Abstract
AbstractTwo populations of Poa annua L., one from an irrigated area and one from a dry area, were studied to test the following hypothesis: different life cycles should be selected in habitats of contrasting mortality regime. Plants from the irrigated area exhibited a typical perennial habit (indeterminate growth and low reproductive effort) while those from the dry area showed traits characteristic of annuals (determinate growth, high reproductive effort and flowering synchrony) although total annuality was not achieved. The antagonism between growth and reproduction was expressed at the individual tiller level and was reflected in the life history traits of the populations. The characters studied exhibited a high broad sense heritability which is required for a rapid evolution of life cycles when selection pressures are strong. Different levels of constraints (ecological, evolutionary, physiological and developmental) have to be taken into account in order to understand this evolution.
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