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The importance of hybridization in the evolution of plant species is widely accepted, but its contributions to animal species evolution remain less recognized. Here we review evidence that hybridization has contributed to the evolution of reef corals, a group underpinning the coral reef ecosystem. Increasingly threatened by human and climate-related impacts, there is need to understand the evolutionary processes that have given rise to their diversity and contribute to their resilience. Reticulate evolutionary pathways among the ecologically prominent, mass-spawning genus Acropora suggest that hybridization, although rare on ecological timescales, has been instrumental in their diversification on evolutionary timescales. Evidence that coral hybrids colonize marginal habitats distinct from those of parental species' and that hybridization may be more frequent at peripheral boundaries of species' ranges supports a role for hybridization in range expansion and adaptation to changing environments. We conclude that outcomes of hybridization are significant for the future resilience of reef corals and warrant inclusion in conservation strategies.

Phylogeographic studies provide critical insight into the evolutionary histories of model organisms; yet, to date, range-wide data are lacking for the rough periwinkle Littorina saxatilis , a classic example of marine sympatric speciation. Here, we use mitochondrial DNA (mtDNA) sequence data to demonstrate that L. saxatilis is not monophyletic for this marker, but is composed of two distinct mtDNA lineages (I and II) that are shared with sister species Littorina arcana and Littorina compressa . Bayesian coalescent dating and phylogeographic patterns indicate that both L. saxatilis lineages originated in the eastern North Atlantic, around the British Isles, at approximately 0.64 Ma. Both lineages are now distributed broadly across the eastern, central and western North Atlantic, and show strong phylogeographic structure among regions. The Iberian Peninsula is genetically distinct, suggesting prolonged isolation from northeastern North Atlantic populations. Western North Atlantic populations of L. saxatilis lineages I and II predate the last glacial maximum and have been isolated from eastern North Atlantic populations since that time. This identification of two distinct, broadly distributed mtDNA lineages further complicates observed patterns of repeated incipient ecological speciation in L. saxatilis , because the sympatric origins of distinct ecotype pairs on eastern North Atlantic shores may be confounded by admixture of divergent lineages.