113 The Rate of Evolutionary Change

Key Idea: New species may arise gradually through accumulation of differences over a long period of time or they may arise relatively suddenly. Two common models, gradualism and punctuated equilibrium, are proposed for the rate at which new species arise (below). It is likely that both operate at different times for different taxa in different situations. There is evidence for both models in the fossil record and in living populations.

Punctuated equilibrium

New species New species Parent species Parent species Phyletic gradualism

New species New species New speciesNew species Parent species Parent species

Each species undergoes gradual changes in its genetic makeup and phenotype.

New species bud off from the parent species and undergo rapid change, followed by a long period of stability. A typical pattern

New species diverges from the parent species. A typical pattern

There is abundant evidence in the fossil record that, instead of gradual change, species stayed much the same for long periods of time (called stasis). These periods were punctuated by short bursts of evolution which produce new species quite rapidly. According to the punctuated equilibrium theory, most of a species’ existence is spent in stasis and little time is spent in active evolutionary change. The stimulus for evolution occurs when a crucial aspect of the environment changes, creating new selection pressures. Phyletic gradualism assumes that populations slowly diverge by accumulating adaptive characteristics in response to different selective pressures. If species evolve by gradualism, there should be transitional forms seen in the fossil record, as is seen with the evolution of the horse. Trilobites, an extinct marine arthropod, are another group of animals that have exhibited gradualism. In a study in 1987, a researcher found that there was a gradual change in eight lineages over a period of three million years.

1. Suggest the kinds of environments that would support the following paces of evolutionary change:

(a) Punctuated equilibrium:

(b) Gradualism:

2. In the fossil record of early human evolution, species tend to appear suddenly, linger for often very extended periods before disappearing suddenly. There are few examples of smooth inter-gradations from one species to the next. Which of the models (punctuated equilibrium or gradualism) best describes the rate of human evolution?

3. Some species, such as sharks, horseshoe crabs, and tuatara, apparently show little evolutionary change over long periods of time (hundreds of millions of years).

(a) What term given to this lack of evolutionary change?

(b) Suggest why such species have apparently changed little over evolutionary time: