Flower size evolution in the Southwest Pacific

Despite accelerating interest in island evolution, the general evolutionary trajectories of island flowers remain poorly understood. In particular the island rule, which posits that small organisms become larger and large organisms become smaller after island colonization, while tested for various plant traits, has never been tested for flower size. Here, we provide the first test of the island rule for flower size for animal- and wind-pollinated flowers, and the first evidence for generalized in situ evolution of flower size on islands.

Focusing on ten archipelagos in the Southwest Pacific, we amassed a dataset comprising 129 independent colonization events, by pairing each island endemic to its closest mainland relative. We then tested for the island rule in flower size and for gigantism/dwarfism in floral display for animal- and wind-pollinated flowers.

Animal-pollinated flowers followed the island rule, while wind-pollinated flowers did not, instead showing evidence of gigantism. Results remained consistent after controlling for breeding system, mainland source pool, degree of taxonomic differentiation, taxonomic family and island type.

While in situ evolution of flower size is widespread on islands in the Southwest Pacific, animal- and wind-pollinated flowers exhibited unexpected and markedly different evolutionary trajectories. Further studies are needed to understand the mechanisms behind these patterns.