Differentiation Among Nine Populations of Phlox: Response to Environmental Gradients

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Abstract
To investigate the extent of ecological differentiation in the annual Phlox drummondii, nine populations were characterized for response along six controlled environmental gradients. Statistically significant heterogeneity among populations in survival, vegetative growth, and flowering was observed in response to different levels of moisture, light, nutrients, and competition. Differences in germination were observed in response to temperature and nutrient levels. Analysis of response along each gradient indicated that some populations are more sensitive to different environments than others, a significant genotype—environment interaction. Differences among populations were teste for adaptive significance by examining the relationships between response along gradients and a set of environmental measurements performed at the site of each population. Soil was characterized for five nutrients, organic matter, and texture. Percent plant cover, extent of surface disturbance, density of P. drummondii, and species diversity were also measured. Climatic differences were established from meteorological records. Correlation was used to examine relationships between site characteristics and population response along gradients. Despite highly significant differences among populations in response along gradients and significant differences in the environment at the sites of populations, we found no evidence that the differentiation among populations is adaptive. These results suggest that in situations where no single, consistent environmental factor has an overriding influence on the distribution and abundance of a species, a certain amount of indeterminancy arises between environment and adaptations exhibited by the organism. This situation may be common among plant species.