Sexual Selection and Morphological Variation Across Animal Taxa

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Research Primer

Background

Sexual selection shapes some of the most striking features of the animal world: the iridescent throats of hummingbirds, the pigmented wings of butterflies, and the acoustic courtship songs of flies. At the Rocky Mountain Biological Laboratory (RMBL) and across the Gunnison Basin, researchers have used a remarkable variety of animals to ask how mating-related traits evolve, how they vary across populations, and how animals actually perceive the signals their mates produce. Understanding these processes matters for mountain ecosystems because sexual selection influences how populations diverge, how species boundaries are maintained, and how sensitive species such as pollinators respond to environmental change.

Several key concepts anchor this area. Courtship displays are behaviors performed to attract mates or outcompete rivals, and they often involve the coordinated production of sound, motion, and color. Sexual dimorphism, the set of differences between males and females, emerges when selection acts differently on each sex — for example, when males evolve elaborate ornaments while females evolve preferences for them. Because populations of the same species can experience different ecological and social environments, biogeographic variation in courtship traits and body shape is common. When populations diverge genetically as well as behaviorally, researchers describe this as population genomic divergence, measurable through genome-wide differences in DNA sequence.

A second set of concepts concerns how signals are perceived. Birds, unlike humans, have four color-sensing cone types in the eye, and the mathematical framework called avian tetrachromatic color space describes how they see colors — including "nonspectral" combinations like UV+green that humans cannot experience. Chromatic contrast refers to how distinguishable two colors appear to a given visual system, and visual modeling uses measurements of light and cone sensitivities to estimate what an animal actually sees. Finally, bill morphology — the length, width, and curvature of a beak — links sexual selection to ecology, because beaks serve both in feeding and in species recognition. Together, these concepts let researchers move beyond human intuition and ask how animals themselves experience courtship.

Foundational work

Early RMBL-linked work on sexual selection emerged strongly through studies of Drosophila montana, a fly species distributed across the northern hemisphere whose Colorado population breeds in the Gunnison Basin. Phylogeographic analyses established clear genetic differentiation between North American and Scandinavian populations, with Finnish, Canadian, and Colorado flies forming genetically distinct groups (Mirol et al., 2007). Against this genetic backdrop, researchers documented that male courtship song varied among populations in ways that did not simply track genetic distance, and that females expressed preferences that sometimes disfavored extreme local male traits (Klappert et al., 2007). A companion study showed that song, wing, and genital morphology all varied among Colorado, Vancouver, and Finnish populations, with Colorado males most divergent in song and Vancouver males most divergent in wing shape — a pattern inconsistent with neutral evolution and suggestive of directional or diversifying selection (Routtu et al., 2007). Microsatellite tools developed in parallel made it possible to reliably identify sibling species in the field (Routtu et al., 2007).

Later foundational work extended these findings into the realm of reproductive isolation. Crosses among allopatric D. montana populations revealed that premating (behavioral) barriers and postmating barriers both contributed to incipient speciation, with different population pairs showing different mixes of isolation mechanisms and no evidence of intrinsic hybrid breakdown (Jennings et al., 2014). Together, these studies established the Gunnison Basin Drosophila as a model for watching speciation in action.

Key findings

Across taxa, a consistent theme is that courtship traits diverge among populations under selection rather than drift. In D. montana, phenotypic divergence in song and wing traits did not mirror genetic divergence, implying that these characters are shaped by selection (Routtu et al., 2007), and Colorado males in particular carried the most distinctive song frequency in the species (Klappert et al., 2007). Reproductive isolation among populations is already partially in place, driven by a mix of behavioral and postmating incompatibilities (Jennings et al., 2014). Genome-wide data reinforce this picture: populations at higher latitudes and altitudes show greater cold tolerance, and thousands of genetic variants across the genome — concentrated on the X and fourth chromosomes — are associated with climatic variables, overlapping with previously identified cold-adaptation genes (Wiberg et al., 2021). Photoperiodic responses also vary dramatically with latitude, with northern strains entering reproductive diapause under nearly all day lengths while southern strains are more selective (Lankinen et al., 2021).

