Summary

Chapter Summary

Reproduction is the process that allows a species to persist across generations, and organisms achieve it through two broad strategies. Asexual reproduction, seen typically in simply organised organisms such as fungi, algae and many invertebrates, involves no gamete fusion and produces offspring — clones — that are genetically identical to the single parent; its common mechanisms include cell division/binary fission in unicellular and prokaryotic organisms, spore-type structures such as zoospores and conidia, budding and gemmule formation in simple animals, and vegetative propagation (via runners, rhizomes, suckers, tubers and offsets) in many flowering plants.

Sexual reproduction, the near-universal strategy in higher animals, is slower and more complex because it depends on the formation and fusion of gametes from (typically) two parents, producing genetically varied offspring. Its events fall into three stages: pre-fertilisation (gametogenesis, which produces haploid gametes by mitosis in haploid organisms or by meiosis in diploid organisms, and gamete transfer, which brings the gametes together — directly in bisexual organisms, via pollination in flowering plants, or via mating in unisexual animals); fertilisation (syngamy), which may occur externally in water or internally within the body, and which produces the diploid zygote; and post-fertilisation events, in which the zygote — whose subsequent fate depends on the organism's life cycle — undergoes embryogenesis (mitotic division plus cell differentiation) to build the embryo. Animals are described as oviparous or viviparous depending on where this development is completed, wi …