A new study in Nature Communications offers a fresh explanation for one of evolution’s most persistent puzzles: why some groups of mammals become extraordinarily diverse while others remain represented by only a handful of species. The research, led by I. Rey-Rodríguez, G. Sotelo and S. Gamboa, finds that two processes often treated as interchangeable may actually follow different evolutionary rules. The emergence of new species—diversification—is associated with climatic specialisation, while the gradual accumulation of species within a lineage is more closely tied to how long that lineage has existed. The distinction could reshape how scientists interpret biodiversity maps, evolutionary family trees and the biological consequences of climate change.
Small mammals provide an especially revealing system for studying these patterns. Rodents, shrews, bats and other small-bodied species occupy nearly every terrestrial environment, from deserts and grasslands to tropical forests and high-elevation ecosystems. Their rapid generation times, varied diets and often limited dispersal abilities can produce sharp differences in how populations respond to temperature, rainfall and seasonal change. These traits also make small mammals sensitive indicators of environmental disruption. By examining their evolutionary histories, researchers can ask whether species-rich groups are diverse because they evolve rapidly, because they have simply had more time to accumulate species, or because particular ecological conditions repeatedly encourage evolutionary splitting.
The study’s central finding separates “diversification” from “species accumulation,” two concepts that are related but not identical. Diversification generally refers to the net production of species through the combined effects of speciation and extinction. A lineage can diversify rapidly if populations repeatedly become isolated and evolve into distinct species, provided those new species persist. Species accumulation, by contrast, describes the number of species present in a lineage or region as time passes. A very old lineage may contain many species even if its current rate of speciation is modest, simply because it has had millions of years in which new species could arise. In evolutionary biology, confusing these processes can make age appear to be an ecological explanation—or ecology appear to be a substitute for time.
According to the findings, climatic specialisation is associated with the generation of evolutionary diversity among small mammals. A species with a narrow climatic niche is adapted to a relatively restricted range of environmental conditions, such as a particular temperature regime, moisture level or pattern of seasonality. When populations become distributed across contrasting climates, natural selection can push them in different directions. Differences in physiology, behaviour, body size, reproductive timing or habitat use may accumulate over generations. If gene flow between populations is reduced, those differences can eventually contribute to reproductive isolation, the key biological barrier that allows separate species to persist.
Climate, however, does not act as a simple machine that automatically produces new species. Specialisation can create opportunities for divergence, but it can also increase vulnerability. A narrow climatic niche may help a species compete effectively in its preferred environment while leaving it poorly equipped to survive rapid warming, altered rainfall or extreme weather. The same ecological precision that may promote evolutionary differentiation over long timescales can therefore become a liability during abrupt environmental change. The research highlights this tension: climatic specialisation may be linked to diversification, yet highly specialised species may face elevated risks when the climate moves beyond the conditions under which their adaptations evolved.
The second pattern identified by the researchers concerns evolutionary time. Lineages that have existed longer have had more opportunities to accumulate species, even if their present-day diversification dynamics are not exceptional. This is sometimes described as a “time-for-speciation” effect. Imagine two evolutionary lineages experiencing similar rates of species formation and extinction, but one beginning tens of millions of years earlier. The older lineage has had more opportunities for geographic isolation, ecological shifts and genetic divergence. Over deep time, those repeated opportunities can produce a larger species inventory. The result is a crucial warning against interpreting species richness as direct evidence that a lineage is currently evolving faster than its relatives.
This distinction also helps explain why biodiversity hotspots can contain such a mixture of evolutionary patterns. Some regions may harbour many species because ancient lineages have persisted there for long periods. Others may support rapid diversification because environmental gradients—such as sharp changes in elevation, temperature or rainfall—repeatedly divide populations into distinct ecological zones. A third pattern may combine both: old lineages that continue to generate species in areas where climate varies dramatically across short distances. By distinguishing the causes of species richness, researchers can move beyond simple rankings of “most diverse” groups and begin identifying the historical and ecological mechanisms behind those rankings.
The implications extend beyond evolutionary theory. Conservation planning often prioritises areas with high numbers of species, but species counts alone do not reveal how vulnerable that diversity may be. A region filled with climatically specialised mammals could contain unique evolutionary adaptations that cannot easily be replaced if species disappear. At the same time, a region dominated by ancient lineages may represent an irreplaceable record of evolutionary history, even if its current rate of species formation is low. Recognising whether diversity reflects climatic specialisation, long-term persistence or both could improve decisions about which habitats to protect and which species require urgent monitoring.
The study also adds nuance to predictions about future biodiversity. Climate change is not expected to affect all evolutionary lineages equally. Species with broad climatic tolerances may be better able to track shifting conditions, while specialists may be unable to move, adapt or adjust their ecological requirements quickly enough. Yet specialists can also represent the evolutionary products of past climatic diversification, making them disproportionately important for understanding how life responds to environmental change. The researchers’ framework therefore connects macroevolution—the study of large-scale patterns across the history of life—with immediate conservation concerns. It suggests that the traits associated with past species formation may not be the same traits that guarantee survival in a rapidly changing world.
By showing that climatic specialisation and evolutionary age explain different aspects of small-mammal diversity, Rey-Rodríguez, Sotelo, Gamboa and colleagues provide a more precise vocabulary for discussing how biodiversity forms. The message is both straightforward and profound: a group can be species-rich because it has been evolving for a very long time, because its members repeatedly diverged along climatic boundaries, or because both forces acted together. Separating those possibilities allows scientists to reconstruct the evolutionary history behind today’s biodiversity—and to better anticipate which parts of that history may be most at risk in the future.
Subject of Research: Evolutionary diversification and species accumulation in small mammals, with a focus on climatic specialisation and evolutionary time.
Article Title: Diversification in small mammals is associated with climatic specialisation, whereas species accumulation reflects evolutionary time.
Article References: Rey-Rodríguez, I., Sotelo, G., Gamboa, S. et al. “Diversification in small mammals is associated with climatic specialisation, whereas species accumulation reflects evolutionary time.” Nature Communications (2026). https://doi.org/10.1038/s41467-026-77109-8
Image Credits: AI Generated
DOI: 10.1038/s41467-026-77109-8
Keywords: small mammals, biodiversity, evolution, diversification, species accumulation, climatic specialisation, climate niches, speciation, macroevolution, conservation biology
