Patterns of Macroevolution

Macroevolution (large-scale evolutionary change) occurs in defined patterns, including stasis, speciation, lineage character change, and extinction (a loss of all members of a particular group). The patterns may be the result of environmental changes, such as an ice age or an increase in predation, the preying of one animal upon others. Geological changes, such as earthquakes or volcanic eruptions, may change the land in such a way that evolutionary changes occur.

Stasis is a pattern in which a species in the fossil record (fossilized artifacts within geological strata) shows minimal change over time, and their lineage remains stable, or static. Connected to this concept is the idea that after long periods of stability, a species might have a burst of evolutionary changes, which may appear to be rapid. Examples of stasis include many species that do not change for many thousands of years, such as horseshoe crabs, which are largely unchanged from 450 million years ago to today. Coelacanths are another example of a organism that has lived for millions of years but shows little change through its lineage. Speciation (the process through which a species evolves into two or more new species) may result from an altered gene pool (the total set of all genes of all individuals in a population), geographic isolation, changes in food supplies, the presence of predators, or the development of natural or human-made barriers. Allopatric speciation occurs when a population of one species becomes geographically isolated from other members of the same species, and a new species evolves. Sympatric speciation is the development of a new species even though there is no geographic boundary. For example, one species of fly lays its eggs only on hawthorn trees. Over time, some females begin laying eggs on apples, and both male and female offspring look for mates on apples. The population has divided and created a separate population because of its own niche.

Within a species, lineage character change may occur either quickly or very slowly. The changes that happen may follow a constant line or show a reversal of traits. Trilobites, a group of extinct marine arthropods, for example, show changes of character through their lineage. Fossil records of trilobites have been found that prove trilobite morphology remained static over several hundred million years, followed by periods of punctuated equilibrium (periods of stasis interrupted by rapid evolutionary change) during which the species added new segments in the thorax. Early fossils of trilobites show fewer thorax segments than trilobites that existed just prior to extinction.

Extinction, the condition in which no more individuals of a species exist, is an ever-present pattern in macroevolution. A mass extinction, an extinction that is rapid and widespread and which dramatically decreases the biodiversity on Earth, has occurred five times, but localized extinction is ongoing. Baseline level extinctions, those that cause the loss of a single species at a time, occur all the time. Today, extinctions are occurring at an accelerated rate because of the overall warming of Earth as well as the other impacts of human populations on Earth. Without including prokaryotes, scientists estimate a biodiversity level of about 8.7 million living species and an annual extinction rate of 100 extinctions for every million species. Evidence suggests this rate will continue to accelerate as humans continue to have an increasing impact on Earth's environment, possibly reaching as high as 1,000 extinctions per year. In terms of macroevolution, 99% of all species that ever lived have gone extinct. Human activity accelerates the rates of extinction, primarily through changes humans make to an animal's natural environment, and mass extinctions cut short the evolutionary opportunities of many species.