XIII EVOLUTION OF SEXUAL REPRODUCTION
From the viewpoint of evolutionary biology, two biological phenomena are apparently most interesting: the tendency of most organisms to form distinct species, and sexual reproduction.Our conception of these two phenomena has undergone a great many changes in recent years.New experimental facts have been obtained and, in particular, a number of hypotheses have been formulated to explain the emergence of the species and also the emergence of sexual reproduction.Nonetheless, this subject is far from a closed matter and the opinions of experts frequently differ diametrically, even on the most basic subjects.
This chapter will be concerned with the evolutionary importance and mechanism of the emergence of sexual reproduction and the processes that provide for the maintenance of sexual reproduction in competition with asexual reproduction.Here, we will be concerned not only with mutual competition between sexually reproducing and asexually reproducing species, but also mutual competition of sexually reproducing and asexually reproducing individuals within a single population.The aspect of the emergence of species will be discussed in Chapters IXX and XX; here, this subject will be considered only in discussion of hypotheses that place the emergence of species in direct connection with the emergence of sexual reproduction.
Where not stated otherwise, the term sexual reproduction as used in this chapter will mean amphimixis, i.e. alternation of reduction division of meiosis with renewal of diploidy of the zygotes, through syngamy, i.e. combination of the nuclear material of two haploid sex cells, usually derived from two individuals of the same species.Automixis, i.e. reproduction encompassing renewal of diploidy by combination of two haploid products of the same meiosis, similarly to apomixis, i.e. reproduction not encompassing meiosis, will be considered to constitute asexual reproduction.A number of authors place the borderline between sexual and asexual reproduction elsewhere; however, the above classification predominates in the evolutionary literature.