For population biological research, a top-down approach to unravel the internal mechanisms underlying adaptations to the outer environment seems more promising. Adaptation is usually not confined to a single trait, but often involves suits of traits. Then, questions arise how coherent adaptive complexes are brought about physiologically and developmentally and how selection can modify these internal mechanisms. The environment of organisms varies in time and space. Plants are sessile during a large part of their life cycle and even dispersal is passive from the plant's point of view.
Plants simply have to cope with environmental changes as they come.
Environmental factors set limits on the distribution of plant species and act as selective factors to which plants adapt. Plants may also depend on plasticity to adapt to varying circumstances. Their reaction norm is subject to selection and, depending on circumstances, plastic generalist plants or specialists, that are adapted to a smaller range of environmental conditions, may evolve. Biotic interactions are special in that change in one species may affect other species. Good examples of coevolution are known, but in the many cases the relations are less clear.