Evolutionary trends, whether macroevolutionary events across higher taxa and large spans of geologic time, or more circumscribed events within smaller clades, have long played a prominent role in evolutionary theory. The identification of trends in the history of life has been used as evidence for the increasing adaptedness of life, for evolutionary progress, and more recently as support for selection at multiple levels. Thus, trends are a key feature of many discussions of evolutionary patterns and process. For many decades the existence of a trend was often simply asserted by an author, sometimes accompanied by an interpretive sketch. More recently, however, standards for the elucidation of trends have become considerably more rigorous. Researchers are expected to distinguish active or driven trends from passive trends. An active or driven trend is one in which there is a shift in the mean value of some character because of selection or some other factor. Passive trends are those associated with an increase in “variance,” and from random fluctuations in trait values. Trends have been described in many different variables, most frequently in body size (Cope’s Rule), as well as in complexity; in rates of speciation, extinction, or diversification; and in a variety of ecological factors. Among some groups of anthropologists, trends in cultural patterns have also been described, although generally without the rigorous analysis now expected in biological systems.
Article. 6378 words.
Subjects: Evolutionary Biology
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