Spatio-temporal relationships of the Macaronesian endemic flora: a relictual series or a window of opportunity?

Convolvulus is represented in Macaronesia by two endemic clades. The first (Clade I) has a predominantly western Mediterranean sister group whilst the second (Clade II) is nested within a predominantly eastern Mediterranean clade. The Relictual Series Hypothesis (RSH) predicts that degree of geogrpahical disjunction between Macaronesian groups and their sister taxa is related to time available for continental extinction to occur and this would provide a possible explanation for the observed pattern if Convolvulus Clade I is younger than Clade II. To test this hypothesis, mean relative divergence times for the two Macaronesian Convolvulus clades are estimated using a nrITS dataset. The results do not support the age-disjunction hypothesis and indicate that the mean divergence time for Clade I predates that of Clade II, although the difference between the mean divergence times is small and the standard deviations are large. The inconsistency between this result and the RSH might be explained by the stochastic nature of evolution: with continental extinction events occurring during a similar time period, there may have been inadequate time for a clear spatio-temporal pattern to develop. An alternative explanation, termed the 'Colonisation Window Hypothesis' (CWH), is also presented. This hypothesis emphasises island establishment rather than continental extinction as the prime determinant of spatio-temporal relationships of Macaronesian groups and predicts that opportunities for colonisation into Macaronesia has been temporally constrained due to discrete waves of colonisation. Whilst the Convolvulus results are consistent with both RSH and CWH, a framework for establishing the ability of these two hypotheses to explain the spatial relationships of the Macaronesian endemic flora is presented.