Common measures of the species rank exhibit widely different correlations with gene content similarity in Streptomycetaceae. The 16S rRNA sequence showed little correlation with gene content similarity (R2 = 0.38), whereas the rpoB gene sequence displayed good correlation (R2 = 0.75). Two genome-wide measures of similarity, average nucleotide identity (ANI) and alignment fraction (AF), were strongly correlated with gene content similarity. Horizontal dashed lines denote commonly used species-level cutoffs for each measure (e.g., > 97% 16S sequence identity)