The key innovation concept in evolutionary biology has generated both tremendous interest and controversy. The idea of key traits that promote differential evolutionary success of the lineages that bear them has an immediate and intuitive appeal. Birds, by nearly any measure, are a tremendously successful group of vertebrates and have radiated into a vast array of ecological niches that are largely inaccessible to many other taxa. It is tempting to think that a particular trait or complex of traits might underlie this seeming pattern of evolutionary exceptionalism. In perhaps the first formal usage of the term, Miller described key innovations as adjustments to morphological and physiological traits that allow organisms to radiate within a new ecological plane, unconstrained by competitors that may have inhibited their diversification in the ancestral plane. In Miller's view, innovations would also come to characterize major taxa because species whose ecologies spanned both adaptive zones would be negatively impacted by interactions with species in both. Hence, gaps would develop between members of the ancestral adaptive zone and the new adaptive zone. Beginning in the 1980s, the rise of modern phylogenetic biology led to a shift in the way evolutionary biologists conceptualized and operationalized the idea of key innovations. Became widespread, biologists increasingly recognized the significance of widespread variation in species richness across the tree of life. The availability of phylogenies, and the desire to extract evolutionary inferences from them, provided a catalyst for the development of sophisticated statistical tools for studying the tempo and mode of lineage diversification. These tools could be used to determine whether the disparities in species richness observed across the tree of life could be explained by chance variation resulting from a common stochastic process, or whether it was necessary to invoke deterministic factors to explain differential diversity..  

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