Differentiation-Hybridization Cycles and Polyploidy in Achillea

Abstract

The concept of differentiation-hybridization cycles was first elaborated in a group of diploid Galium species from the Eastern Mediterranean (Ehrendorfer 1958). In this group crossing barriers between the very distinct Galium graecum and G. canum remain practically intact on some Aegean Islands, e.g., Rhodes, where strong competition tolerates only a small amount of hybrid introgression in spite of extensive sympatric occurrence. On the Anatolian mainland, however, crossing barriers between G. graecum and G. canum have broken down under less rigorous competition and more possibilities for ecological and geographical expansion. Hybridization has led to the origin of the highly polymorphous G. dumosum, a hybrid complex which has widely replaced the parental species and is in full course of new differentiation, forming a center of variability. The data concerning this and other groups of Galium have been incorporated into a generalized diagram, conveying the concept of differentiation-hybridization in pictorial manner (fig. 1)...