Behavioural, physiological and neural adaptations to high-altitude environments in Heliconius butterflies

Abstract

Habitat heterogeneity entails shifts in climate, vegetation structure and sensory environments. These changes can set the conditions for divergent natural selection to operate across neighbouring populations. The resulting local adaptations can in turn promote species divergence, even if geographic barriers are absent. However, little is known about the nature and the timing of these adaptations in relation to the completion of speciation. Here, I explore trait variation driven by shifts in forest type along gradients of altitude in two pairs of incipient Heliconius butterfly species that have recently diverged but maintain ongoing gene-flow in hybrid zones at mid-altitude. First, I characterise the abiotic and biotic characteristics that define the habitats of these taxa. Then, I quantify variation in behavioural, physiological and neural traits across them. I next use a comparative approach to identify parallel trait-shifts between low to high-altitude species, which I interpret as local adaptations to high-altitude conditions that have independently evolved across separate locations. I additionally explore natural selection against hybrids as a mechanism preventing gene-flow in the absence of geographic isolation between low and high-altitude populations. I find evidence that the high-altitude Heliconius chestertonii and Heliconius himera have evolved adaptations that differentiate them from the low-altitude Heliconius erato venus and Heliconius erato cyrbia respectively, potentially preventing gene-flow across the hybrid zones due to fitness deficits in migrants stemming from altered foraging windows, development time, flying behaviour and neuroanatomy. I show that hybrids between Heliconius erato cyrbia and Heliconius himera are likely to also experience fitness deficits related to habitat mismatches in flying behaviour and neural traits, further contributing to reduced gene-flow. These findings shed light on the environmental factors linked to diversification, the role of different evolutionary forces during early species divergence and the timing of the evolution of adaptations in relation to the completion of speciation.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Sinead English (Supervisor) & Stephen H Montgomery (Supervisor)