Incipient radiation versus multiple origins of the GalápagosCroton scouleri(Euphorbiaceae)

Abstract

AbstractAimIsland 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 Galápagos flora. Our hypothesis is that the phenotypic variation ofCroton scouleriis a sign of incipient radiation, in which a single colonization has generated new lineages with considerable morphological differentiation.LocationThe Galápagos Islands and Neotropics.MethodsOne hundred and forty‐four nuclear ribosomalDNA(ITS) and plastidtrnL‐Fsequences ofCrotonsect.Adenophylliwere used to test the hypothesis of a single ancestry (monophyly) ofC. scouleriusing a phylogenetic approach. Sequence data were analysed using Bayesian inference (BI) and maximum likelihood (ML). A complementary phylogeographical analysis ofC. scouleriand phylogenetically related species was also performed using 123 plastid sequences (trnL‐F,petL‐psbE,trnH‐psbA) in search for common ancestry of Galápagos lineages.ResultsThe phylogenetic approach revealed that the closest relatives ofC. scouleriwereC. alnifolius,C. pavonisandC. rivinifolius. However, we failed to support monophyly ofC. scouleripopulations. Despite finding numerous haplotypes (14 polymorphic sequences/9 substitution‐based sequences), their distribution acrossCrotonspecies prevented us from inferring common ancestry forC. scouleri. The phylogeographical reconstruction revealed multiple lineages related to the origin ofC. scouleri.Main conclusionsLack of monophyly likely indicates that an incipient radiation from a single ancestor does not account for the striking infraspecific phenotypic variation inC. scouleri. This morphological diversity could be explained by recurrent biogeographical connections between Galápagos 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.