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Abstract
Species flocks are proliferations of closely-related species, usually after colonization of depauperate
habitat. These radiations are abundant on oceanic islands and in ancient freshwater lakes, but rare
in marine habitats. This contrast is well documented in the Hawaiian Archipelago, where terrestrial
examples include the speciose silverswords (sunflower family Asteraceae), Drosophila fruit flies,
and honeycreepers (passerine birds), all derived from one or a few ancestral lineages. The marine
fauna of Hawai‘i is also the product of rare colonization events, but these colonizations usually
yield only one species. Dispersal ability is key to understanding this evolutionary inequity. While
terrestrial fauna rarely colonize between oceanic islands, marine fauna with pelagic larvae can
make this leap in every generation. An informative exception is the marine fauna that lack a pelagic
larval stage. These low-dispersal species emulate a “terrestrial” mode of reproduction (brooding,
viviparity, crawl-away larvae), yielding marine species flocks in scattered locations around the
world. Elsewhere, aquatic species flocks are concentrated in specific geographic settings,
including the ancient lakes of Baikal (Siberia) and Tanganyika (eastern Africa), and Antarctica. These
locations host multiple species flocks across a broad taxonomic spectrum, indicating a unifying
evolutionary phenomenon. Hence marine species flocks can be singular cases that arise due to
restricted dispersal or other intrinsic features, or they can be geographically clustered, promoted
by extrinsic ecological circumstances. Here, we review and contrast intrinsic cases of species
flocks in individual taxa, and extrinsic cases of geological/ecological opportunity, to elucidate the
processes of species radiations.