In biology, alternation of a sexual phase ( gametophyte ) and a nonsexual phase ( sporophyte ) in the life cycle of an organism.
The two phases, or generations, are often distinct in structure and sometimes in chromosome makeup. Alternation of generations is common in algae , fungi (see fungus ), moss es, fern s, and seed plant s. The character and extent of the two phases vary greatly among different groups of plants and algae. During the course of evolution, the gametophyte stage has been progressively reduced. Thus in higher ( vascular ) plants, the sporophyte is the dominant phase; in more primitive, nonvascular plants the gametophyte is dominant. Among animals, many invertebrates (e.g., protozoan s, jellyfish , flatworm s) have an alternation of sexual and asexual generations.