![]() ![]() It is part of evolutionary theory that all processes take place at the level of populations. Others use terms like macroevolution, or even megaevolution, as if the processes are different from those which occur below species level. Robert L Carroll prefers to use the term major evolutionary transitions, though it turns out that all or most of these could also be described as adaptive radiations. The term was introduced and discussed by George Gaylord Simpson, the palaeontologist who contributed to the modern evolutionary synthesis. This is an evolutionary process driven by natural selection. With less competition, groups diversify to fill available habitats and niches. The greatest radiation of all took place early in the Cambrian period, when most of our animal phyla evolved: see List of animal phyla. The Ediacaran biota were the result of an early metazoan radiation. This must have something to do with the availability of ecological niches and relative little competition. However, in the most striking cases, such as occurred in the Triassic after the greatest extinction event in Earth history, many lines underwent rapid radiation simultaneously. Some definitions phrase it in terms of a single clade: "Adaptive radiation is the rapid proliferation of new taxa from a single ancestral group". It produces more new species, and those species live in a wider range of habitats. It is an increase in the number and diversity of species in each lineage. There are at least 9,000 living species of birds, far more than mammals.Īdaptive radiation is rapid evolutionary radiation. The synthesis of ecological, phylogenetic, experimental, and genomic advances promises to make the coming years a golden age for the study of adaptive radiation natural history data, however, will always be crucial to understanding the forces shaping adaptation and evolutionary diversification.The evolution of bird beaks and feeding methods led to a great increase in the number of bird species. Whether radiations usually unfold in the same general sequence is unclear because of the unreliability of methods requiring phylogenetic reconstruction of ancestral events. Contingencies of a variety of types may usually preclude close similarity in the outcome of evolutionary diversification in other situations. Although predicted by theory, replicated adaptive radiations occur only rarely, usually in closely related and poorly dispersing taxa found in the same region on islands or in lakes. ![]() Nonetheless, many clades fail to radiate although seemingly in the presence of ecological opportunity until methods are developed to identify and quantify ecological opportunity, the concept will have little predictive utility in understanding a priori when a clade might be expected to radiate. Ecological opportunity is usually a prerequisite for adaptive radiation, although in some cases, radiation can occur in the absence of preexisting opportunity. In this review, I examine two aspects of adaptive radiation: first, that it results from ecological opportunity and, second, that it is deterministic in terms of its outcome and evolutionary trajectory. Adaptive radiation refers to diversification from an ancestral species that produces descendants adapted to use a great variety of distinct ecological niches. ![]()
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