Justen Whittall and Scott Hodges of the University of California-Davis said the flowers' long petals might have evolved in a predictable way but not for the reason Darwin predicted.

Whittall and Hodges assessed the evolutionary relatedness of Aquilegia -- a group of herbaceous perennial plants in which each petal is modified into an elongated nectar spur. The scientists report they have determined the nectar spurs evolved in a recurring and directional fashion, becoming longer and longer over time.

The team said that shape evolved in a punctuated manner, with plants adapting to new species of unrelated pollinators, each with a longer tongue than its predecessor. That theory, however, is at odds with Darwin's prediction that a co-evolutionary conflict drove the increase in length of both nectar spur and pollinator's tongue.

The study appears in the journal Nature.