Spatial and temporal stability of demes in Diploneis smithii/D. fusca (Bacillariophyta) supports a narrow species concept

Abstract An earlier study demonstrated the presence of 11 morphologically distinct demes (‘morphotypes’) within the Diploneis smithii/D. fusca species complex in a single sample from Ganavan near Oban on Scotland's west coast. Two of these demes were subsequently shown to occur elsewhere around the British coastline and to keep their identity wherever they occurred together. However, the conclusion that the demes were sufficiently distinct and widespread for them to be described as separate species has been criticized on the grounds that no evidence exists that there are reproductive barriers between them which could provide independent evidence for their relationships. It has not been possible, thus far, to grow the species in culture (a prerequisite for controlled breeding experiments), and so independent evidence has been sought elsewhere. It is shown here that nine of the 11 demes from Ganavan also occur in Cumbrae (in the Firth of Clyde, some 80 km SSE of Oban), not only in recent samples but also in...