Work on hummingbirds has transformed understanding of how courtship signals are perceived. Male broad-tailed hummingbirds perform U-shaped dives in which speed, mechanical sounds produced by wing and tail feathers, and the color of the iridescent throat patch are all synchronized within 300 milliseconds, with males reaching speeds near 23.5 m/s (Hogan & Stoddard, 2018). Because the throat is iridescent, its color shifts dramatically — from bright red to black — as the male changes orientation relative to the female, meaning the "signal" is inseparable from motion. Behavioral experiments at RMBL further showed that wild hummingbirds can discriminate nonspectral colors such as UV+red, UV+green, and purple, and that roughly 30% of plumage colors and 35% of plant colors fall into this nonspectral range invisible to humans (Stoddard et al., 2020). These findings imply that the color world of both courtship and foraging is richer than previously recognized.

Studies of butterflies add a complementary perspective on color signaling. In Speyeria mormonia, males approach more pigmented female decoys more often, consistent with the idea that pigmentation signals female quality; however, adding a black achromatic border alters male approach behavior, indicating that contrast — not pigment alone — shapes mate assessment (Maass, 2022).

Current frontier

Work published in the 1990s and 2000s established the basic patterns of population divergence in Drosophila song and morphology and the phylogeographic structure underlying them. Research since 2018 has shifted in two directions. First, signal research has become increasingly multimodal and perceptual: rather than measuring a trait in isolation, recent studies reconstruct how receivers experience a dynamic display, whether that means synchronized sound, color, and motion in a hummingbird dive (Hogan & Stoddard, 2018) or how achromatic borders alter color perception in butterflies (Maass, 2022). Behavioral experiments with free-flying wild birds using programmable LED devices have opened a frontier in tetrachromatic color perception (Stoddard et al., 2020). Second, work on fly populations has moved from phenotypes and a few genetic markers toward whole-genome approaches that link environmental gradients to specific genomic regions under selection (Wiberg et al., 2021), and toward dissecting how photoperiodic physiology varies across latitudes (Lankinen et al., 2021).

Open questions

Several promising directions remain. It is still unclear how perceptually salient nonspectral colors actually shape mate choice and pollinator decisions in the wild, or how climate change might reshape the pigments and iridescent structures that produce those signals. In Drosophila, the genomic basis of female preference — as opposed to male traits — is largely unresolved, as is the question of whether cold-adaptation genes and courtship divergence share genomic architecture. Across taxa, linking multimodal display components to receiver psychology under natural light and motion conditions remains a major methodological challenge. Finally, as the Gunnison Basin warms, understanding how locally adapted populations of flies, butterflies, and hummingbirds will respond — and whether sexual selection accelerates or constrains adaptation — is an urgent question for the coming decade.

References

Hogan, B. G., & Stoddard, M. C. (2018). Synchronization of speed, sound and iridescent color in a hummingbird aerial courtship dive. Nature Communications. →

Jennings, J. H., Snook, R. R., & Hoikkala, A. (2014). Reproductive isolation among allopatric Drosophila montana populations. Evolution. →

Klappert, K., Mazzi, D., Hoikkala, A., & Ritchie, M. G. (2007). Male courtship song and female preference variation between phylogeographically distinct populations of Drosophila montana. Evolution. →

Lankinen, P., Kastally, C., & Hoikkala, A. (2021). Nanda-Hamner curves show huge latitudinal variation but no circadian components in Drosophila montana photoperiodism. Journal of Biological Rhythms. →

Maass, E. (2022). Does an achromatic border alter color discrimination of male Speyeria mormonia butterflies during mate searching? →

Mirol, P. M., Routtu, J., Hoikkala, A., Butlin, R. K., et al. (2007). Phylogeographic patterns in Drosophila montana. Molecular Ecology. →

Routtu, J., Hoikkala, A., & Kankare, M. (2007). Microsatellite-based species identification method for Drosophila virilis group species. Hereditas. →

Routtu, J., Mazzi, D., Van Der Linde, K., Mirol, P., Butlin, R. K., & Hoikkala, A. (2007). The extent of variation in male song, wing and genital characters among allopatric Drosophila montana populations. Journal of Evolutionary Biology. →

Stoddard, M. C., Eyster, H. N., Hogan, B. G., Morris, D. H., Soucy, E. R., & Inouye, D. W. (2020). Wild hummingbirds discriminate nonspectral colors. Proceedings of the National Academy of Sciences. →

Wiberg, R. A. W., Veltsos, P., Snook, R. R., & Ritchie, M. G., et al. (2021). Cold adaptation drives population genomic divergence in the ecological specialist, Drosophila montana. Molecular Ecology. →

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