ABSTRACT
Not all animals reproduce sexually. For example, the cnidarian Hydra, an animal that lives as a solitary polyp and possesses a diffuse nerve net, can reproduce by the outgrowth of a body part that fragments itself to form a new individual. This process, called budding, occurs in other cnidarians (e.g., sea anemones), and in some sponges and worms. In other animals, including some species of insects, fish, frogs, and lizards, females are capable of producing viable offspring from unfertilized eggs, a process called parthenogenesis. These examples of asexual reproduction do not involve meiosis or fertilization and can lead to a fast reproductive rate. Cell division, or mitosis, is the basis for the proliferation of cells leading to an adult. Descendants are all clones, possessing the same genotype and constituting a genetically uniform population. Some species are known to be exclusively parthenogenetic, such as the whiptail lizard, Cnemidophorus
uniparens, a species consisting only of females (Crews & Moore, 1986). In fact, one-third of the forty-five lizard species of this genus are parthenogenetic! In some species, sexual and asexual modes of reproduction coexist in the same individual, which can switch from one modality to the other depending on environmental factors, such as the abundance of food resources. This exemplifies a trade-off between genetic diversity (a key consequence of sexual reproduction) and speed of reproduction (the hallmark of asexual reproduction), each of which maximizes fitness under different ecological conditions.
