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(A) A column containing four positions: Xi, −Xj, −Yk, and Zl. The overlaid trees show the position's phylogenetic relationships. (B) The mirror of the column in A. (C) A most parsimonious history for the base pairs in A. A substitution is inferred to have occurred along the lineage marked by a star, but the base pairs are all still considered homologous. (D) The mirror history of C.
