Abstract
Discrete color polymorphisms represent a fascinating aspect of intraspecific diversity. Color morph ratios often vary clinally, but in some cases, there are no marked clines and mixes of different morphs occur at appreciable frequencies in most populations. This poses the questions of how polymorphisms are maintained. We here study the spatial and temporal distribution of a very conspicuous color polymorphism in the club‐legged grasshopper Gomphocerus sibiricus. The species occurs in a green and a nongreen (predominately brown) morph, a green–brown polymorphism that is common among Orthopteran insects. We sampled color morph ratios at 42 sites across the alpine range of the species and related color morph ratios to local habitat parameters and climatic conditions. Green morphs occurred in both sexes, and their morph ratios were highly correlated among sites, suggesting shared control of the polymorphism in females and males. We found that in at least 40 of 42 sites green and brown morphs co‐occurred with proportions of green ranging from 0% to 70% with significant spatial heterogeneity. The proportion of green individuals tended to increase with decreasing summer and winter precipitations. Nongreen individuals can be further distinguished into brown and pied individuals, and again, this polymorphism is shared with other grasshopper species. We found pied individuals at all sites with proportions ranging from 3% to 75%, with slight, but significant variation between years. Pied morphs show a clinal increase in frequency from east to west and decreased with altitude and lower temperatures and were more common on grazed sites. The results suggest that both small‐scale and large‐scale spatial heterogeneity affects color morph ratios. The almost universal co‐occurrence of all three color morphs argues against strong effects of genetic drift. Instead, the data suggest that small‐scale migration–selection balance and/or local balancing selection maintain populations polymorphic.
Highlights
Balanced intraspecific color polymorphisms have fascinated researchers for a long time, because they demand eco-evolutionary explanations for how polymorphisms are maintained in the long run (Fisher, 1930; Ford, 1965; Huxley, 1955; Svensson, 2017)
Which mechanisms maintain populations polymorphic? Polygenic traits can remain polymorphic in mutation–selection–drift balance when selection at individual loci is weak and the mutational target is large (Bürger & Lande, 1994; Bürger, Wagner, & Stettinger, 1989), but, as we argue below, we consider it unlikely that the green–brown color polymorphisms is a highly polygenic trait
All populations were green– brown polymorphic with the possible exception of two sites in the east of the range that possibly lack green morphs
Summary
Balanced intraspecific color polymorphisms have fascinated researchers for a long time, because they demand eco-evolutionary explanations for how polymorphisms are maintained in the long run (Fisher, 1930; Ford, 1965; Huxley, 1955; Svensson, 2017). The old age of the clade and the immense number of green–brown polymorphic species, interspersed with some exclusively brown and some exclusively green taxa, makes Orthopterans suitable for studying the mechanisms that maintain balanced color polymorphisms. Even if selection is fluctuating, the system needs to be very fine-tuned with sufficiently strong selection to counter genetic drift effects and regular switches in the sign of selection in order to protect populations against accidental loss of one of the morphs. While such systems may occur, it is not trivial to find a situation in which selection varies in magnitude, and in direction on a regular basis. We combine large-scale with small-scale spatial and temporal sampling that allows us to
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