Evolution of the seed habit : Is niche construction a missing component?

Evolution of land plants is one of the major transitions in the history of life on Earth. In this process, evolution of seeds constitutes one of the key events, liberating plants from dependence of free external water for fertilization, thus promoting colonization of dry environments and the build-up of terrestrial ecosystems. Previous explanations of evolution of seeds from heterosporous predecessors have been based on a framework of kin and sexual selection theory. Here I suggest that that niche construction is a missing component in these explanations. During colonization of increasingly drier habitats, the heterosporous life cycle was subjected to strong gradients in water availability. The ancestral condition of separate niches of the sporophyte and female gametophyte generations changed into a situation where the sporophyte generation provided the only means by which female gametophytes could develop, in effect ‘constructing’ the recruitment niche for the female gametophyte, attached to the sporophyte. Selection favored modifications in the developmental program, altering the relative timing of fertilization and dispersal. Kin and sexual selection processes could then play out in the context of a plant life cycle where fertilization preceded dispersal, eventually forming the seed habit. Niche construction by the sporophyte removed the ecological independence of the two generations; the sporophyte provided the female gametophyte with a recruitment niche, transforming the biphasic life cycle into a unitary life cycle, and enabled an expansion of the ecological niche zone for land plants, eventually leading to a vegetation covering most parts of the land mass.