Abstract
We used a colour-space model of avian vision to assess whether a distinctive bird pollination syndrome exists for floral colour among Australian angiosperms. We also used a novel phylogenetically based method to assess whether such a syndrome represents a significant degree of convergent evolution. About half of the 80 species in our sample that attract nectarivorous birds had floral colours in a small, isolated region of colour space characterized by an emphasis on long-wavelength reflection. The distinctiveness of this ‘red arm’ region was much greater when colours were modelled for violet-sensitive (VS) avian vision than for the ultraviolet-sensitive visual system. Honeyeaters (Meliphagidae) are the dominant avian nectarivores in Australia and have VS vision. Ancestral state reconstructions suggest that 31 lineages evolved into the red arm region, whereas simulations indicate that an average of five or six lineages and a maximum of 22 are likely to have entered in the absence of selection. Thus, significant evolutionary convergence on a distinctive floral colour syndrome for bird pollination has occurred in Australia, although only a subset of bird-pollinated taxa belongs to this syndrome. The visual system of honeyeaters has been the apparent driver of this convergence.
Highlights
Selection exerted by pollinators on floral traits is one of the best studied examples of natural selection arising from biotic interactions [1]
The reality of pollination syndromes has been contested, because many angiosperm species have generalized relationships with multiple classes of pollinators [1,44], and because floral traits do not seem to cluster into distinct regions of morphospace corresponding to the putative syndromes [45]
The colours of Australian flowers visited by birds are not, as a whole, very different from the colours of flowers visited only by insects, suggesting that there is no general syndrome for floral colour that is distinct to bird-pollination
Summary
Selection exerted by pollinators on floral traits is one of the best studied examples of natural selection arising from biotic interactions [1]. We consider whether the colours of bird-pollinated flowers show evidence of convergent evolution, based on the floral reflectance spectra of 234 native Australian angiosperms, including species visited by birds (80 spp.) and, for comparison, insect-pollinated species (154 spp.). We defined target regions for convergence (such as the space containing all bird-visited species) and used the inferred colour loci of internal nodes in the phylogenetic tree of all sample species to count the number of lineages that evolved rspb.royalsocietypublishing.org Proc. Because apparent convergence can occur even in the absence of adaptation, especially in low-dimensional morphospaces such as the colour tetrahedrons, we tested the significance of the observed convergence indices by simulating floral colour evolution on the phylogenetic tree according to a Brownian motion model of evolution, with the evolutionary variance –covariance matrix estimated from the observed data. Stayton), which uses routines from the ape, cluster, geiger and phytools packages [38 – 41]
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