Information about Radiation (biology)

Evolutionary radiation is a term used by biologists and palaeontologists to describe a dramatic and rapid (in geological terms) increase in the taxonomic diversity of a particular group of organisms. Typically, this diversification reveals a trend from a small number of similar ecological niches to a wide range of dissimilar ones. In other words, where a particular taxonomic group initially occupied one particular way of life, after a few million years it occupies many different ways of life.

Perhaps the most familiar example of an evolutionary radiation is that of placental mammals immediately after the extinction of the dinosaurs at the end of the Cretaceous, about 65 million years ago. At that time, the placental mammals were mostly small, insect-eating animals similar in size and shape to modern shrews. By the Eocene (58-37 million years ago), they had evolved into such diverse forms as bats, whales, and horses. ^[1]

Evolutionary radiation in the fossil record

Much of the work carried out by palaeontologists studying evolutionary radiations has been using marine invertebrate fossils simply because these tend to be much more numerous and easy to collect in quantity than large land vertebrates such as mammals or dinosaurs. Brachiopods, for example, underwent major bursts of evolutionary radiation in the Early Cambrian, Early Ordovician, to a lesser degree throughout the Silurian and Devonian, and then again during the Carboniferous. During these periods, different species 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 by convergent evolution: when subjected to similar selective pressures, organisms will often evolve similar adaptations. ^[2]. Further examples of rapid evolutionary radiation can be observed among ammonites, which suffer a series of extinctions from which they repeatedly re-diversify; and trilobites which, during the Cambrian, rapidly evolved into a variety of forms occupying many of the niches exploited by crustaceans today ^[3], ^[4], ^[5]

Recent evolutionary radiations

A number of groups have undergone evolutionary radiation in relatively recent times. The cichlids in particular have been much studied by biologists. In places such as Lake 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. ^[6] Grasses have been another success, evolving in parallel with grazing herbivores such as horses and antelope [1].

What causes evolutionary radiation?

In some cases, evolutionary radiation was likely driven by the sudden removal of whatever group of animal was dominant at the time. For the mammals, it seems most likely that they were limited to shrew-like lifestyles because the dinosaurs occupied all the other niches that the primitive mammals might have exploited. In other cases, evolutionary 'breakthroughs' suddenly allowed new groups of organisms to occupy niches that simply didin't exist beforehand. This is likely what drove the rapid evolution of birds and terrestrial plants, for example ^[7], ^[8].

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