Biological diversification often includes burst of lineage splitting. Such “radiation” has been known to act as evolutionary arenas with the potential to generate unique phylogenetic clusters and further novel groups. Although these radiations when accompanied by ecological diversification, so-called “adaptive radiation” have persisted as a central premise in evolutionary biology, the ecological and genetic mechanism of such rapid diversification has remained unclear. There are several critical definitions for the pattern of adaptive radiation, and those provide delimitation of adaptive and nonadaptive radiation. That being said, only a few studies have provided any clear demarcations in our understanding of the adaptive and nonadaptive causes of radiation from the mechanism of speciation. Here, we review the current consensus for the causes of adaptive radiation, especially along with the recent theoretical synthesis of “ecological speciation.” Further, we suggest the signature of adaptive and nonadaptive radiation in the earliest stages of diversification from the viewpoint of speciation. These criteria from the speciation view are useful to find the cases with the signatures of adaptive/nonadaptive radiation.