Evolutionary rates and adaptive radiations

The term adaptive radiation has been recurrently used to describe evolutionary patterns of several lineages, and has been proposed as the main driver of biological diversification. Different definitions and criteria have been proposed to distinguish an adaptive radiation, and the current literature shows disagreements as to how radiating lineages should be circumscribed. Inconsistencies increase when authors try to differentiate a clade under adaptive radiation from clades evolving under regular' speciation with adaptation, a pattern anticipated and predicted by the evolutionary theory in any lineage. The most important disagreement is as to which evolutionary rate (phenotypical or taxonomical) authors analyze to characterize a radiation; a discussion embedded in a prevailing inability to provide mechanistic explanations of the relationship among evolutionary rates. The union of pattern and process in the same term, the inadequacy of reported null hypotheses, and the frequent use of ad hoc comparisons between lineages have also contributed to the lack of consensus. A rigorous use of available terms and the articulation of solid criteria with objective methodologies in distinguishing evolutionary patterns are imperative. Given the difficulties in detecting adaptation, the use of the adaptive' term to qualify a radiation should be avoided unless methodologically tested. As an unambiguous method to distinguish radiating lineages, the statistical detection of significant increases in taxonomic diversification rates on monophyletic lineages can be considered a distinctive signature of a radiation. After recognizing this pattern, causal hypotheses explaining them can be stated, as well as correlates with other rates of evolution.