25.2: There are costs and benefits to having a partner: sexual versus asexual reproduction.

The term “reproduction,” the process by which new organisms are produced from existing organisms, usually conjures images of a male and a female, producing offspring together. And for most animals, this is how it’s done. But recall from Section 6-13 that there are two fundamentally different ways in which organisms can reproduce. While the vast majority of plants and animals reproduce sexually, prokaryotes and many plant and animal species reproduce asexually—and some can reproduce in both ways.

Sexual reproduction involves two individuals contributing genetic material to produce offspring (FIGURE 25-2). The genetic material is contained in gametes, the reproductive cells. The male gamete is called a sperm (or sperm cell) and the female gamete is called an egg or ovum (pl. ova). Recall from Section 6-10 that the cellular division process of meiosis produces gametes, cells that contain only half as many sets of chromosomes as other body cells (the somatic cells). When the male and female gametes fuse in fertilization, the full chromosome number is restored. Both the production of gametes and the combination of genetic material from two individuals tend to increase genetic diversity among offspring. We explore sexual reproduction in greater detail in the remainder of this chapter.

An important feature of sexual reproduction is that it leads to offspring that differ genetically from each other and from either parent (see Figure 6-28). This genetic diversity can be an evolutionary adaptation, increasing fitness in changing environments. If an environment is gradually changing, individuals producing diverse offspring increase the likelihood that one of their offspring will be suited, genetically, to the new environment. There are, however, disadvantages to sexual reproduction. The two main drawbacks are that (1) finding a partner and mating can be time-consuming and challenging, and (2) an individual contributes only half of the alleles that its offspring carry (FIGURE 25-3).

In contrast to sexual reproduction, asexual reproduction involves the production of offspring by a single individual without a contribution of genetic material from another individual. There are several types of asexual reproduction in animals, including parthenogenesis, budding, and fragmentation (FIGURE 25-4).

1. Parthenogenesis. In parthenogenesis, a female’s egg develops into a new organism without having to be fertilized by a sperm cell. Some species, including the desert grassland whiptail lizard, are exclusively asexual, and all of the individuals in the species are female. Other species can reproduce either asexually or sexually, depending on environmental conditions; parthenogenesis allows them to utilize resources—particularly food—as quickly as possible. For example, in the spring, when food is plentiful, aphids produce diploid maternal cells that develop into normal adults without fertilization. When food is limited, they produce haploid eggs that must be fertilized before development. Hammerhead sharks and, occasionally, turkeys also have the ability to reproduce by parthenogenesis (although turkeys resulting from asexual reproduction tend to be less healthy).

2. Budding. In budding, an offspring grows directly out of the body of the parent. Hydras, which are predatory cnidarians, reproduce by budding.

3. Fragmentation. In fragmentation, one individual breaks into multiple pieces, each of which develops into a fully functioning, independent individual. Fragmentation is seen among many species of flatworms, as well as some sea stars. Among some sea stars, for example, if even a tiny part of one arm breaks off, it can develop into a complete individual.

Asexual reproduction can be fast and easy, because it involves only a single individual. And if an organism’s environment is stable, it is beneficial for offspring to carry all of the genes that their parent carried. If an environment is changing, however, asexually reproducing organisms may be at a disadvantage.