Adaptation in the eye and brain contributes to species divergence in visual perception in Heliconius butterflies
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Sensory systems mediate the interaction between organisms and their environment, but how complex sensory pathways evolve and relate to variation in perception and behavior across ecological contexts, remains poorly understood, especially for terrestrial taxa. Here, we investigate whole-visual-system adaptation in Heliconius erato butterflies. Using continent-wide sampling, we demonstrate that within H. erato , facet count significantly decreased with increasing elevation. Common-garden rearing of low-elevation H. erato populations from Ecuador and their high-elevation sister species, H. himera , showed that eye and brain morphology are heritable, and comparisons to genomic measures of divergence indicates that this variation is due to divergent selection. Parallel comparisons from Colombia involving H. chestertonii (high elevation) and H. erato venus (low elevation) further revealed that eye and brain morphology can evolve as independent, decoupled traits. For both locations, differences in visual acuity correlated with variation in facet count. We also observed parallel evolution of spectral sensitivity, with independent high-elevation populations having fewer red-reflecting lateral filtering pigments. To experimentally link visual system morphology to behavior, we assessed visual acuity in second-generation H. erato cyrbia - H. himera hybrids. Overall, acuity was influenced by facet count, and when analyzed together with brain morphology, by a positive interaction between facet count and optic lobe volume, demonstrating that structural investment in the eye and neural expansion combine to maximize visual perception. This work shows that visual adaptation is a multi-layered process whereby sensory traits can evolve independently under localized ecological pressures, but evolution across the visual pathway contributes to refinements in behavioral performance.
Significance statement
By integrating broad surveys of wild populations, common-garden rearing, and behavioral assays in Heliconius butterflies, we show that visual adaptation along elevational gradients is a multi-layered process. Eye morphology, neuroanatomy, and spectral sensitivity display independent, habitat-associated shifts, with peripheral and neural components specifically evolving under divergent selection. Crucially, behavioral tests in hybrid crosses reveal that eye and brain traits interactively influence visual acuity, showing that neural expansion must accompany peripheral changes to improve visual performance. This modular yet co-evolving response across the visual pathway provides a flexible mechanism for sensory adaptation across terrestrial environments.
