ABSTRACT
Taxonomy is intended to be a tool for biologists, allowing categorization of species into groupings that reflect their relative degree of relationship to each other. Evolutionary biologists have shown that many of these similarities shared within these groupings are due to common ancestry: descent from a single population of organisms. Macroevolution, however, encompasses processes that often take thousands of generations, resulting in major changes in the average phenotypes of populations. When observing macroevolutionary events, some processes become important besides the microevolutionary forces of selection, mutation, and genetic drift. The gradual increase in atmospheric oxygen permitted evolution of aerobic metabolism, a more efficient energy use in organisms, and was, therefore, a necessary but not sufficient reason for the origins of multicellular organisms. Reptiles can live their entire lives outside of water, in part due to the evolution of the amniote egg, which can be laid, and hatched, on land.
