Alternation of generations (also known as
metagenesis) is a term primarily used to describe the
life cycle of
plants (taken here to mean the
Archaeplastida). A multicellular
gametophyte, which is
haploid with
n chromosomes, alternates with a multicellular
sporophyte, which is
diploid with 2
n chromosomes, made up of
n pairs. A mature sporophyte produces
spores by
meiosis, a process which reduces the number of chromosomes to half, from 2
n to
n. Because meiosis is a key step in the alternation of generations, it is likely that meiosis has a fundamental adaptive function. The nature of this function is still unresolved (see
Meiosis), but the two main ideas are that meiosis is adaptive because it facilitates repair of DNA damage and/or that it generates genetic variation.