisms in the past few million years. Thus, it is possible that both gradualism and punctuated equilibrium have operated. The important contribution of punctuated equilibrium is that there can be alternative ways of interpreting the fossil record and that the pace of evolution can be quite variable. However, both approaches take into account the importance of genetic diversity as the raw material for evolution and the mechanism of natural selection as the process of determining which gene combinations fit the environment. The gradualists point to the fossil record as proof that evolution is a slow, steady process. Those who support punctuated equilibrium point to the gaps in the fossil record as evidence that rapid change occurs.
14.5. CONCEPT REVIEW
13. What is the difference between gradualism and punctuated equilibrium?