Summary

Islands provide classic model biological systems. We review how growing appreciation of geoenvironmental dynamics of marine islands has led to advances in island biogeographic theory accommodating both evolutionary and ecological phenomena. Recognition of distinct island geodynamics permits general models to be developed and modified to account for patterns of diversity, diversification, lineage development, and trait evolution within and across island archipelagos. Emergent patterns of diversity include predictable variation in island species–area relationships, progression rule colonization from older to younger land masses, and syndromes including loss of dispersability and secondary woodiness in herbaceous plant lineages. Further developments in Earth system science, molecular biology, and trait data for islands hold continued promise for unlocking many of the unresolved questions in evolutionary biology and biogeography.

ADVANCES

Classification and analysis of marine islands by their geophysical dynamics, and of their species by how they colonized, provides a step toward a more nuanced biogeography out of which new insights are already emerging. This perspective is exemplified by the general dynamic model of oceanic island biogeography, which predicts how immigration, speciation, and extinction respond to the typical life cycle of hotspot islands, with phases of emergence, development, and submergence. The model successfully predicts such emergent patterns as the occurrence of peak diversification on youthful, expanding islands with maximum vacant niche space. Diversity patterns analyzed for large numbers of data sets have confirmed the importance of in situ evolutionary dynamics on remote archipelagos, which typically possess steep island species–area relationships, especially for endemic taxa. We may infer that variations in propagule flow among islands within archipelagos are important in modulating these emergent diversity patterns. There is, for example, good support for an “island progression rule” in which older land masses donate colonists to younger islands (consistent with the generalization of islands as “sinks”), but there is also increasing evidence of “reverse colonization,” including from islands to continental regions. Advances are also being made in linking such island biogeographical models with the classic traits and syndromes of insular species, although this first demands that previous generalizations are rigorously reexamined using expanded data sets and modern techniques of analysis. A classic insular syndrome is the loss of dispersability of formerly dispersive species following island colonization, for which there is now good evidence for several taxa, including many genera of land birds. Yet, paradoxically, and perhaps controversially, it has also been inferred that many species of plants lacking specialized dispersal adaptations can colonize quite remote islands, often by nonstandard means of transport. Unfortunately, island evolutionary syndromes, such as loss of flight in birds, frequently predispose species to heightened extinction risk when islands are colonized and transformed by humans, as we also document.