Key innovations versus key opportunities: Identifying causes of rapid radiations in derived ferns

Abstract

Biological radiations, e.g., adaptive radiations and rapid radiations, are widely accepted as one of the major events contributing to the diversification of the tree of life, but many aspects of these events are poorly understood (Schluter 2000; Gavrilets and Losos 2009). The classical examples for biological radiations are adaptive radiations, in which a lineage occupies a range of niches by diversifying in a relative short time until all niches are filled. However, not all radiations necessarily fulfil this pattern and alternative scenarios such as rapid radiations are widely considered. A major setback is the lack of a generally accepted definition of biological radiations (Schluter 2000). Empirical studies on the frequency and contribution of radiation, however, need concise theoretical concepts and criteria such as temporal increase of diversification rate or the absolute number of species to define biological radiations (Schluter 2000; Gavrilets and Losos 2009). Thus, our knowledge is limited by both the disparity of applied concepts and the limited amount of studies scrutinizing evidence for as many lineages of organisms as possible. It is therefore impossible to estimate quantitatively the contribution of radiations to the diversity of life on earth today. A recent study on 101 phylogenies discovered evidence for a hypothesis that explains the constant accumulation of biodiversity through rare single speciation events instead of species radiations (Venditti et al. 2010).