Mechanistic validation of the Climatic Variability Hypothesis: does climatic variability shape geographic range extent through physiological tolerance?

The Climatic Variability Hypothesis (CVH) — the proposal that species from more climatically variable habitats evolve broader environmental tolerances and consequently occupy larger geographic ranges — is the classical mechanistic explanation for Rapoport's rule, the poleward increase in species' range sizes. Pintor, Schwarzkopf and Krockenberger (2015) supported the CVH at the pattern level in three Australian skink clades: occurrence-derived maximum annual temperature variability (MAV) correlates positively with latitudinal and elevational range extent. However, the causal core remains unverified: MAV is an unvalidated proxy that has never been tested against directly measured physiological tolerances; the historical origin of broader tolerances (variable source habitats versus adaptation during expansion) is unresolved; the contributions of alternative range-limiting mechanisms and of climatic axes other than annual temperature are unquantified; and generality beyond these clades and Australia is untested. The problem is to establish, through combined physiological, phylogeographic, and comparative analysis across taxa and regions, whether climatic variability causally shapes range extent via evolved or plastic physiological tolerance, and to quantify its importance relative to other mechanisms.

Background

Rapoport's rule is the observation, formalized by Stevens (1989) after remarks by Eduardo Rapoport, that the geographic ranges of species tend to be larger at higher latitudes. Stevens proposed as its mechanism the Climatic Variability Hypothesis (CVH), an idea with roots in Janzen (1967): because seasonal climatic variation increases away from the tropics, high-latitude taxa experience wider annual temperature swings and must evolve broader environmental tolerances; broadly tolerant species can then occupy extensive ranges along climatic gradients, while taxa adapted to narrow, stable tropical conditions are restricted to small ranges. If true, the CVH would make physiological tolerance a first-order determinant of range extent, with consequences for explanations of the latitudinal diversity gradient and for predicting species' vulnerability to climate change.

Most tests of the CVH have been pattern-level, correlating range size with latitude, elevation, or occurrence-derived climate statistics. Pintor, Schwarzkopf and Krockenberger (2015) introduced a refined occurrence-based test in three monophyletic clades of medium-sized Australian skinks (Carlia, Ctenotus, and Egernia) — lizards, i.e., ectotherms, whose body temperature follows environmental temperature and whose physiology is therefore closely tied to climate. For each species they computed the maximum annual variability (MAV): the annual temperature range, $MAV = T_{\mathrm{max}} - T_{\mathrm{min}}$ (long-term mean temperature of the warmest period minus that of the coldest period), at the species' single most climatically variable occurrence point, derived from climate layers mapped at museum occurrence records. Relating MAV to latitudinal and elevational range extents, with a clade-pooled null model controlling for the fact that wide-ranging species sample more places by chance, they found positive MAV–range correlations in all six clade-by-dimension comparisons and concluded that the CVH was supported — even in taxa showing no classical Rapoport effect, because Australia's climatic gradients are complex and not simply latitudinal.

Physiological evidence relevant to the CVH exists but is coarse and indirect. Addo-Bediako, Chown and Gaston (2000), compiling published data across the globe, found that upper thermal limits of ectotherms vary little with latitude while lower (cold) limits track poleward winter severity, so tolerance breadth increases toward the poles — a necessary but not sufficient condition for the CVH. The critical gap is construct validity: an occurrence-derived measure like MAV has never been shown to correspond to a species' actual physiological tolerance. A species can record a high MAV by merely occurring in a variable site while behaviorally avoiding extremes through microhabitat use (behavioral buffering of climate). Directly measured tolerances — critical thermal limits (CTmax and CTmin, the body temperatures at which an animal loses motor control and cannot escape lethal conditions), desiccation resistance, metabolic limits, and acclimation capacity (the reversible adjustment of physiological limits to prior thermal experience) — are difficult to compare across species: the same authors showed (Pintor et al. 2016) that acclimation in reptiles is slow (95% complete only after weeks to months) and large, so measured tolerances depend heavily on acclimation history, and literature values taken under inconsistent acclimation cannot simply be mined for a validation. Interpretation of pattern-level results is further clouded by null-model work: Saupe et al. (2019) simulated virtual species under realistic Quaternary climate dynamics and showed that latitudinal gradients in realized niche breadth can arise purely from the spatial arrangement and temporal dynamics of climates, without any differences in fundamental physiological tolerances. What remains unresolved is the mechanistic core of the hypothesis itself: whether the Climatic Variability Hypothesis is supported as the mechanism underlying such patterns, given that the support for it rests on occurrence-derived temperature variability rather than on directly measured physiological tolerances.

