In Britain, the hexaploid perennial grass Koeleria vallesiana (Honck.) Bertol. is confined to the western end of the Mendip Hills but is widespread in western Europe. Comparisons of the marginal British populations with continental populations from France indicate that there are no differences in breeding behaviour or genetic variability. There are, however, greater geneti differences in morphological characters between British and French material than between British populations. In addition, the Mendip populations, which are topographically isolated, show significant differences in morphology which are genetic in origin.

Comparisons of floret fertility under different systems of mating indicate that K. vallesiana is a self-incompatible outbreeder. Florets collected from the wild show low fertility (c. 5%) in common with other outbreeding perennial grasses.