Extrapolating from local ecological processes to genus-wide patterns in colour polymorphism in South AfricanProtea

Abstract

Polymorphic traits are central to many fundamental discoveries in evolution, yet why they are found in some species and not others remains poorly understood. We use the African genus Protea-within which more than 40% of species have co-occurring pink and white floral colour morphs-to ask whether convergent evolution and ecological similarity could explain the genus-wide pattern of polymorphism. First, we identified environmental correlates of pink morph frequency across 28 populations of four species. Second, we determined whether the same correlates could predict species-level polymorphism and monomorphism across 31 species. We found that pink morph frequency increased with elevation in Protea repens and three section Exsertae species, increased eastward in P. repens, and increased with seed predation intensity in section Exsertae. For cross-species comparisons, populations of monomorphic pink species occurred at higher elevations than populations of monomorphic white species, and 18 polymorphic species spanned broader elevational gradients than 13 monomorphic species. These results suggest that divergent selection along elevational clines has repeatedly favoured polymorphism, and that more uniform selection in altitudinally restricted species may promote colour monomorphism. Our findings are, to our knowledge, the first to link selection acting within species to the presence and absence of colour polymorphism at broader phylogenetic scales.

Flower colour polymorphism in the Mediterranean Basin: occurrence, maintenance and implications for speciation

“…). Thus, Carlson & Holsinger () found that flower colour polymorphic species of Protea inhabiting the Cape region showed broader elevational gradients than monomorphic species. We postulate that the evolutionary processes maintaining polymorphism in spatially and temporally variable environments, such as the Mediterranean Basin (Thompson ), will result in a continuum of geographic patterns of FCP (morph compositions and relative frequencies).…”

“…; Dick et al . ; Carlson & Holsinger ), resulting in a geographic pattern of morph distribution that contributes to divergence.…”

“…While flower colour is diverse throughout the angiosperms, variation of flower colour within species is traditionally considered uncommon; and species that show a stable, genetically based polymorphism are comparatively rare (Wheldale ; Kay ). However, an increasing body of literature suggests that the occurrence of flower colour polymorphism (FCP hereafter) is far from unusual in certain groups of species (Warren & Mackenzie ; Carlson & Holsinger ; Ellis & Field ; Table ). Here, we use polymorphism to refer to discrete variants within a population in a frequency high enough not to be the result of recurrent mutations (Ford ; Huxley ); thus, a species is considered as colour polymorphic when possessing at least one population where two or more morphs coexist.…”

“…Most of the colour polymorphic species show variation in morph frequencies, usually related to environmental heterogeneity (McLean & Stuart‐Fox ; Carlson & Holsinger ; Forsman & Wennersten ). The Mediterranean Basin is a spatial environmental mosaic due to the highly heterogeneous climate, orography and soils (Thompson ); in addition, climate in the Mediterranean Basin is highly unpredictable, giving rise to a temporal environmental heterogeneity.…”

Signals of selection in conditionally expressed genes in the diversification of three horned beetle species

“…Divergence between species is enabled by phenotypic variation present in natural populations (Gompel et al, 2005;Wittkopp et al, 2009;Sanger et al, 2013;Carlson et al, 2015) and biased along both genetic and developmental lines of least resistance (Schluter, 1996), such as those arising through sex-specific development or developmental processes biased towards particular reproductive or trophic morphs (Lorch et al, 2003;West-Eberhard, 2003;Smith et al, 2008;Aubin-Horth & Renn, 2009). For example, many invertebrate and vertebrate species have diverged most obviously in the expression of secondary sexual traits, which also account for the greatest phenotypic variation within populations (Emlen et al, 2012).…”