Colour pattern divergence in reef fish species is rapid and driven by both range overlap and symmetry.

Signal divergence is an important process underpinning the diversification of lineages. Research has shown that signal divergence is greatest in species pairs that possess high geographic range overlap. However, the influence of range-size differences within pairs is less understood. We investigated how these factors have shaped signal divergence within brightly coloured coral reef butterflyfishes (genus: Chaetodon). Using a novel digital imaging methodology, we quantified both colouration and pattern using 250 000 sample points on each fish image. Surprisingly, evolutionary age did not affect colour pattern dissimilarity between species pairs, with average differences arising in just 300 000 years. However, the effect of range overlap and range symmetry was significant. Species-pair colour patterns become more different with increasing overlap, but only when ranges are similar in size. When ranges differ markedly in area, species-pair colour patterns become more similar with increasing overlap. This suggests that species with small ranges may maintain non-colour-based species boundaries.