Comparative phylogeography of endemic Azorean arthropods

Abstract

Background: For a remote oceanic archipelago of up to 8 Myr age, theAzores have a comparatively low level of endemism. We present ananalysis of phylogeographic patterns of endemic Azorean islandarthropods aimed at testing patterns of diversification in relation tothe ontogeny of the archipelago, in order to distinguish betweenalternative models of evolutionary dynamics on islands. We collectedindividuals of six species (representing Araneae, Hemiptera andColeoptera) from 16 forest fragments from 7 islands. Using three mtDNAmarkers, we analysed the distribution of genetic diversity within andbetween islands, inferred the differentiation time-frames andinvestigated the inter-island migration routes and colonizationpatterns.Results: Each species exhibited very low levels of mtDNA divergence,both within and between islands. The two oldest islands were notstrongly involved in the diffusion of genetic diversity within thearchipelago. The most haplotype-rich islands varied according to speciesbut the younger, central islands contributed the most to haplotypediversity. Colonization events both in concordance with and incontradiction to an inter-island progression rule were inferred, while anon-intuitive pattern of colonization from western to eastern islandswas also inferred.Conclusions: The geological development of the Azores has followed aless tidy progression compared to classic hotspot archipelagos, and thisis reflected in our findings. The study species appear to have beendifferentiating within the Azores for <2 Myr, a fraction of the apparentlife span of the archipelago, which may indicate that extinction eventslinked to active volcanism have played an important role. Assuming thatafter each extinction event, colonization was initiated from a nearbyisland hosting derived haplotypes, the apparent age of speciesdiversification in the archipelago would be moved closer to the presentafter each extinction-recolonization cycle. Exploiting these ideas, wepropose a general model for future testing.