Figure 1. Diagrammatic representation of complex life cycles involving alternation of asexual and sexual reproduction. (a) Multiplication of embryos (polyembryony); (b) replication of larvae; (c) fission or budding; (d) parthenogenesis – each parthenogenic diploid female may produce further diploid eggs as shown. Eventually, in a sexual phase of the life cycle (not shown), meiosis may occur so that haploid eggs and sperm are produced and sexual reproduction involving the fusion of haploid gametes may occur.

Figure 2. Life cycle of the scyphosoan cnidarian Aurelia auretia, the common European jellyfish. Many medusae are derived from a single fertilized egg, but each has the same diploid genotype.

Figure 3. Life cycle of an aphid with primary and secondary host plants.

Figure 4. Representation of the trade-off between investment in male and female functions in relation to fitness benefit and the evolution of gonochorism and hermaphroditism. (a) Direct relationship between investment and benefit (no saturation). (b) Trade-off curve concave; maximum fitness (*) associated with investment of resources in either male or female function. Under these circumstances natural selection favours gonochorism. (c) Declining relationship between investment and benefit (saturation of the male or female gain curve). (d) Trade-off curve convex; maximum fitness (*) associated with allocation of some proportion of resources, q_f, to female function and some proportion, q_m, to male function. Under these conditions selection favours simultaneous hermaphroditism.

Figure 5. Simultaneous hermaphroditism in the earthworm Lumbricus. (a) The internal anatomy includes male and female reproductive organs, ovaries and testes, as well as seminal vesicles for the storage of ‘self sperm’ and spermatheca for storage of a partner's sperm. To engage in mating behaviour, the worms leave the burrows in the soil to find a mate. (b) During the copulatory behaviour, mutual exchange of sperm occurs and sperm moves from the seminal vesicles of the donor worm to the spermatheca of the partner. (c) After mating the worms can fertilize their eggs with the stored sperm of a partner. The eggs are released from the oviducts and move back to the clitellum, which will form a protective covering containing albumen to sustain the developing worms. (d) After the cocoon has been secreted, it moves forwards and, as it passes over the spermatheca, spermatozoa are released to fertilize the eggs. (e) The cocoon is shed over the head of the earthworm and forms a waterproof protective environment in which the eggs develop into young worms.