Islands are natural laboratories of evolution, where it is possible to observe evolutionary trajectories that differ from those characterising continental masses. One of the aspects that has always aroused interest and controversy among experts is the so-called 'insularity rule' (or Foster's rule) proposed in 1964, which attempts to explain the processes of gigantism and dwarfism that characterise many island animal species.

According to this biological principle, members of a species inhabiting islands tend to increase or decrease in size over time compared to those living on the continents: think of the now-extinct hippopotamuses and dwarf elephants in the Mediterranean islands, examples of island dwarfism. Just as there are small species that can evolve into giants after colonising islands, giving rise to oddities such as the St. Kilda Island mouse in Scotland (twice the size of its continental ancestor), the Komodo dragon, a large lizard species found on the Indonesian islands, or the dodo, a giant pigeon on the Mauritius islands, which became extinct in the 17th century due to human presence.

An international research team led by Ana Benítez-López from the Doñana Biological Station (EBD-CSIC) in Andalusia, with Luca Santini from the "Charles Darwin" Department of Biology and Biotechnology at the Sapienza University of Rome, has compared some 2,400 populations of over 1,000 island species with their corresponding continental populations, re-evaluating the postulate underlying the insularity rule and showing how geographical and climatic factors strongly influence this principle. The study was published in the journal Nature, Ecology and Evolution.

In particular, researchers have shown that the extent of island dwarfism and gigantism depends on the degree of isolation and the size of the islands, with the effects more pronounced in mammals and reptiles inhabiting small islands far from the mainland. The mechanisms explaining these factors most likely derive from the small number of species present, resulting in reduced levels of predation and competition, as well as reduced gene exchange with mainland populations on islands far from the mainland.

Furthermore, the influence of climatic factors on the extent of the island effects has been observed: both mammals and birds (warm-blooded animals) show more pronounced gigantism and less pronounced dwarfism on cold islands, presumably to reduce heat loss. In addition, seasonality was found to be an important factor for small reptiles, which exhibit more pronounced gigantism on islands with high seasonality, probably to cope with long periods of resource scarcity. 

By using sophisticated statistical techniques, the authors were able to control several problems inherent in biodiversity data, such as the variability of errors in the source data and the evolutionary relationship between species; the result was clear evidence in favour of the new hypotheses.

"There is a long history of biogeographical research on the processes of gigantism and island dwarfism, yet incredibly no consensus has ever been reached among experts as to whether these processes could actually be considered a rule," says Luca Santini of Sapienza University. "Our study addresses this question, showing not only that gigantism and island dwarfism are general mechanisms acting consistently across all vertebrates, and therefore that they are not single evolutionary events in a few known species, but also that there are many processes that help explain size diversity in species."

References:

The island rule explains consistent patterns of body size evolution in terrestrial vertebrates. Ana Benítez-López, Luca Santini, Juan Gallego-Zamorano, Borja Milá, Patrick Walkden, Mark A J Huijbregts, Joseph A Tobias. Nature, Ecology and Evolution 2021. doi: 10.1038/s41559-021-01426-y

Further Information

Luca Santini 
Department of Biology and Biotechnology "Charles Darwin"
luca.santini@uniroma1.it