10.12: The pace of evolution is not constant.

If you listened in on some scientists debating their research, you might hear one side describing “evolution by jerks” and the other side making light of “evolution by creeps.” They’re not gearing up for a fight out in the schoolyard. They’re just debating the pace of evolution.

The traditional model of how evolutionary change occurred was that populations changed slowly but surely, gradually accumulating sufficient genetic differences for speciation—hence the phrase “evolution by creeps.” Spurred on by findings from the fossil record that do not always support this view, however, researchers have come to believe that evolution may often occur in a different way: brief periods of rapid evolutionary change immediately after speciation, followed by long periods with relatively little change—hence “evolution by jerks.” This newer view of the pace of evolution, in which long periods of relatively little evolutionary change are punctuated by bursts of rapid change, is called punctuated equilibrium (FIGURE 10-23).

In nature, we can find examples of both gradual change and the irregular pattern of punctuated equilibrium. For many groups of organisms, such as many mollusks and mammals, we see in the fossil record a long period in which particular species seem to go through very little change, followed by the appearance of a large number of newer, but clearly related, species, without seeing any fossils representing the gradual transition between the newer and older fossils. For other groups, such transitional fossils do exist. For example, very complete sets of fossils reveal the transitional sequence from fox-like ancient terrestrial mammals to modern whales.

In the end, there is no single rate at which evolution occurs across all species. Some species have spent vast periods of time with little change at all. The coelacanth species of fish, for example, seems to have undergone almost no physical changes over more than 300 million years. Other species have clearly changed gradually, over time, for their entire evolutionary history. For any organism, the rate of evolutionary change depends on the selective forces acting on the population. Strong and directional forces may act over long periods of time and lead to rapid change, while stabilizing selection may lead to very little change.

Darwin foreshadowed the discussion about the pace of evolution in The Origin of Species, writing that “the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form.”