What is adaptive radiation? Many manifestations of the phenomenon in an iconic lineage of Hawaiian spiders.

ABSTRACT

Adaptive radiation provides the ideal context for identifying and testing the processes that drive evolutionary diversification. However, different adaptive radiations show a variety of different patterns, making it difficult to come up with universal rules that characterize all such systems. Diversification may occur via several mechanisms including non-adaptive divergence, adaptation to novel environments, or character displacement driven by competition. Here, we characterize the ways these different drivers contribute to present-day diversity patterns, using the exemplary adaptive radiation of Hawaiian long-jawed orbweaver (Tetragnatha) spiders. We present the most taxonomically comprehensive phylogenetic hypothesis to date for this group, using 10 molecular markers and representatives from every known species across the archipelago. Among the lineages that make up this remarkable radiation, we find evidence for multiple diversification modalities. Several clades appear to have diversified in allopatry under a narrow range of ecological conditions, highlighting the role of niche conservatism in speciation. Others have shifted into new environments and evolved traits that appear to be adaptive in those environments. Still others show evidence for character displacement by close relatives, often resulting in convergent evolution of stereotyped ecomorphs. All of the above mechanisms seem to have played a role in giving rise to the exceptional diversity of morphological, ecological and behavioral traits represented among the many species of Hawaiian Tetragnatha. Taking all these processes into account, and testing how they operate in different systems, may allow us to identify universal principles underlying adaptive radiation.