The rate at which species evolve has been an issue of importance even before Darwin's contributions to evolutionary theory. The concept of spontaneous generation was embraced by Lamarck and a similar idea, saltation, is considered by some even today as a possible explanation (by way of genetic mutations) for sudden appearances of species. Darwin himself was troubled by the gaps in the fossil record, going so far as to profess this phenomenon as a serious obstacle to his theory.
Today many of these gaps have been, and continue to be, filled, but debates still continue concerning evolutionary rates of lineages. This is in part due to the fact that where there are complete pictures the results are mixed and invite a variety of interpretations.
Different lineages can, and often do, evolve at different rates. Even subfamilies of a common family can exhibit marked differences, as demonstrated by two subfamilies of oreodonts, Merycochoerinae and Merychyinae, in the Miocene. The fact that there is a significant evolutionary rate among certain species at all is curious; one would expect, if environmental conditions and habitat were unchanging, that natural selection would not be very active and the resulting rate would be steady and close to zero.
Recorded examples of these kinds of conditions do exist and they confirm somewhat slow rates but are not static. A study of the sequences of molar size among three functionally similar early primates in the Eocene establishes moderate rates of evolution for these animals. The physical characteristics of Brachiopod ribs in a line descent during ten million years in the Silurian actually alternated between rapid and slow rates. Similarly, the measurement of lungfish rate of change from the Devonian to present shows a variable rate, beginning rapidly and then leveling off almost, but not quite, to zero after a considerable length of time (300 my).
Thus it appears to follow that a relative stasis in environment does not correspond to a stasis in evolutionary rate in taxa, although the gradual unfoldment of evolution as conceived by Darwin can be interpreted from these and many more examples.
However, in contrast, other examples show that gradualism is not always the rule, and this has fanned the flames of the old saltorial debates, but in more modern terms of quantum evolution and punctuated equilibrium.
Quantum evolution may seem to support saltation theory but the examples found in the fossil record are not altogether common to all species as a whole. Instead, they reflect special circumstances in which there is a shift, or breakthrough, from one ecological "zone" to another (this is often associated with adaptive radiation). A notable example is the sudden appearance of functional wing structures on a certain mammal, resulting in the Chiroptera order (bats) about 50 million years ago. This example involves origins of lineages. When a similar phenomenon of sudden origination affects the species level it may be termed quantum speciation. An example is the very high rate of speciation of the elephant family, not only in the direct lineage but among the side branches as well.
This last example, and the fact that there are still many gaps in the record, has led to explanations of punctuated equilibrium and to the old saltorial models which seem to refuse to die. The latter, however, have found some restraints that were not available to Darwin's time. Genetically, an entire mutational makeover in one step is very unlikely; speciation is the result of a complex and much slower genetic rearrangement. Furthermore, some proponents of punctuated equilibrium see very rapid speciation followed by long periods of no change as a reason for fossil record gaps. But the earlier examples given show clearly that long periods of no change are quite rare.
Examples given of high rates of evolution such as those found during periods of adaptive radiation suggest that gradualism is not necessarily the rule (although that term is, arguably, a relative one), but explanations involving radical leaps in speciation appear particularly shaky. They arise in most part as an attempt to explain lapses in recorded history. However, these gaps are being filled and the pictures that are being completed, in turn, are not very supportive of these hypotheses. As might be expected, the truth is probably somewhere in between, dependent on several types of circumstances and not one single rule. As is usually discovered when observing any evolutionary mechanism in depth, it is difficult to impose narrowly-defined limits or restrictions in order to the simplify the picture.
S. Brown