Behavioural adaptations are often some of the first to evolve during population divergence, and can play a key role during the evolution of new species. For example, although many organisms can interbreed with close relatives, they often ‘choose’ not to. Our research focuses on sensory and behavioural adaptations: how ecological, genetic and developmental factors influence their evolution, and how they contribute to speciation and other population level processes
© Carolin Bleese
We are interested in how sensory and behavioural phenotypes contribute to population divergence and speciation. To address this, we take an interdisciplinary approach, using methods ranging from ecological and behavioural experiments in the tropics, to neuroanatomy and population genomics.
Our work is largely focused on speciation in Heliconiusbutterflies, which show a striking radiation of bright warning patterns across the Neotropics. Closely related taxa often display divergent wing patterns, and because males almost invariably prefer to court females that share their own colour pattern, this contributes to an important pre-mating reproductive barrier between species.
Organisms often use visual signals to attract and recognise suitable mates. Although the evolution of these cues is increasingly understood at the molecular level, we know little of the genetic and developmental mechanisms underlying variation in the corresponding preferences, or of visually guided behaviours more broadly. Heliconius butterflies are excellent animals to study how visual preference behaviours are generated during development and across evolutionary time.
Our work on visual mating preferences addresses four major questions:
1) How do visual preferences contribute to the evolution of reproductive isolation, and what is the role of genetic architecture and ecology? 2) How do visual preferences interact with other sensory modalities? 3) How is behavioural variation encoded in the genome, and how have preference and colour pattern genes co-evolved across the group? And finally, 4) how are mating preferences mediated through changes in the neural and sensory systems?
Rossi, M., Hausmann, A.E., Alcami, P., Möst, M., Wright, D.S., Kuo, C.-Y., Lozano-Urrego, D., Maulana, A., Melo-Flórez, L., Rueda-Muñoz G., McMahon, S., Linares, M., McMillan, O., Pardo-Diaz, C., Salazar, C. & Merrill, R.M. (2024) Adaptive introgression of a visual preference gene. Science 383: 1368-1378
Hausmann, A.E., Kuo, C.-Y., Freire, M., Rueda-M, N., Linares, M., Pardo-Diaz, C., Salazar, C., & Merrill, R.M. (2021) Light environment influences mating behaviours during the early stages of divergence in tropical butterflies. Proc Roy Soc B288: 20210157
Merrill R.M., Rastas P., Martin S.H., Melo, M.C., Barker S., Davey, J., McMillan W.O., Jiggins, C. (2019) Genetic dissection of assortative mating behavior. PLoS Biology 17: e2005902
The compound eyes of insects consist of numerous independent photosensitive units, ommatidia, each of which receives light information and transfers it to the brain. Variation in insect eye size can directly affect visual perception. We have been investigating how eye size varies both within and between related Heliconius species. In particular, our recent results suggest that this variation is driven by divergent selection, and that it relates to visual acuity (i.e. the ability to perceive detail in a visual scene. We are currently further leveraging the Heliconius system to better understand the evolutionary genetics and development of variation in insect eye size, and also plan to expand these analyses across butterflies.
Wright, D.S., Rodriguez-Fuentes, J., Ammer, L., Darragh, K., Kyu, C-Y., McMillan, W.O., Jiggins, C., Montgomery, S. & Merrill, R.M. (2024) Selection drives divergence of eye morphology in sympatric Heliconius butterflies Evolution 78: 1338-1348
Wright, D.S., Manel, A.N., Guachamin-Rosero, Chamba_vaca, P., Bacquet, C.N, & Merrill, R.M. (2023) Quantifying visual acuity in Heliconius butterflies Biology Letters 19: 20230476
Selection should favour divergent sensory phenotypes in populations exposed to different sensory conditions. This can generate reproductive isolation either by affecting mate choice, or through reduced fitness of poorly adapted immigrants and/or hybrids. Despite the central role of the brain in modulating sensitivity or integration of stimuli, existing speciation studies have focussed on the sensory periphery. Alongside our collaborators at the University of Bristol, we are combining comparisons of neuroanatomy, gene expression and behaviour between recently diverged Heliconius species and their hybrids, with broader scale comparative analyses to address four broad questions: 1) Are changes in neural investment driven by ecological selection? 2) How do these changes relate to shifts in sensory behaviours? 3) To what extent have different neural and sensory traits coevolved, and is their evolution constrained by developmental and/or fitness trade-offs? And, 4) what is the genetic architecture, and ultimately the specific genetic changes, underlying shifts in neural adaptation?
Montgomery, S.H., Rossi, M., McMillan, O. & Merrill, R.M. (2021) Neural divergence and hybrid disruption between ecologically isolated Heliconius butterflies. PNAS 116: e2015102118
Dalbosco Dell’Agli, D.•, Rivas, D.F.•, Wright, D.S.•, Merrill, R.M.* & Montgomery, S.H.* (2025) The sensory ecology of speciation Cold Spring Harbour Perspectives in Biology 16: a0411428
YP Toh, F Cicconardi, G Bianchini, RM Merrill*, SH Montgomery* (2025) Evolution of the olfactory system during the radiation of Heliconiini butterflies Journal of Evolutionary Biology: voaf114
Borrero., J., Mogollon Perez, E., Wright, D.S., Lozano, D., Reuda, G., Pardo-Diez, C., Salazar, C. & Merrill, R.M. (2024) Weighting of sensory cues reflect changing patterns of visual investment during ecological divergence in Heliconius butterflies. Biology Letters 20: 20240377
We have recently started a collaboration with Jonna Kulmuni to study adaptation, speciation and hybridisation in Formica ssp. wood ants in the Alps. Our main field site for this project is a little more than an hour by train from our department in Munich (a very different situation to ‘our’ Neotropical butterflies). Species normally only found in the north of Europe are present at higher elevations in this region, allowing us to explore how these populations have diverged from closely related ‘southern’ lower elevation species. We plan to compare these populations with those in Finland, where these species are known to hybridise.
For a full up-to-date list of our publications please see GoogleScholar. Feel free to email for pdfs.
We are lucky to collaborate with passionate and talented scientists across the world, including Stephen Montgomery(Bristol, UK), Camilo Salazar & Carolina Pardo-Diaz (Rosario, Colombia), Caroline Bacquet (IKIAM, Ecuador), Jonna Kulmuni & Patrick Krapf (Amsterdam, Nethelerlands), Owen McMillan (Smithsonian Tropical Research Institute, Panama), Chris Jiggins (Cambridge, UK), Nicola Nadeau (Sheffield, UK), Basil El Jundi (Oldenburg, Germany) and Gregor Beluśič (Ljubljana, Slovenia) amongst others
We are also part of the ESEB Special Topic Network for the Integration of Speciation Research
