The costs of sex

While sexual reproduction plays a crucial role in increasing genetic diversity, it comes with significant costs that can impact the survival and reproductive success of organisms. Understanding these costs helps explain why sexual reproduction is not universal among all species. This section explores the major costs associated with sex, including the cost-of-males (two-fold cost of sex) and the cost of meiosis (genome dilution), as well as additional challenges like competition for mates, reduced reproductive opportunities, and the risk of sexually transmitted diseases.

Ultimately, these costs raise two fundamental evolutionary questions: how did sexual reproduction evolve and why does it persist? We will start by outlining the costs in more detail below, and tackle the how and why in subsequent chapters.

The Two-Fold Cost of Sex (Cost-of-Males)

The two-fold cost of sex, also known as the cost-of-males, is a significant disadvantage of sexual reproduction first articulated by evolutionary biologist John Maynard Smith. He reasoned that in a sexually reproducing population—where there are distinct male and female individuals—a mutation causing asexual reproduction should spread rapidly (Figure 1). This is because asexual individuals produce only individuals capable of reproducing independently, leading to a higher per-capita birth rate. In contrast, sexual females invest resources in producing both male and female offspring, but only female individuals contribute directly to the next generation. Thus, the reproductive potential of a sexually reproducing population is effectively halved compared to an asexual one, as males do not directly produce offspring.

Empirical studies support Maynard Smith’s model. For example, research on mixed populations of sexual and asexual snails in large outdoor environments found that asexual snails increased in frequency at a rate consistent with the predicted two-fold cost of sex. This aligns with the assumption that all else is equal between sexual and asexual females, including survival and reproductive rates.

The Cost of Meiosis (Genome Dilution)

Another major cost of sexual reproduction is the cost of meiosis, also known as genome dilution. This refers to the reduction in an individual’s genetic contribution to the next generation due to the mixing of genes during sexual reproduction.

During sexual reproduction, meiosis reduces the genetic material by half, producing gametes (sperm and eggs) that contain only one set of chromosomes. When two gametes fuse during fertilization, the resulting offspring inherits half of its genetic material from each parent. Thus, an individual passes on only 50% of its genes to each offspring, compared to 100% in asexual reproduction. This dilution means that an individual’s genetic influence on future generations is lessened, which can be a disadvantage in terms of passing on advantageous alleles.

Additional Costs of Sex

Beyond the two fundamental costs of producing males and genome dilution, sexual reproduction introduces several other challenges that can affect an organism’s fitness. These include factors such as:

• Competing for Mates. Finding and attracting a mate requires significant time and energy. This can involve developing elaborate displays, calls, or behaviors to attract the opposite sex. Resources invested in mating efforts could otherwise be used for growth, survival, or caring for existing offspring.
• Reduced Reproductive Opportunities. Not all individuals are successful in finding mates due to competition, skewed sex ratios, or sparse populations. Mating often requires synchronization of reproductive cycles, which may not always align due to environmental factors or individual health. Time spent searching for a mate reduces time available for foraging, avoiding predators, and other vital activities.
• Risk of Sexually Transmitted Diseases. Close physical contact during mating provides an avenue for pathogens to spread between individuals. Sexually transmitted diseases can reduce an individual’s health, fertility, and lifespan, negatively affecting their reproductive success. High prevalence of diseases can impact overall population health and growth rates.

Despite the considerable costs associated with sexual reproduction—including the two-fold cost of producing males, genome dilution, and additional challenges like mate competition and disease transmission—sex remains a dominant mode of reproduction in many species. The persistence of sexual reproduction suggests that there must be some adaptive benefits. In the next chapter, we will explore these benefits in more detail.

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