rates and trends in evolution Evolution is change through time, and hence it is convenient to divide its patterns into rate and direction of change. Rate is defined as the amount of change over a period of time. In graphical terms, this corresponds to the slope of a line tracing the phenomenon through time. If the line is straight, the rate is constant; if the line is curved, then the rate is changing. Direction is the tendency for a phenomenon to increase or decrease persistently over a period of time. A common word for such a tendency is ‘trend’.
Evidence from many fossil studies has given rise to a major debate over whether evolutionary rates are generally rapid or gradual. Darwin originally argued for ‘gradualistic evolution’. Work by Stephen J. Gould of Harvard and Niles Eldredge of the American Museum of Natural History indicates, however, that a different process, termed ‘punctuated equilibrium’, is more common. This process is one in which very little change takes place during much of a species' existence, but rapid brief bursts of change punctuate this stasis or equilibrium. It is thought that the change is forced by rapid evolution in small isolated populations during periods in which their range has been reduced by changing environmental conditions. A survey of fossil lineages in which rates of evolution have been studied shows punctuated rather than gradualistic patterns in the majority of cases. There are, however, problems that impede a clear interpretation of the data. In particular, there are gaps in the fossil record and there is disagreement over what is meant by ‘rapid’. As the fossil record is very incomplete, we do not have a continuous record of any species. If intermediate forms are missing, a gradual rate of evolution may appear to be punctuated. This is exacerbated by the difficulty of precisely dating sedimentary rocks: we cannot tell the length of the gaps. If they were short, then it could be assumed that evolution really was rapid. Rapid evolution can be shown in some modern examples, as in Death Valley, where small lakes that have been separated for only 20 000 years now harbour different species of fish. It is also clear that evolutionary rates can be very low in some lineages, for it is possible to find modern forms, termed ‘living fossils’, that differ little from fossil forms. Sequoia trees have changed little in the past 70 million years, and the inarticulate brachiopod
Lingula has changed little in over 350 million years.
Trends can be seen either as lineage trends in evolving lineages, or biosphere trends affecting the evolution of life as a whole. Lineage trends show directional tendencies in various traits and organs that change through time, and result from consistent directional selection. Thus, the evolution of the horse shows correlated changes in traits resulting in increases in tooth and body size, and reduction in the number of toes from four to one that occurs over a period of 60 million years. This is apparently due to the adaptation of the horse to the spreading grasslands of the Cenozoic. Biosphere trends can be seen in the increasing diversity and size of organisms through time, which indicates a steady increase in their complexity.
David K. Elliott
