Title: evolution - Why is one phase of a plant dominant (Alternation of generation)? - Biology Stack Exchange
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Why is one phase of a plant dominant (Alternation of generation)?
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Content: I would suspect this is due to the evolutionary advantages that are associated with the sporophyte life stage in land plants. Multiple advantages developed in the sporophyte stage of plants as they evolved from simple gametophyte-dominant bryophyte ancestors to the vast array of much more successful and dominant ferns, gymnosperms, and angiosperms. Shift from gametophyte-dominated life cycle (in bryophytes) to sporophyte-dominated life cycle. Source: Cengage Learning (2016)
Evolutionary developments that have occurred across almost 500 million years in land plants include the development of stomata, true leaves, pollen and seeds, and flowers and fruits. Each of these developments occurred in the sporophyte life stage. Each of these evolutionary traits had significant effect on the success of the various plant types that possess them, as can be inferred from the increasing success and dominance of increasingly more evolved plant Divisions. Each of these sporophyte-focused evolutionary traits increased the reproductive success of plants (therefore swaying increasing advantage toward plants possessing increasingly sporophyte-dominant life cycles) due to their ability to increase plant size, growth, and success living in arid (i.e., terrestrial) environments. stomata allow for increased gas exchange with more limited water loss (note: these were necessary with the development of cuticles in sporophytes of early bryophytes as well)
vascular tissue 2 allow plants to grow taller/bigger/more complex while still receiving water and nutrients from the soil
true leaves allow for greater energy capture and growth (and therefore increased competitive advantages)
pollen and seeds allow for greater dispersal and removes/limits the constraints of aridity on fertilization
flowers and fruits increase pollination and seed dispersal
Reducing the gametophyte stage to small cells within larger sporophyte plants allows this otherwise water-necessary process to occur in the dry conditions of living on land.