Phylogram of nine species of Pemphigus. The three species that were subject to detailed ecological study are shown in bold typeface. Eriosoma ulmi is the outgroup. A parenthetical "S" indicates that the species possesses soldiers and, where underlined, highly aggressive soldiers with specialised defensive morphology. The absence of soldiers is denoted by a parenthetical "N". Arrow (A) indicates the likely origin of soldiers, arrow (B) indicates the evolution of highly aggressive soldiers, and arrow (C) indicates the evolution of highly aggressive soldiers along with loss of host-alternation and the gain of dimorphic nymphs. The numbers on the branches give the number of times out of 100 that a bootstrap replicate recovered the branch, and the scale indicates units of 0.01 expected substitutions per nucleotide position.