Evolutionary Modification of Ecology in a Common Lichen Species

Abstract

The very common foliose lichen Pseudoparmelia caperata (Ascomyco- tina, Parmeliaceae) has an exceptionally broad amplitude of ecological tolerance, growing on trees and noncalcareous rocks on all continents. In most of eastern North American, however, it is primarily epiphytic, its place on rock being taken by the closely related, exclusively American P. baltimorensis. Juveniles of both species are morphologically indistinguishable but the frequent presence of gyrophoric acid in P. baltimorensis in much of its range gives a chemical marker for identifying most immature thalli. Juveniles are distributed approximately like adults, indicating that the factors responsible for excluding most P. caperata from rock and most P. balti- morensis from tree trunks must act very early, perhaps at the stage of first growth of the asexual propagules that account for most individuals in populations. Pseudopar- melia caperata in eastern North America may have sustained genetic impoverishment through the loss of gamodemes adapted to the microhabitat of rock as a result of intense competition with P. baltimorensis in the past. Each species still occasionally grows in the other's preferred microhabitat and both may grow in approximately equal numbers on tree bases, which seem to be ecologically intermediate, having the chemistry and texture of bark and the reduced light and increased moisture of the forest floor. In spite of the importance of asexual reproduction in propagating the best genotypes, apothecia with spores are still regularly produced in both species. Widely distributed or disjunct species in which long-isolated popula- tions have seemingly not diverged are evolutionary enigmas. This phe- nomenon has been interpreted as evidence that gene flow is not the major evolutionary force that integrates the biological species (Ehrlich and Raven 1969). Many species of cryptogams, including lichen-forming fungi, consist of such old discontinuities without obvious divergence, a situation traditionally dismissed as reflecting genetic stagnation in these organisms. The present paper analyzes a widely distributed lichen species that would superficially seem to fit the model of ancient discontinuity without differentiation. Closer examination, however, shows that the representation of this species in one part of its range has actually sub- stantially diverged-not in morphology, which would be easier to per- ceive, but rather in amplitude of ecological tolerance, which is more elusive. We believe that this modification resulted from past competition with a similar and closely related species that is still extant today. Pseudoparmelia caperata (L.) Hale is one of the best-known of all foliose lichens. It is native to all continents and in many parts of the world is among the commonest of all lichen species, growing on trees and non- calcareous rocks. It varies little morphologically, is usually sterile, and produces soredia in characteristic erumpent soralia (fig. 1). In the late 1960s we became interested in a morphologically different species then