Flower colour polymorphism in Cirsium palustre 1

In seacliff and mountain populations the thistle Cirsium palustre (L.) Scop. is polymorphic for flower colour, occurring as the typical purple flowered morph, a white flowered morph, and two intermediate flowered morphs. The white morphs are homozygous. The polymorphism is subject to selective forces associated with exposure and fluctuation in population size. Selection is sufficiently intense to produce intrapopulational differentiation.