Anevolutionary radiation is an increase intaxonomic diversity that is caused by elevated rates ofspeciation,[1] that may or may not be associated with an increase inmorphological disparity.[2] A significantly large and diverse radiation within a relatively shortgeologic time scale (e.g. aperiod orepoch) is often referred to as anexplosion. Radiations may affect oneclade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment, they are termedadaptive radiations.[3]
Perhaps the most familiar example of an evolutionary radiation is that ofplacental mammals immediately after theextinction of the non-aviandinosaurs at the end of theCretaceous, about 66 million years ago. At that time, the placental mammals were mostly small, insect-eating animals similar in size and shape to modernshrews. By theEocene (58–37 million years ago), they had evolved into such diverse forms asbats,whales, andhorses.[4]
Other familiar radiations include theAvalon Explosion, theCambrian Explosion, theGreat Ordovician Biodiversification Event, theCarboniferous-Earliest Permian Biodiversification Event, theMesozoic–Cenozoic Radiation, the radiation of land plants after theircolonisation of land, the Cretaceousradiation of angiosperms, and the diversification of insects, a radiation that has continued almost unabated since theDevonian,400 million years ago.[5]
Adaptive radiations involve an increase in a clade's speciation rate coupled with divergence of morphological features that are directly related to ecological habits; these radiations involve speciation not driven by geographic factors and occurring in sympatry; they also may be associated with the acquisition of a key trait.[6]Nonadaptive radiations arguably encompass every type of evolutionary radiation that is not an adaptive radiation,[7][8] although when a more precise mechanism is known to drive diversity, it can be useful to refer to the pattern as, e.g., a geographic radiation.[1] Geographic radiations involve an increase in speciation caused by increasing opportunities for geographic isolation.[1] Radiations may be discordant, with either diversity or disparity increasing almost independently of the other, or concordant, where both increase at a similar rate.[2] Where the mechanism of diversification is ambiguous and the species seem to be closely related, sometimes the terms "species radiation," "species flock" or "species complex" are used.[9]
Much of the work carried out bypalaeontologists studying evolutionary radiations has been using marineinvertebratefossils simply because these tend to be much more numerous and easy to collect in quantity than large landvertebrates such asmammals ordinosaurs.Brachiopods, for example, underwent major bursts of evolutionary radiation in the EarlyCambrian, EarlyOrdovician, to a lesser degree throughout theSilurian andDevonian, and then again during theCarboniferous and earliestPermian. During these periods, differentspecies of brachiopods independently assumed a similar morphology, and presumably mode of life, to species that had lived millions of years before. This phenomenon, known as homeomorphy, is explained byconvergent evolution: when subjected to similar selective pressures, organisms will often evolve similar adaptations.[10] Further examples of rapid evolutionary radiation can be observed amongammonites, which suffered a series of extinctions from which they repeatedly re-diversified; andtrilobites which, during the Cambrian, rapidly evolved into a variety of forms occupying many of theniches exploited bycrustaceans today.[11][12][13]
A number of groups have undergone evolutionary radiation in relatively recent times. Thecichlids in particular have been much studied bybiologists. In places such asLake Malawi they have evolved into a very wide variety of forms, including species that are filter feeders, snail eaters,brood parasites, algal grazers, and fish-eaters.[14]Caribbean anoline lizards are another well-known example of an adaptive radiation.[15]Grasses have been a success, evolving in parallel withgrazingherbivores such ashorses andantelope.[16]