Macroevolutionary Trends: New Perspectives on the Roles of Adaptation and Incidental Effect

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Science  22 Jul 1983:
Vol. 221, Issue 4608, pp. 387-389
DOI: 10.1126/science.221.4608.387


Trends, long-term directional tendencies in evolution, are traditionally interpreted as selected for and adaptive. Alternatively, trends may be unselected effects of characters and processes within species: the effect hypothesis. Thus adaptations of organisms, varying among species, were selected for immediate fitness, but they may also incidentally determine different speciation and extinction rates and trends.