Triangulating from the cairns…
Investigates how geographic isolation and cold climate adaptation drive divergence in mating signals, morphology, and genomic structure across wild Drosophila populations.
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Fruit flies in the genus Drosophila have long served as windows into how new species form and how animals adapt to challenging environments. One species in particular, Drosophila montana, is a cold-tolerant fly found in birch and aspen forests across the northern hemisphere, including the high-elevation aspen groves around Gothic, Colorado. Because populations of D. montana live in places as different as Finland, coastal Canada, and the Rocky Mountains, they offer a natural experiment: how do flies that share recent ancestry diverge when isolated in distinct climates? Understanding this matters for the Gunnison Basin because the Colorado population of D. montana lives near the southern, high-altitude edge of the species' range, where cold tolerance and short growing seasons shape life-history decisions every year.
Several concepts are essential to follow the findings below. Courtship displays are the songs, movements, and chemical signals that males use to attract females; in D. montana, males produce a courtship song by vibrating their wings, and females are choosy about specific song features. Biogeographic variation refers to consistent differences in traits — body shape, song, behavior — among populations living in different regions. Population genomic divergence describes how genetic differences accumulate across the genome between isolated populations; researchers measure it with statistics like FST, which quantifies how allele frequencies differ between groups. To reconstruct the history behind those differences, scientists use coalescent simulation, a computational approach that traces sampled DNA sequences backward in time to estimate when populations split. And to actually measure genetic relationships among individual flies, researchers often use microsatellite DNA analysis, which reads highly variable short DNA repeats that mutate quickly enough to distinguish closely related populations.
Recording male Drosophila courtship songs by placing flies in Petri dish chambers and capturing wing vibration sounds with directional microphone posi...
Wild-caught flies are used to establish laboratory breeding lines maintained under standardized conditions. Lines are kept in multiple vials with cont...
Together these tools let researchers ask whether courtship traits, cold tolerance, and reproductive compatibility are diverging because of random drift in isolated populations or because natural and sexual selection are actively pulling populations apart — the first steps toward the formation of new species.
Early work in this area established both the geographic structure of D. montana and the basic phenotypic differences among its populations. Phylogeographic analysis using mitochondrial sequences and microsatellite loci revealed clear genetic differentiation between North American and Scandinavian populations, with Finnish, Canadian, Colorado, and Japanese flies forming genetically distinct groups, and divergence times between Scandinavian and North American clades estimated at roughly 450,000 to 900,000 years (Mirol et al., 2007). A companion methodological paper developed a microsatellite-based identification system using 14 polymorphic loci that could reliably distinguish D. montana from its close relatives in the D. virilis group, providing a practical tool for fieldwork (Routtu et al., 2007).
Building on this genetic framework, researchers documented striking phenotypic differences across populations. Male courtship song, wing shape, and genital morphology all varied among Colorado, Vancouver, and Oulanka (Finland) flies, but the pattern of phenotypic divergence did not match the pattern of genetic divergence — a strong clue that selection, rather than neutral drift, was shaping these traits (Routtu et al., 2007). Colorado males in particular stood out for their unusual song characteristics, while Vancouver males diverged most in wing traits. Female preference tests showed that Colorado females actually disfavored the extreme song features of their own males, hinting at complex sexual selection dynamics within populations (Klappert et al., 2007).
A central finding across the research program is that courtship song is evolving rapidly and is not tracking neutral genetic history. Carrier frequency, a key song trait under sexual selection, differs sharply among Oulanka, Vancouver, and Colorado populations, and the Colorado population shows the most distinct song frequency despite not being the most genetically distant (Klappert et al., 2007); (Routtu et al., 2007). The mismatch between trait divergence and genetic divergence implies that directional or diversifying selection — rather than chance — is sculpting male signals. Female preferences vary both within and between populations, adding another layer of evolutionary pressure that can either reinforce or oppose male trait change (Klappert et al., 2007).
These behavioral differences appear to be early steps toward speciation. Crosses among allopatric Colorado, Vancouver, and Finnish populations revealed premating (sexual) isolation in every pairing, and in some pairings a strong postmating-prezygotic barrier in which sperm were transferred and stored but the majority of eggs went unfertilized (Jennings et al., 2014). Notably, there was no evidence of intrinsic postzygotic effects such as hybrid sterility, suggesting that mate choice and fertilization compatibility — not hybrid breakdown — are the leading edge of reproductive isolation in this system.
At the same time, cold tolerance varies systematically across the species range. Critical thermal minimum, the lowest temperature at which flies retain coordinated movement, is lower in populations from higher latitudes and altitudes, indicating that northern and montane flies (including those near Gothic) are genuinely better at coping with cold (Wiberg et al., 2021). Genome-wide scans identified thousands of SNPs whose allele frequencies track climate, with the X and fourth chromosomes showing the strongest signals of climatic selection and significant overlap with genes previously implicated in cold adaptation (Wiberg et al., 2021). Photoperiodic responses show a parallel pattern: northern strains enter reproductive diapause under almost any day length, while southern strains do so only near the natural 24-hour cycle, reflecting deep latitudinal tuning of the seasonal clock (Lankinen et al., 2021).
Early work in the 2000s built the phylogeographic and phenotypic scaffolding for this system, and a productive middle period from roughly 2010 to 2014 used quantitative genetics and crossing experiments to dissect reproductive isolation and trait architecture. Recent studies since 2020 have shifted decisively toward genome-scale analyses and physiology. Population genomic work now directly links environmental variables — temperature, latitude, altitude — to specific regions of the D. montana genome, identifying candidate SNPs for cold adaptation and demonstrating that the X chromosome plays an outsized role in climatic differentiation (Wiberg et al., 2021). Complementary physiological experiments are refining our understanding of how day length, night length, and temperature interact to trigger diapause, with surprising results showing that classical circadian rhythms may not underlie photoperiodism in this species after all (Lankinen et al., 2021).
The frontier is therefore moving from describing differences to mapping their genetic and ecological causes. New methods include genome-wide environmental association scans, fine-scale photoperiod manipulations, and integrative approaches that connect behavior, physiology, and genomics within the same populations.
Several major questions remain. First, which specific genes on the X and fourth chromosomes actually drive cold tolerance differences, and do the same loci shape the courtship and reproductive barriers that separate populations? Second, how will Colorado populations — already at the warm, southern edge of the species range — respond as the Gunnison Basin warms and snowpack declines, given that their cold-adapted physiology and distinctive courtship songs evolved under historical climate conditions? Third, are the premating and postmating-prezygotic barriers among populations stable, or could renewed contact (through climate-driven range shifts) erode them or push them toward full speciation? Answering these questions will require longer-term sampling of Rocky Mountain populations, experimental crosses across the full geographic range, and genomic monitoring as climate change reshapes the environments to which these flies have so finely tuned themselves.
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. →
Mirol, P. M., Schäfer, M. A., Orsini, L., Routtu, J., Schlötterer, C., Hoikkala, A., Butlin, R. K. (2007). Phylogeographic patterns in Drosophila montana. Molecular Ecology. →
Routtu, J., Hoikkala, A., Schlötterer, C. (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. →
Wiberg, R. A. W., Veltsos, P., Snook, R. R., Ritchie, M. G., Kankare, M. (2021). Cold adaptation drives population genomic divergence in the ecological specialist, Drosophila montana. Molecular Ecology. →