American Society of Naturalists

Molly C. Hetherington-Rauth and Marc T. J. Johnson (July 2020)

Insect pollinated plants evolve smaller flowers on some (sub)tropical islands and in and in specific plant families

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Was Darwin right when he called the flowers of island plants ‘insignificant, ugly little flowers’?

Islands often play host to “unusual” organisms. For example, giant tortoises, swimming iguanas, flightless birds, and tame animals are all features commonly associated with animals inhabiting islands. These unusual suites of traits, such as gigantism and tame behavior, are referred to as island syndromes and have been well characterized for island animals; however, the presence of island syndromes for plants has been poorly studied.

One observation repeatedly made by naturalists, including Charles Darwin and Alfred Russel Wallace, is that island plants have small, non-showy flowers compared to plants living on the mainland. An often-cited explanation for this is that islands typically have less diverse pollinator communities and often lack large pollinators compared to mainland pollinator communities. Despite this explanation, it has yet to be tested whether island plants do, in fact, evolve small flowers.

In this study, Hetherington-Rauth and Johnson put these historical claims to the test by examining island plants from across seven tropical and subtropical island systems in the Pacific Ocean. Using herbarium specimens, they measured the flowers of 556 plant species found on islands, as well as their closest living mainland relatives.

Contrary to naturalists’ predictions, they find that on average island plants did not evolve smaller flowers relative to mainland plants; however, the study did find that on specific islands, including the Galápagos Islands where Darwin visited, plants did evolve smaller flowers. The study also found that certain plant families tended to evolve smaller flowers. The authors conclude that no general island syndrome exists for flower size, but instead floral morphology likely evolves in response to the features of specific islands and the differing evolutionary history among plant families.

Abstract

Animals frequently evolve unique suites of traits on islands, but whether plants evolve comparable island syndromes remains unresolved. Here we test the prediction based on natural history observations that insect pollinated plants evolve smaller flowers on islands than mainland communities. We examined 556 plant species representing 136 phylogenetically independent contrasts between island and mainland sister taxa. We focused on endemic taxa originating from the Americas associated with seven tropical and subtropical islands of the Pacific Ocean. Contrary to conventional wisdom, flowers were not on average smaller on islands than the mainland. On specific archipelagos (the Galápagos Islands and Revillagigedo Islands), however, island taxa did evolve smaller flowers. Divergence in flower size between island and mainland taxa also varied among taxonomic families, such that some plant families evolved smaller flowers on islands, other families evolved larger flowers on islands, while some families exhibited no divergence in flower size between island and mainland taxa. Overall, our results show that there is no general island syndrome for flower size, but instead the evolution of floral morphology is complex and context dependent, depending on variation among islands and plant families. Our results also suggest that if island floras are typically dominated by small flowered species, as suggested by natural history observations, then ecological filtering is a more likely explanation of this pattern than evolutionary divergence post-colonization. We propose future studies that could disentangle the relative roles of ecological filtering and evolution in the distribution of floral traits on islands.