According to evolutionary theory, the fossil record is supposed to show the development of life over long periods of time. However, construction of evolutionary phylogenies often depends on conclusions drawn from fossil morphology alone. With more detailed morphologic analyses, with comparisons of fossils to living species, and with genetic analyses of both fossil and living representatives, large-scale changes and unequivocal transitions are difficult to perceive. Using several recent studies, this lack of change is attributed to limits on variation through time and among similar organisms.