PHENOTYPIC SELECTION AND POPULATION DIFFERENTIATION IN RELATION TO HABITAT HETEROGENEITY IN ARRHENATHERUM-ELATIUS (POACEAE)

1 Population differentiation in variable environments is often related to the different local selection pressures that plants may experience . We examine the relation between reproductive success and morphologic al and phenological traits in woodland and open habitat populations of the tetraploid grass Arrhenatherum elatius. 2 The reproductive correl ates of morphological and phenological variation were estimated in woo dland and open habitats, and on plants from each of three populations from woodland and three from open habitats grown in a garden environme nt at two light intensity levels. Genetic variation in these populatio ns was estimated in uniform conditions, 3 Temporal and spatial differe nces in phenotypic selection on morphology and phenology were observed within and between the two sites. Plants from the two sites showed si gnificant phenotypic and genetic differences, and there is a dramatic reduction in reproductive success in the woodland habitat and in plant s from this habitat grown in the two garden environments. 4 In control led conditions, selection patterns were similar to those observed in n atural habitats, suggesting that light availability may be an importan t ecological factor involved in population differentiation in A, elati us. The presence of significant genetic variability for the studied tr aits indicates the presence of a genetic potential to respond to the d etected selection pressures,5 We argue that a clear understanding of p atterns of local adaptation and phenotypic selection in this species r equires that they are examined in the light of regional processes rela ted to seed dispersal and relative abundance in open and woodland habi tats.