INTERFERENCE BETWEEN INDIVIDUALS IN PURE AND MIXED CULTURES OF STEPHANOMERIA MALHEURENSIS AND ITS PROGENITOR

Abstract

The diploid annual plants Stephanomeria exigua ssp. coronaria and S. malheurensis are related as progenitor and derivative. Stephanomeria malheurensis is known only from a single locality in eastern Oregon where it grows alongside (cohabits) its parent. Although the species are morphologically similar, they can be distinguished under uniform garden conditions by differences in root/shoot ratio, duration of rosette growth, branch structure and number of flower heads, seed weight, and other characters. The species were grown in pure and mixed cultures at several densities in two experiments to test the hypothesis that the distinctive phenotype of S. malheurensis could have evolved to reduce competition with its parent. The experiment revealed that the expression of the S. malheurensis phenotype did not change when it was grown alone or in mixed cultures with ssp. coronaria. Stephanomeria malheurensis neither gained nor lost an advantage. Since the measured characters related directly to fitness components, this suggests that the phenotype of the derivative probably did not evolve as a result of character displacement brought about by interference with ssp. coronaria. The consequence of sympatry between a newly arisen species and its parent appears to depend on the nature of the environment at the time of its origin. In an open habitat such as the Oregon locality of S. malheurensis, competition with the progenitor need not be a primary factor influencing the evolution of a new species.