Problem Statement

Determine whether the Climatic Variability Hypothesis is a valid mechanistic explanation for geographic range extent, at the generality of the hypothesis itself — across taxa and regions, beyond the three Australian skink clades and the Australian climate geometry in which it has so far been supported. A complete resolution requires establishing the mechanism directly, from measured physiological tolerances rather than occurrence-derived proxies, and quantifying how much of the variation in range extent it explains relative to alternative mechanisms. The five dimensions below are therefore jointly required rather than optional, and a complete answer addresses all of them; answering one of them alone — for example the proxy-to-tolerance check — does not resolve the task. The causal question spans the following dimensions:

1. Construct validity of the proxy: do species whose occurrence-derived climate variability (e.g., maximum annual variability, the annual temperature range at the most variable occurrence point) is high actually possess broader physiological tolerances — measured directly, with thermal tolerance breadth (critical thermal limits after controlled, complete acclimation) as the primary measure and desiccation resistance, metabolic limits, and acclimation capacity as supporting ones — rather than merely buffering variable climates behaviorally or through microhabitat choice?
2. Historical origin: did broader tolerances originate in more climatically variable source habitats and persist through range expansion, or evolve (adaptively or plastically) during expansion — a distinction that requires phylogeographic, paleoclimatic, and experimental evidence together (e.g., common-garden comparisons, in which individuals from different source populations are raised under identical conditions) rather than present-day occurrence data alone?
3. Relative contribution of alternative mechanisms: as a further comparative dimension, how much of the variation in range extent does climatic variability explain compared with biotic interactions, habitat specialization, and dispersal barriers, including for the exceptional small-range species that occur in highly variable habitats?
4. Other climatic axes: do climatic axes beyond annual temperature variability — such as precipitation, humidity, diurnal and microclimatic variability — shape range extent in the same direction and with comparable strength?
5. Generality: does the mechanism operate beyond the three Australian skink clades (Carlia, Ctenotus, Egernia) and the Australian climate geometry, across taxa with different life histories and regions with different spatial climate patterns?

These dimensions bear on one question: whether, and through which mechanism — physiological tolerance evolution or plasticity, or an alternative — climatic variability shapes geographic range extent, and with what relative importance.

