Of Brobdingnag and Lilliput, or how the area of an island may determine the size of the bodies and genomes that inhabit it, along with their mutation rates

There are recurrent observations of insular gigantism and dwarfism, often explained by the island rule: small organisms would become larger on islands, while large organisms become smaller. But why is the latter? In addition, not all the observations fit this rule. Here I propose a neutral null expectation linking existing frameworks to understand body and genome size variation. Following the Island Biogeography Theory (IBT), insular area influence census N . Under Damuth’s law, changes in N are deduced from changes in body size. Because collapses in N are associated with collapses in effective population size N e , and because Ne may decline at a different rate than island area, change in area may alter effective density D e . A prediction of the drift-barrier hypothesis is that N e is affecting mutation rates. Consequently, body mass, genome size, and μ may covary with island area through changes in D e and N e . Falsification of the latter model is feasible by determining changes in genomic features of insular species. We now have the opportunity to interrogate the extensive data available. Here I ask whether decreasing island area predicts body size, genome size, and μ in a global dataset. The results are broadly consistent with such a model, although the observed effects are modest. Significance statement Naturalists have long reported that insular species tend to become unusually large or small compared to their mainland relatives. Despite the familiarity of this “island rule”, there is still no broad mechanistic explanation for why these changes occur so consistently across different groups of organisms. This work proposes that an important neutral factor can be the change in effective density of isolated populations. By combining the expectations of Damuth’s law, Island Biogeography Theory, and the drift-barrier hypothesis, this synthetic framework provides a null expectation for how variation in island area may be associated with variation in body size, genome size, and evolutionary rates.