This is the fourth of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
Charles Darwin was a visionary in more ways than one. In 1862, Darwin was studying a Malagasy orchid called Angraecum sesquipedale, whose nectar stores lie inaccessibly at the bottom of a 30cm long spur (tube). Darwin predicted that the flower was pollinated by a moth with tongue long enough to raid the spur.
Few people believed him, but in 1903, zoologists discovered Darwin’s predicted moth, Xanthopan morgani praedicta, and it did indeed have a very long tongue. Darwin accurately predicted the extraordinary but matching lengths of moth tongue and orchid spur, but his explanation for them is another story.
He suggested that the two species were locked in an ‘evolutionary arms race’. Orchids and pollinators gradually co-evolved over time, lengthening both tongues and spurs in response to each other. Orchids with the longest spurs have an advantage. Their nectar stores are only just within reach of pollinators, so they are tempting but don’t sacrifice too much valuable nectar. For pollinators, the advantage belongs to those with the longest tongues because they have access to the most food.
The arms race model has become widespread and popular since Darwin’s time. It helps to explain relationships between predators and prey, parasites and hosts and even males and females. But its use in explaining the relationship between flowers and pollinators has been called into question.
Justen Whittall and Scott Hodges from the University of California, Santa Barbara, tested the arms race theory by looking at another long-spurred flowering plants – the columbines (Aquilegia sp). In these flowers, every petal carries its own elongated nectar spur and the advent of these spurs coincided with the recent and rapid diversification of this group. In this group, the duo found that evolution happened in a stop-start ‘punctuated’ way, as the flowers encountered new pollinators with increasingly long tongues.