Vertebrate Reproduction: An Outline
II. General features of vertebrate reproduction.
C. Polygamy versus monogamy.
D. Oviparity, ovoviviparity, and viviparity.
The preservation of species requires reproduction. Among the vertebrates, a diverse array of reproductive mechanisms have evolved. These include both sexual and asexual fertilization events. Moreover, offspring may be produced by oviparous, ovoviviparous, and viviparous processes. The different vertebrate classes additionally exhibit considerable variation in the amount of care which is provided to the young. Relatively complex mechanisms for nurturing offspring occur in the Aves and Mammalia classes.
Most vertebrate species have separate sexes. Genetic variation is the "raw material" of evolution (6:593). Sexual reproduction ensures the combination of different genotypes. In male vertebrates, the testes produce spermatozoa; in females, the ovaries produce eggs. For the most part, both male and female vertebrate gonads are paired organs. The sperm are flagellated and motile. They "swim" in fluids produced by specialized glands, and eventually leave the body via the spermiducts (1:19). In contrast, vertebrate eggs are generally surrounded by follicle-forming nurse cells. During ovulation, the follicle ruptures and the egg is transported through the oviducts.
Sexual reproduction involves the combination of sperm and eggs. The actual binding of a sperm cell to an egg is species-specific: In practically all organisms, the egg's sperm receptors only recognize specific sperm cells (10:1988). Fertilization occurs when the nuclei of an egg and a sperm cell fuse to form a zygote. The zygote then undergoes a series of cell divisions and differentiations, and ultimately forms a new individual (1:19).
Most vertebrates reproduce periodically. Therefore, individual organisms typically alternate between periods of reprod...