Table 2.

Genes in new operons are no more likely than other genes to have trees that are discordant with the species tree

	In a new operon?
	Yes	No
# Concordant	345	692
# Discordant	22	69
% Concordant	94.0%	90.9%

As described in the text, we used the one-sided Kishino-Hasegawa test to determine whether genes in new operons had trees that were concordant with the species tree. To avoid discordant trees due to paralogs, only genes present as unique members of a COG were included. The two percentages shown are not significantly different (P = 0.08, Fisher exact test).