The traditional understanding of evolution is that species, over time, evolve into new species. Speciation is the result of a gradual transition of an ancestral population. This theory has withstood the test of time as the accepted model for speciation.

The controversey surrounding this theory is that it often contradicts other proven principles. Homeostasis proposes that species are resistant to change by nature, and should therefore be in equilibrium with their environments. Fossils with widespread distribution and with a large vertical range thus represent organisms which were the most succesful in resisting change.

Source: Eldredge and Gould (Kauffman and Hazel 1977) p62