While the apparent mutual exclusiveness of anthocyanins and betalains in the Caryophyllales has given rise to considerable taxonomic debate, historical factors affecting the present distribution of these compounds have rarely been discussed. An understanding of pigment evolution in the order is hindered by a number of unresolved systematic issues and a lack of knowledge of the importance of anthocyanins and betalains beyond their roles in pollination and seed dispersal. The hypothesis that betalains arose in an unpigmented ancestor of the Chenopodiinae in response to selection from pollinators cannot be rejected, but scant evidence exists in favor of it. Questions persist regarding whether the most recent ancestor to the Chenopodiinae presented a pigmented floral display and whether the appropriate pollinators were present at this time to select for floral pigmentation. In view of these ambiguities and the possible non-monophyly of the Chenopodiinae we consider some alternative scenarios and suggest potentially rewarding avenues for future research. We discuss roles for anthocyanins and betalains beyond their use as optical attractants, possible costs and benefits associated with producing each pigment type, and the possibility that they may have cooccurred in an ancestor for some period of time.

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