Incipient radiation vs. multiple origins of the Galápagos Croton scouleri (Euphorbiaceae)

Abstract

Aim: Island radiations imply the emergence of numerous species in a short period of time. Downscaling at the infraspecific level, considerable differentiation among populations can be a sign of ‘incipient radiation’. However, this process remains largely unexplored. We focus on one of the most outstanding cases of infraspecific morphological variation in the Galapagos flora. Our hypothesis is that the phenotypic variation of Croton scouleri is a sign of incipient radiation, in which a single colonization has generated new lineages with considerable morphological differentiation. Location: The Galapagos Islands and Neotropics. Methods: One hundred and forty-four nuclear ribosomal DNA (ITS) and plastid trnL-F sequences of Croton sect. Adenophylli were used to test the hypothesis of a single ancestry (monophyly) of C. scouleri using a phylogenetic approach. Sequence data were analysed using Bayesian inference (BI) and maximum likelihood (ML). A complementary phylogeographical analysis of C. scouleri and phylogenetically related species was also performed using 123 plastid sequences (trnL-F, petL-psbE, trnH-psbA) in search for common ancestry of Galapagos lineages. Results: The phylogenetic approach revealed that the closest relatives of C. scouleri were C. alnifolius, C. pavonis and C. rivinifolius. However, we failed to support monophyly of C. scouleri populations. Despite finding numerous haplotypes (14 polymorphic sequences/9 substitution-based sequences), their distribution across Croton species prevented us from inferring common ancestry for C. scouleri. The phylogeographical reconstruction revealed multiple lineages related to the origin of C. scouleri. Main conclusions: Lack of monophyly likely indicates that an incipient radiation from a single ancestor does not account for the striking infraspecific phenotypic variation in C. scouleri. This morphological diversity could be explained by recurrent biogeographical connections between Galapagos and the mainland, involving multiple colonizations to the islands from the continent rather than back colonizations from the islands to the mainland. Morphological, reproductive, geographical and ecological evidence better support the scenario of recurrent colonizations from the continent in different periods of time.