A study of DNA sequences from more than 1,800 organisms on Mount Kinabalu in Borneo reveals the evolutionary mechanisms that led to the mountain's high and unique biodiversity. See Letter p.347
Mountains occupy only about one-eighth of the world's land surface outside Antarctica, yet they are home to around one-third of all terrestrial species1. However, little is known about when, where and how this mountain biodiversity developed. In a paper published on page 347, Merckx et al.2 investigate the origins of species on a highly diverse tropical mountain, Mount Kinabalu on the island of Borneo. They find that most endemic species — those found nowhere else — arose relatively recently on the mountain, deriving from both local ancestors and distant immigrants that were pre-adapted to cool environments. These observations have direct implications for our understanding of montane biodiversity, and may offer clues on how to protect both the unique organisms found there and the habitats that provide the stage for speciation.
Ever since the first botanical documentation of a tropical mountain by Alexander von Humboldt more than two centuries ago3, naturalists have been fascinated by the diverse and unusual variety of life forms found on mountains. But we still lack answers to fundamental questions about the evolution of mountain biodiversity. Did the species living on mountains originate in the surrounding lowlands, where their ancestors became successively adapted to higher altitudes? Or are montane species mostly pre-adapted immigrants from far-away mountains? And are the mountain-dwelling species ancient or recent?
Merckx and colleagues got to the root of these questions by using a new comparative approach. Rather than studying the formation of species in a single group of organisms and extrapolating from there, which has been the standard approach so far, the researchers collected a large variety of organisms — from frogs and snails to insects, plants and fungi — that inhabit the iconic Mount Kinabalu and its surroundings. They then sequenced DNA from the approximately 1,850 collected specimens, compared these sequences with others in their own collections and public databases, and calculated their relationships, ages, geographical origins and ancestral environments.
The first striking result is that most of the montane organisms examined are relatively young. They started to speciate during the past 6 million years, after, or at the same time as, the rise of the mountain they inhabit. Unlike the ancient creatures found on a remote tropical mountain in Arthur Conan Doyle's novel The Lost World, this finding suggests a recent origin for montane species across the domains of life, and supports the recent speciation documented for alpine plants on several continents4.
The second major finding is the dual origin of montane organisms (Fig. 1). Some of the species, in particular those found at the highest elevations of Mount Kinabalu, have their closest relatives outside of Borneo. Their ancestors were often good at dispersing, such as plants or fungi that produced large quantities of light seeds or spores that could be transported with the wind. Other species — about twice as many — derive from local ancestors at lower altitudes on the same island. The location of Mount Kinabalu, surrounded by an exceedingly diverse tropical forest at the crossroads of Asia and Oceania, two regions that have their own distinct fauna and flora, apparently provided the mountain with a rich initial stock for the evolution of its unique biodiversity.
Finally, the authors' analysis shows an overarching role for niche conservatism — the tendency for organisms to maintain their environmental preferences over evolutionary time. This result is evident both from the immigrant and the local lineages that gave rise to Mount Kinabalu's biodiversity. Most of the ancestral species were already adapted to cool conditions, either in temperate regions or in other montane habitats. Even the lineages that 'climbed up' Mount Kinabalu often remained in the same broadly defined vegetation zone. The niche conservatism and pre-adaptation shown for the inhabitants of this tropical mountain are in line with previous findings across the Southern Hemisphere5 and with observed patterns of plant movement into cold environments around the world6.
Merckx and colleagues' study thus provides a textbook example of how biodiversity originates from the interplay between long-distance dispersal and local recruitment, followed by adaptation and speciation through interaction with changes in the landscape, climate and environment7. Its limitations are shared with other studies that are based on living organisms and current species distributions. Biological surveys sample only a fraction of the total biodiversity of an ecosystem, and large organisms found in easily accessible sites are typically over-represented. Furthermore, estimates of speciation events, geographical history and niche conservatism obtained from phylogenetic trees are prone to large error intervals and many assumptions, and largely disregard the confounding effects of extinction. The integration of recent and past data — from DNA, fossils and environmental and geological proxies — could remedy these shortcomings8, but such data are still scarce or not readily available.
Much could be gained by applying Merckx and colleagues' whole-community approach to other systems around the world. As well as increasing our fundamental knowledge of the diversity and distribution patterns of species, such eco-evolutionary studies would shed light on why some regions are much more biodiverse than others. A better understanding of the past may also, at least to some extent, help scientists to manage the present and predict the future. Taking climate changes as an example, evolutionary studies may help to inform us about the resilience of species and ecosystems9. They may also help to ascertain the role of mountains as potential reservoirs of biodiversity, because montane species need to move only short distances to keep their preferred niche10. Finally, we may need to devote more resources to preserving the natural corridors that link vegetation zones along altitudinal slopes if we are to safeguard the biotic interchange reported by Merckx et al., and thus enhance our protection of the world's unique and rich mountain biodiversity.Footnote 1
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