Megaevolution
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Megaevolution is a term which describes the most dramatic events in evolution. By that is not meant a different kind of evolution; rather, it means evolution which produces a tremendous effect. It is not suggested that the evolutionary processes involved are special although, in some cases they might be. Since the word 'macroevolution' can apply to relatively modest changes at the species and genus level, 'megaevolution' may be used for really huge changes.
The adaptive radiation of birds in the Lower Cretaceous, teleosts in the Cretaceous, flowering plants in the Upper Cretaceous, mammals in the Eocene, moths in the Cretaceous are striking examples of macroevolution. However, there are even more important events in the history of life. An interesting list was prepared by Maynard Smith and Szathmáry, which they called the major transitions in evolution. They did the list twice.[1][2]
- 1999 list
- Replicating molecules: change to populations of molecules in protocells
- Independent replicators leading to chromosomes
- RNA as gene and enzyme change to DNA genes and protein enzymes
- Bacterial cells (prokaryotes) leading to cells with nuclei and organelles (eukaryotes)
- Asexual clones leading to sexual populations
- Single-celled organisms leading to fungi, plants and animals
- Solitary individuals leading to colonies with non-reproducing castes (termites, ants & bees)
- Primate societies leading to human societies with language
Some of these topics have been discussed before.[3][4][5][6][7][8][9][10]
Numbers one to six on the list are about events which are of huge importance, but about which we know relatively little. All occurred before (and mostly very much before) the fossil record started, or at least before the Phanerozoic eon.
Numbers seven and eight on the list are of a different kind from the first six, and are generally not considered by the other authors. Number four is of a type which is not covered by traditional evolutionary theory, The origin of eukaryotic cells is probably due to symbiosis between prokaryotes. This is a kind of evolution which must be a rare event.[11][12][13]