Sexual Reproduction

Sexual reproduction is built around the fusion of a male gamete and a female gamete to form a diploid zygote, which then develops into a completely new organism; because two different parental genomes combine in this fusion, sexual reproduction - unlike the single-parent asexual strategies covered earlier - is the source of genetic variation, the raw material evolution and adaptation depend on.

Animals show two broad forms of sexual reproduction: syngamy, more commonly called fertilization, in which two haploid gametes fuse directly to produce a diploid zygote, and conjugation, a temporary union between two individuals in which nuclear material is exchanged rather than gametes fused outright. The sections that follow work through the different kinds of syngamy recognised across the animal kingdom, the internal-versus-external classification of fertilization, conjugation itself, the phases every organism's reproductive life cycl …