References

1. Pintor, A. F. V., Schwarzkopf, L., Krockenberger, A. K. (2015). Rapoport's Rule: Do climatic variability gradients shape range extent? Ecological Monographs 85(4): 643-659. DOI 10.1890/14-1510.1, https://doi.org/10.1890/14-1510.1
2. Pintor, A. F. V., Schwarzkopf, L., Krockenberger, A. K. (2016). Extensive Acclimation in Ectotherms Conceals Interspecific Variation in Thermal Tolerance Limits. PLoS ONE 11(3): e0150408. DOI 10.1371/journal.pone.0150408, https://doi.org/10.1371/journal.pone.0150408
3. Stevens, G. C. (1989). The Latitudinal Gradient in Geographical Range: How so Many Species Coexist in the Tropics. The American Naturalist 133(2): 240-256. DOI 10.1086/284913, https://doi.org/10.1086/284913
4. Janzen, D. H. (1967). Why Mountain Passes are Higher in the Tropics. The American Naturalist 101(919): 233-249. DOI 10.1086/282487, https://doi.org/10.1086/282487
5. Addo-Bediako, A., Chown, S. L., Gaston, K. J. (2000). Thermal tolerance, climatic variability and latitude. Proceedings of the Royal Society of London B 267(1445): 739-745. DOI 10.1098/rspb.2000.1065, https://doi.org/10.1098/rspb.2000.1065
6. Mumladze, L., Asanidze, Z., Walther, F., Hausdorf, B. (2017). Beyond elevation: testing the climatic variability hypothesis vs. Rapoport's rule in vascular plant and snail species in the Caucasus. Biological Journal of the Linnean Society 121(4): 753-763. DOI 10.1093/biolinnean/blx027, https://doi.org/10.1093/biolinnean/blx027
7. Saupe, E. E., Myers, C. E., Peterson, A. T., Soberón, J., Singarayer, J., Valdes, P., Qiao, H. (2019). Non-random latitudinal gradients in range size and niche breadth predicted by spatial patterns of climate. Global Ecology and Biogeography 28(7): 928-942. DOI 10.1111/geb.12904, https://doi.org/10.1111/geb.12904
8. Zacaï, A., Brayard, A., Laffont, R., Dommergues, J.-L., Meister, C., Fara, E. (2018). The Rapoport effect and the climatic variability hypothesis in Early Jurassic ammonites. Palaeontology 61(6): 963-980. DOI 10.1111/pala.12389, https://doi.org/10.1111/pala.12389
9. Radomski, T. (2021). Rapoport effects are inconsistently demonstrated through time. Palaeogeography, Palaeoclimatology, Palaeoecology 577: 110568. DOI 10.1016/j.palaeo.2021.110568, https://doi.org/10.1016/j.palaeo.2021.110568
10. Radomski, T. (2025). Revisiting Rapoport's Rule: The History of an Idea and Alternative Explanations for Latitudinal Variation in Geographic Range Size. The Quarterly Review of Biology 100(2): 63-81. DOI 10.1086/736000, https://doi.org/10.1086/736000
11. Gallou, A., Jump, A. S., Lynn, J. S., Field, R., Irl, S. D. H., Steinbauer, M. J., Beierkuhnlein, C., Chen, J.-C., Chou, C.-H., Hemp, A., Kidane, Y., König, C., Kreft, H., Naqinezhad, A., Nowak, A., Nuppenau, J.-N., Trigas, P., Price, J. P., Roland, C. A., Schweiger, A. H., Weigelt, P., Flantua, S. G. A., Grytnes, J.-A. (2023). Diurnal temperature range as a key predictor of plants' elevation ranges globally. Nature Communications 14: 7890. DOI 10.1038/s41467-023-43477-8, https://doi.org/10.1038/s41467-023-43477-8
12. López-Reyes, K., Yáñez-Arenas, C., Villalobos, F. (2024). Exploring the causes underlying the latitudinal variation in range sizes: Evidence for Rapoport's rule in spiny lizards (genus Sceloporus). PLoS ONE 19(7): e0306832. DOI 10.1371/journal.pone.0306832, https://doi.org/10.1371/journal.pone.0306832
13. Montaño-Centellas, F. A., Fuentes, A. F., Cayola, L., Macía, M. J., Arellano, G., Loza, M. I., Nieto-Ariza, B., Tello, J. S. (2024). Elevational range sizes of woody plants increase with climate variability in the Tropical Andes. Journal of Biogeography 51(5): 814-826. DOI 10.1111/jbi.14783, https://doi.org/10.1111/jbi.14783
14. Barandun, M., Paz, A., van Tiel, N., van den Hoogen, J., Pellissier, L., Crowther, T. W., Maynard, D. S. (2025). Global patterns in plant environmental breadths. Ecography 2025: e07637. DOI 10.1111/ecog.07637, https://doi.org/10.1111/ecog.07637
15. Youngflesh, C., Kapsar, K., Uscanga, A., Williams, P. J., Doser, J. W., Kounta, L., Zarnetske, P. L. (2025). Environmental Variability Shapes Life History of the World's Birds. Ecology Letters 28(2): e70077. DOI 10.1111/ele.70077, https://doi.org/10.1111/ele.70077