Since the mid-1800s evolutionary biologists have speculated on the pace at which speciation typically occurs. Most of the evidence regarding past speciation events is present in the fossil record, which is all the fossilized artifacts taken in the context of their placement within Earth's geological strata. The fossil record spans such vast periods of time that events that seem instantaneous in geological time have taken millions of years to occur. For example, fossils that are only a few feet lower than others in the strata may illustrate a dramatic shift in environment and the species that existed and, hence, may seem to indicate that there was a rapid change. In reality, it would have taken thousands or millions of years for the sediment to build up over the fossils of one species and be compressed into the rock in which we now find them. Other limitations of the fossil record, such as gaps and a general lack of organisms that do not fossilize well, have added additional room for debate about how speciation happens in nature. Gradualism and punctuated equilibrium are two models of the tempo of speciation. Evidence exists to support both hypotheses.
Some argue that speciation involves the accumulation of small genetic changes over a long period of time, a concept known as gradualism. Proponents of gradualism argue that huge changes over a short period of evolutionary time are rare. When a gradual change is noticed in the fossil record, it is difficult to tell exactly when a new species formed. This is because the fossil record has many inconsistencies and is incomplete. One species, however, does show some evidence of gradual transitions. The fossil stickleback fish Gasterosteus doryssus provides sufficient evidence to show such transitions. Many fossils have been found and dated, which leads scientists to believe that G. doryssus resembles the similar, modern three-spined stickleback Gasterosteus aculeatus. The body of Gasterosteus aculeatus is covered with armored plates, but G. doryssus is only minimally covered with armor. The fossils suggest that this extinct species evolved two variations, including a near-shore, bottom-feeder and an offshore species that ate plankton. The bottom-feeding species expanded its range into the off-shore region of the lake. Here, it apparently switched to eating plankton and lost most of its armor within c. 5000 years. The evidence suggests a gradual evolution of the mean phenotypes.
The idea of a different path to speciation has become popular among evolutionary biologists in recent decades. Punctuated equilibrium describes a pattern in the fossil record in which a species undergoes long periods of no change, which are interrupted by a burst of strong selection and rapid changes over relatively short periods of time. This is exemplified in a coral-like animal called a bryozoan. There are currently three living species of the genus Metrarabdotos. Fossil evidence suggests that Metrarabdotos underwent massive speciation some 8-4 million years ago, becoming 12 species. Studies have compared the genetics and body structures of these extinct species with modern ones and determined that the evolution of Metrarabdotos happened through a series of short, dramatic events. Evidence suggests that between 8-7 million years ago, 9 of the 12 species evolved. Most of these have gone extinct. Those that survived, however, continued to evolve into the modern species.Scientists support punctuated equilibrium more consistently than gradualism because of the incompleteness of the fossil record. When additional evidence is found, the ideas about the tempo of speciation are revisited and adjusted.