The short-term changes in allele frequencies within populations that we have emphasized in this chapter, often termed microevolutionary changes, are an important focus of study for evolutionary biologists. These changes can be observed directly, they can be manipulated experimentally, and they demonstrate the actual processes by which evolution occurs.

Long-term patterns of macroevolutionary change can also be strongly influenced by events that occur so infrequently (such as a meteorite impact) or so slowly (such as continental drift) that they are unlikely to be observed during short-term studies. The evolutionary mechanisms at work may change over time with changing environmental conditions. Even among the descendants of a single ancestral species, different lineages may evolve in different directions. Additional types of evidence—evidence demonstrating the effects of rare and unusual events on trends in the fossil record—must be gathered if we wish to understand the course of evolution over billions of years. We will consider these long-term aspects of evolution in the remaining chapters of this section.