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It is abundantly evident that rates of evolution vary. They vary greatly from group to group, and even among closely related lineages there may be strikingly different rates. Differences in rates of evolution, and not only divergent evolution at comparable rates, are among the reasons for the great diversity of organisms on the earth. Among the living primates there are, for instance, some rather unspecialized or primitive prosimians (i.e., little changed from Eocene progenitors), a larger number of divergently specialized prosimians, many monkeys of different degrees of progression and divergence, a few apes, and the unique species of man. Important as is the purely divergent evolution, it is also clear that differential rates are involved. At the extremes, the lineages of the more primitive living prosimians have evolved less rapidly as regards the whole of their structure and adaptive position than has the lineage of man.

Anyone group, large or small, seems to have a fairly characteristic mean or modal rate of evolution and a certain range of less common rates on each side of this. The rates have a distribution pattern which is open to investigation and which is an important element in evolution. It has long been noticed, further, that there are a few lines in most large groups that seem to evolve at altogether exceptional rates. Some hardly change at all over long periods of time. Others change, over short periods of time, at rates so rapid as to be almost beyond comparison with those usual among their relatives. It must be considered whether these extraordinary rates are merely the extremes of normal variation in rates or whether they reflect special circumstances different from those in the distributions of usual rates. In either case they must involve different intensities, combinations, or both as regards evolutionary determinants, and it would be of extreme interest, perhaps even of considerable practical value, to identify these.

Distributions of Rates of Evolution

Examples of how rates may be estimated and represented showed that:

(a) related lines of descent commonly differ in evolutionary rates; 

(b) within larger taxonomic groups such as orders or classes there is generally an average or modal rate typical for the group; and (c) the average rates may differ greatly from one group to another. Since rates do vary broadly around a group mode, rate distributions of different groups tend to overlap widely even though the modes are quite distinct. Mammals seem surely to have a much higher model rate of evolution than molluscs, but some mammals have evolved more slowly than some molluscs. It even seems clear that some mammals have evolved at rates below the mode for molluscs and some molluscs at rates above the mode for mammals..

The two cannot even both be studied authoritatively by one specialist. For instance, it is often said (e.g., again by Hutchinson, 1945)that molluscs may have evolved very rapidly in the soft parts without having this reflected in the shell. But in molluscs the whole skeleton is usually available and studied in fossils (which is very rarely true of, say, fossil mammals) and their skeleton reveals a great deal about the soft parts, directly (muscle scars, etc.) or indirectly (growth rates, thickness and chemical nature of shell, twisting of body, etc.). Percentagewise a mollusc shell probably tells more about the soft parts than do the usually available parts of a vertebrate. It is extremely unlikely that any marked progressive change in soft parts could occur without obvious changes in the shell. We need not, then, bend over backward in denying validity to comparisons of rates between groups, even such very disparate groups as molluscs and mammals, and of course this applies all the more strongly to comparisons between groups that are similar, such as different sorts of molluscs or of mammals. The rate distributions can be compared in any two groups without regard for the absolute values of the rates. Survivorship curves are quite comparable even when the absolute rates involved are altogether different. There is little reason to doubt that both the resemblances and differences in shapes of such curves can be significant facts that are independent of our ability (or inability) to equate individual rates in the groups compared.

Survivorship tends to be negatively correlated with rate of evolution. By and large, the longer a genus, say, endures the more slowly its included populations are evolving. This provides a very convenient way to approach the problems of the distributions of evolutionary rates. By assuming that evolutionary rates are the reciprocals of survivorship, it is possible to calculate from any survivorship curve (or tabular data) what percentage of the population has evolved at any particular rate. e.g., in decile classes of range.

Such rate distributions for pelecypod and mammalian land carnivore genera have been calculated from the survivorship data that are graphically presented in Figure. For reasons that will become clear, these rate distributions are based on extinct genera, only. The two curves have a characteristic difference: the carnivore mode is more strongly peaked and classes just below the mode are correspondingly lower.

FIGURE 1. FREQUENCY DISTRUBUTIONS OF RATES OF EVOLUTION IN GENERA OF PELECYPODS AND LAND CARNIVORES. Histograms based on survivorship of extinct genera and on the postulate of—1.00 correlation of survivorship and rates of evolution. Ranges (abscissal scale) divided into deciles to produce histograms strictly comparable in form although absolute rates are very different. Normal curves equal in area to the histograms drawn for comparison