Evidence for the Evolution of Competition Between Two Species of Annual Plants

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Abstract
Populations of Erodium cicutarium and E. obtusiplicatum, common plant species in the California annual grassland, were studied in regard to ecotypic differentiation as a response to competition. The main hypothesis was that the presence or absence of coexistence in the history of a population would affect its fitness in interspecific competition. Populations of the 2 spp. were collected from isolated and sympatric sites in nature. Competition experiments were done in a greenhouse common garden environment, using the DeWitt replacement series design. The seed yield of both species was measured in 3 types of cultures: sympatric, allopatric and transposed. Two cultures of each type were planted. The total reproductive rates of sympatric cultures were higher than those of allopatric cultures. The total reproductive rates of sympatric cultures were higher than those of the transposed cultures only at low frequencies of E. cicutarium. Evidence was obtained that 2 of the 3 populations of E. obtusiplicatum display specific responses to the genetic composition of the population of E. cicutarium competitors. Ratio-diagram analysis indicated different results for all cultures. These experimental results suggest that evolution in response to competition between 2 spp. of plants occurs at the population level and that this evolution involves the selective response of the gene pool of 1 population to the presence of a specific gene pool of another species.