| Summary: | Deletion sets the stage for reversion, the reappearance during evolution of traits lost earlier in the evolution of a lineage. Reversion can also refer to a return to an ancestral phenotypic state, as when a lineage of multicellular organisms gives rise to a lineage of unicellular ones, even though the single cell that results may not closely resemble the ancestral single cell. Deletions can occur due to changes in regulation that are small relative to their phenotypic effects. So some deleted traits may be subject to atavistic recall and reversion with little environmental or genetic change. An atavism is a low-frequency or sporadic reversion, the reappearance of a lost character of remote ancestors not seen in the parents or recent ancestors of the individuals that express it (Hall, 1984). Like heterochrony and heterotopy (see chapters 13 and 14), reversion, as a category of evolutionary transition, overlaps on all sides with other categories. The reestablishment of a lost, ancestral trait that had evolved via heterochrony, for example, may occur if the heterochrony, or change in timing of expression, is reversed. Such a reversion could itself accurately be called a heterochrony. Many reversions could be classified as deletions, if they involve the loss of a recently evolved trait. One of the examples discussed in this chapter, the reversion to solitary reproduction in lineages of social bees, could be termed a deletion because it involves the loss of worker behavior. Atavisms occur in a wide variety of organisms. An often-cited example of a revealing and useless atavism is the case of one humpback whale (Megaptera nodosa) with two hind limb-like appendages reflecting its terrestrial ancestry. Each limb was over a meter long and contained a nearly complete femur, tibia and vestiges of tarsal and metatarsal bones (Andrews, 1921, cited by Hall, 1984, as M. novaeangliae; Levinton, 1988; John and Miklos, 1988). Skeletal atavisms are relatively well studied in whales because Russian researchers (cited in Yablokov, ...
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