A tradition in biology has been taxonomy, the classification of organisms into hierarchical groupings: the identification of species, the grouping of species into genera, genera into tribes, tribes into families, and so on. Many biologists have long been preoccupied with going further and attempting to construct phylogenies―evolutionary relationships―from that information. But the reverse procedure of building evolutionary trees from molecular data, and then defining taxonomic groupings and levels by dates of common ancestry, has opened up new avenues for studying evolutionary processes. One such process is the occurrence of species radiations, in which certain evolutionary lineages have diversified and produced far more species than others. What factors lie behind this phenomenon? As he describes in Proceedings of the Royal Society, Robert Ricklefs has tackled the question by looking at data for the passerines. This is a group of birds, at the taxonomic level of order (one above families), which includes over 5,000 species and such well-known examples as crows, thrushes and finches. One common assumption in evolutionary biology is that key morphological or behavioural innovations spark off adaptive radiations. For example, plant-feeding in insects has evolved on several occasions and seems generally to have resulted in an increased net rate of speciation2. Ricklefs concludes, however, that key innovations are unlikely to have been the major cause of variation in speciation rates among the passerines. Instead, those species that happened to be in the right place at the right time to exploit ecological opportunities were the progenitors of the major radiations.
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