STYLE MORPH RATIOS IN TRISTYLOUS DECODON VERTICILLATUS (LYTHRACEAE) -SELECTION VS HISTORICAL CONTINGENCY

Tristylous plant populations should exhibit equal frequencies of the t hree style morphs at equilibrium. In contrast, New England and central Ontario populations of Decodon verticillatus (Lythraceae) show a mark ed deficiency of the mid-styled (M) morph. This pattern was also evide nt in an independent sample of 30 populations from eastern Ontario; al l nine cases of morph loss involved the M morph, and this morph also o ccurred at low frequencies, especially in large trimorphic populations . The hypothesis that a fitness disadvantage to the M morph accounts f or its low frequency was not supported by data from a series of held a nd glasshouse experiments. Measures of reproductive success from two p opulations showed no disadvantage to the M morph, but revealed substan tially higher seed set in the mid- (M) and short-styled (S) morphs tha n in the long-styled (L) morph. A pollen addition experiment showed th at low seed set in the L morph can only partly be explained by reduced pollen receipt. Comparison of morph frequencies between adult plants and their open-pollinated progeny in three populations failed to revea l any morph-specific fitness differences. Analysis of growth and flowe ring of open-pollinated progeny from three populations in a 2-yr glass house experiment revealed differences among morphs for some parameters in some populations, but no consistent patterns were evident. The def iciency of the M morph may have resulted from an historical accident d uring post-glacial colonization of parts of the northern range. Comput er calculations examining the return of morph frequencies to equilibri um after major perturbations indicated that populations of long-lived clonal species like D. verticillatus may preserve skewed morph ratios for > 10 000 yr. Moreover, the distribution of fossil seeds indicates that this species has occurred in its post-glacial range in southern O ntario for only approximate to 5000 yr. It is important to distinguish between ecological and evolutionary time scales when testing selectiv e interpretations of microevolutionary patterns involving clonal taxa in glaciated regions.