Successful plant colonizers are characterized by the attributes of weediness but exhibit a variety of life-history traits and genetic systems. Colonizing success would depend on the ability of exploiting niches in competition with earlier inhabitants. Within-population genetic variation plays a role in adaptation to new habitats. An experiment with hybrid derivatives from a cross between wild and cultivated soybean species proved that the ability to establish seedlings in semi-natural conditions was genetically controlled, and was loosely correlated with various traits. A wild rice species, Oryza perennis, shows perennial-annual continuum among populations. The populations markedly differed in life-history traits and niche dimension, as conditioned by seasonal water regime and man's disturbance of the habitats. Intra-populational differentiation was also observed. The data suggested that various life-history traits are selected as a set.