reversal
1. A change of direction, usually by 180°. It commonly refers to a change of polarity of the geomagnetic field.

2. A form of homoplasy; resemblance between two taxa because one of them has gained a new character, then lost it again, and the other taxa has never gained it. Reversal by character loss is common; there is much doubt about whether it ever occurs by regaining a lost character.

REVERSAL, also retronym. Semitechnical terms for a word that spells another word backwards: doom, straw. Deliberate reversals form a new word: the trade name Trebor; the BrE slang yob (‘a backward boy’: a lout). Such reversals are sometimes used for effect in utopian and satirical writing, such as Samuel Butler's novel Erewhon (1872), set in a land where Victorian ideas and values are reversed. Erewhonians have such names as Yram and Nosnibor.

reversal A form of homoplasy; resemblance between two taxa because one of them has gained a new character, then lost it again, and the other taxon has never gained it. Reversal by character loss is common; there is much doubt about whether it ever occurs by regaining a lost character.