Figure 1. A partial phylogeny of theropod dinosaurs and birds to illustrate history-dependent definitions of adaptation. The phylogenetic ‘tree’ represents evolutionary relationships, with earlier origins of lineages represented by splits near the base, and later origins by splits near the top. All the groups represented are extinct, except for the modern birds. The absence of feathers in allosauroids is not an adaptation because this trait has not been modified from the ancestral condition of dinosaurs. The origin of feathers in the ancestor of the lineage of Beipiaosaurus–tyrranosaurid–Caudipteryx–Archaeopteryx–modern birds could be an adaptation, provided feathers brought superior fitness in the environment of that lineage. A reversion to the featherless state in tyrannosaurids (fossil evidence on this question is lacking) could also be an adaptation, because it would be a modification of the ancestral condition in this lineage, even though the same featherless state is not an adaptation for allosauroids.