Experimental Ecological Genetics in Plantago: A Structural Equation Approach to Fitness Components in P. Aristata and P. Patagonica

A modified path-analysis technique, maximum likelihood linear structural equation modelling, is applied to the comparison of fitness components in the annuals Plantago aristata and P. patagonica, grown under controlled environment conditions. The technique is used to determine the magnitude and significance of causative relations in a model scheme where "fitness," an unmea- sured construct of two measured variables, seed size and seed number, is determined by direct and indirect (via reproductive structures) paths from the photosynthetic structures represented by different "leaf areas," unmeasured constructs of leaf number and leaf size, during development. The model provides a reasonable fit to the observed data, and the two species do not differ significantly with regard to the parameter estimates. Inflorescence number and capsule number have large significant effects on fitness, but the effect of number of seeds per capsule is not significant. Leaf areas during midlife influence fitness indirectly via production of reproductive structures; during late-life they have a direct effect on fitness via seed filling. The analysis of fitness components using the method of structural equation model fills an intermediary role between simple morphometric analysis and more explicit modelling of physiological processes.

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