Islands are considered natural laboratories to study evolutionary questions. One of the most striking patterns described on islands is the tendency for species to become giants or dwarfs in comparison with mainland relatives, a pattern called the ‘island rule'. Dwarf hippos and elephants in the Mediterranean islands, or even our miniature relative Homo floresiensis on the Indonesian island of Flores, are examples of dwarfism. On the other hand, small mainland species may evolve into giants after colonizing islands, giving rise to such oddities as the St Kilda field mouse (twice the size of its mainland ancestor), the infamous dodo of Mauritius (a giant pigeon), and the Komodo dragon. Yet, since it was first coined in 1964 by Foster, the ‘island rule' hypothesis remains contentious, with mixed results reported across several studies and authors.
A research team formed by scientists from several institutions in Europe has revisited the island rule (also known as Foster's rule) in order to elucidate its validity for explaining the tendency of small-sized and large-sized species to gigantism and dwarfism, respectively, in insular ecosystems. The study, coordinated by Dr. Ana Benítez-López during her postdoctoral stay at Radboud University (Netherlands) and finished at her current institution, Doñana Biological Station (EBD-CSIC), has been published in the journal Nature, Ecology and Evolution.
In their analysis, the authors compared ca. 2400 island populations and over 1000 insular species with their mainland counterparts, and showed that ‘island rule' effects are widespread across vertebrates, and are evident in mammals, birds and reptiles, but less so in amphibians, which mostly tend towards gigantism. In their study, the authors found that the magnitude of insular dwarfism and gigantism is mediated by island size and isolation, with more pronounced effects in smaller, more remote islands for mammals and reptiles.
The mechanisms underlying these patterns are likely linked to the low number of species and subsequent low levels of predation and competition in small islands, and the reduced gene flow with mainland ancestors in isolated islands. Using state-of-the-art statistical techniques, the authors were able to control for a number of confounding factors that characterize biodiversity data (e.g. variability between sources, phylogenetic relatedness), thereby strengthening the evidence for predictable evolutionary trajectories on islands.
"Our study used old and new measurement data from museum and live specimens to address an old-standing question in evolutionary ecology", adds Ana Benítez, "By applying a more complex statistical approach, we were able to synthesize all heterogeneous data from varying sources into a single analysis that demonstrates that insular gigantism and dwarfism across vertebrates is a generalized pattern and not just an evolutionary fluke".
Yet, there are other factors modulating insular size shifts. For example, mammals and birds (warm-blooded animals) both responded to island temperature in line with the heat conservation hypothesis, with small- and large-sized species exhibiting exacerbated gigantism and diminished dwarfism, presumably to conserve heat in colder, harsher insular environments. Further, seasonality was an important factor for reptiles, with smaller species becoming larger on islands with high seasonality. Hence, while the island rule is pervasive across vertebrates, the implications for body size evolution are nuanced and depend on an array of context-dependent ecological pressures and environmental conditions.
Luca Santini added: "The island rule has a long history of investigation in biogeography, and incredibly no consensus was yet achieved among experts. Our study addressed this long-standing debate, suggesting a multifactorial nature of body mass evolution and the importance of the choice of methods and data quality when addressing such complex questions".
Dr Joseph Tobias, from the Department of Life Sciences at Silwood Park, Imperial College London, said: "Most so-called ‘rules' in evolution turn out to be fairly limited in their scope and predictive power. From that perspective our results are exciting, suggesting that ecological processes can explain unusual evolutionary outcomes on islands worldwide."
Benítez-López, A., Santini, L., Gallego-Zamorano, J., Milá, B., Walkden, P., Huijbregts, M. A. J., Tobias, J. A. The island rule explains consistent patterns of body size evolution in terrestrial vertebrates. (2021) Nature, Ecology and Evolution. doi: 10.1038/s41559-021-01426-y