There have been three fundamental innovations in vascular plants that changed their reproductive biology: the appearance of heterospory, of the seed habit, and of flowering plant reproduction. Although the origins of seeds and flowering plants have received the most study, heterospory was the first major reproductive innovation to occur (by ~405 million years ago), and the only one that has evolved repeatedly (as many as 11 times).
In the broadest sense, heterospory refers to the production of different size classes of spores within a plant species. At least in living taxa, these size classes result in the restriction of sperm production to gametophytes produced by small spores (i.e. microspores, typically <100um in diameter), and of egg production to gametophytes produced by large spores (i.e. megaspores; typically defined as >200um in diameter). Seed plants then simply represent an extreme version of this strategy, in which microspores (pollen) are released but megaspores are retained on the parent plant (in ovules). The ancestral reproductive strategy, homospory, is in contrast characterized by the production of similarly sized spores that germinate into free-living gametophytes with both male and female functionality (although their exact patterns of sex expression can be complex). The appearance of heterospory in various plant lineages therefore represents a major shift in their reproductive biology, because it alters sex expression and spore size, which in turn influence spore dispersal abilities and patterns of resource allocation.