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monophyly The condition in which a group of taxa share a common ancestry, being ultimately derived from a single interbreeding (or Mendelian) population, as opposed to a polyphyletic group, which is derived from many such populations. If the members of a given taxon are descended from a common ancestor they are said to be monophyletic’ (e.g. the families within a class would be monophyletic if they were all descended from the same family or lower taxonomic unit). Under the strictest definition they would all have to be descended from a single species. Monophyly is sometimes used interchangeably with holophyly.