Experimental and phylogenetic evidence for floral mimicry in a guild of fly-pollinated plants

Abstract

Mimicry, as an adaptive explanation for the resemblance between organisms, is not always readily distinguishable from, inter alia, coincidence, shared ancestry, or convergent evolution. We tested the hypothesis that two rare South African orchid taxa Brownleea galpinii ssp. major (nectar-producing) and Disa cephalotes ssp. cephalotes (non-rewarding) are mimics of the nectar-producing flowers of a relatively common species, Scabiosa columbaria (Dipsacaceae), with which they always occur sympatrically. Flowers of the orchids were apparently unscented and had similar dimensions and almost identical spectral reflectance to the flowers of Scabiosa. The orchids were pollinated exclusively by long-proboscid flies (Tabanidae and Nemestrinidae) that feed mainly on nectar in Scabiosa flowers. Choice experiments showed that these flies did not discriminate between the orchids and Scabiosa when alighting on their flat-topped inflorescences. However, flies were not attracted to related orchids dissimilar to Scabiosa, or to inflorescences of B. galpinii that had been artificially reconstructed in the shape of a spike, rather than a flat-topped capitulum. A phylogenetic analysis showed traits that give the orchids a resemblance to Scabiosa, such as a flat-topped inflorescence and cream floral colouration with dark spots and short spurs, to be mostly apomorphic features, and therefore likely to be relatively recent adaptations for mimicry. We caution that the term mimic should not be applied to species whose resemblance to another species is due entirely to plesiomorphic traits that, in all likelihood, evolved prior to the ecological association.