Why Do Some Organisms Use Both Types of Reproduction?

Organisms that can reproduce both sexually and asexually gain a flexible toolkit for survival, one that lets them multiply rapidly when conditions are good and shuffle their genes when conditions get dangerous. This dual capacity, called facultative sex, shows up across an enormous range of life forms, from water fleas and aphids to sharks and flowering plants. The reasons behind it are not reducible to a single evolutionary pressure but instead reflect a tangle of parasite defense, reproductive insurance, environmental responsiveness, and long-term genomic maintenance.

The Steep Price of Sex

To understand why some organisms bother with both options, you first need to appreciate how expensive sexual reproduction actually is. An asexual female can devote all of her offspring to producing more females, each of which can reproduce immediately. A sexual female, by contrast, must produce males, who cannot themselves bear offspring. This means a sexual population grows at roughly half the rate of an otherwise identical asexual one. Biologists call this the “twofold cost of sex,” and experimental work in freshwater snails has confirmed it: when sexual and asexual snails were placed together, the observed shift in population frequencies was consistent with asexuals having about a twofold reproductive advantage.¹ That cost is so severe that, in theory, asexual lineages should rapidly outcompete sexual ones and drive them to extinction.²

Yet sex persists almost everywhere in the animal kingdom, and many lineages that could reproduce entirely without it still maintain sexual capability. That mismatch between theory and reality is one of the longest-running puzzles in evolutionary biology. The persistence of sexual females requires that sex confer some advantage, at least periodically, that offsets the cost of producing males. As it turns out, multiple advantages exist, and different organisms lean on different ones depending on their ecology.

Parasite Pressure and the Red Queen

The most celebrated explanation for why sex endures is the Red Queen hypothesis. The idea is that parasites and pathogens evolve to exploit the most common host genotypes. If a population is full of genetically identical clones, a well-adapted parasite can sweep through all of them. Sexual reproduction, by constantly generating new genetic combinations, produces rare genotypes that parasites have not yet evolved to attack.³

Some of the strongest evidence for this comes from New Zealand’s freshwater snails, where sexual and asexual individuals of the same species coexist. Researchers tracked infection rates by a sterilizing trematode parasite over five years at multiple lake sites. At most sites and in most years, the sexual snails were less infected than the asexual ones. At times, the frequency of uninfected sexual females was more than double that of uninfected asexual females, enough to offset the twofold cost of sex entirely.⁴ The implication is clear: in environments with aggressive, coevolving parasites, sex pays for itself by keeping the host population genetically one step ahead.

For an organism that uses both reproductive modes, this creates a logic for switching. When parasite pressure is low or absent, clonal reproduction lets a female maximize her output. When a well-adapted pathogen arrives and begins decimating common clones, sex becomes the better bet because it generates the rare genotypes most likely to survive. Maintaining the option to do both lets the organism respond to whichever threat is more pressing at the moment.

Reproductive Assurance When Mates Are Scarce

Parasite arms races are not the only thing keeping asexual reproduction in the toolkit. Sometimes the simpler problem is that there is nobody around to mate with. This is where reproductive assurance enters the picture: the ability to produce offspring without a partner functions as a safety net for populations that are small, isolated, or spread thinly across a landscape.

Research on mayflies offers a vivid example. These insects have notoriously short adult lifespans and limited ability to disperse, which means a female emerging from a stream may simply never encounter a male. In natural populations of one facultatively parthenogenetic mayfly species, sex ratios became female-biased only at low population densities, exactly the pattern you would expect if mate limitation were the primary pressure selecting for parthenogenesis.⁵ When males are abundant, the mayflies reproduce sexually and gain the genetic benefits. When males are rare, females switch to producing daughters on their own rather than failing to reproduce at all.

A similar logic applies to sharks and rays held in aquariums or living in fragmented ocean habitats. Several elasmobranch species have been documented reproducing by parthenogenesis in captivity, where males are absent.⁶ Researchers have speculated that this capacity may serve as an adaptive fallback to compensate for the absence of males, preventing a complete reproductive dead end once any stored sperm is used up or becomes unviable.⁷ In whiptail lizards, facultative parthenogenesis has been framed similarly: it could help bridge population bottlenecks when mate encounters are infrequent, though it comes with the trade-off of shrinking the gene pool.⁸

Environmental Cues That Flip the Switch

Many organisms do not simply stumble into one reproductive mode or the other. They actively switch between sex and asexual reproduction in response to environmental signals, often with remarkable precision.

Aphids are the textbook case. During spring and summer, when resources are abundant and days are long, aphids reproduce clonally. Females give birth to live daughters that are already pregnant, enabling explosive population growth across just a few weeks. As autumn approaches and day length shortens, the aphids perceive this change in photoperiod and begin producing sexual males and egg-laying females instead.⁹ The sexual generation mates and lays cold-resistant eggs that can survive the winter, something the fragile clonal offspring cannot do.¹⁰ This seasonal toggling between reproductive modes allows a single aphid genotype to produce both sexual and asexual forms depending on what the environment demands.¹¹

Water fleas in the genus Daphnia follow a related pattern, but with a different trigger. These tiny crustaceans normally reproduce asexually, churning out genetically identical daughters. When conditions deteriorate, they switch to sexual reproduction and produce hardy resting eggs encased in protective shells. Researchers found that what food Daphnia were eating could directly trigger this switch: animals fed one type of algae continued reproducing asexually under crowded conditions, while those fed a nutritionally poorer algae species began producing resting eggs. The trigger turned out to be a dietary deficiency in the lower-quality food, essentially a chemical signal that conditions were worsening.¹² The sexual eggs produced through this switch can lie dormant in lake sediment for years, hatching only when conditions improve.

Hedging Bets in Unpredictable Environments

The aphid and Daphnia examples involve fairly reliable seasonal cues, but what about environments where the future is genuinely unpredictable? Here, maintaining both reproductive modes functions as a form of bet-hedging: rather than committing fully to one strategy, an organism spreads its risk across multiple approaches.

Research on Daphnia populations living in temporary ponds found a telling correlation. Populations inhabiting more unpredictable ponds, where the growing season might end abruptly, showed a higher propensity for sexual reproduction compared to populations in more stable ponds.¹³ The interpretation is that investing in sex and the durable resting eggs it produces is a conservative strategy: you sacrifice some short-term reproductive output for insurance against the pond drying up before your clonal daughters can establish themselves. Modeling work confirms that organisms maximizing long-term fitness over many generations, rather than short-term output in any single season, benefit from reduced fitness variance when they time their sexual reproduction strategically.¹⁴

This helps explain a pattern seen across many organisms that toggle between reproductive modes. The shift to sex often coincides with environmental stress or uncertainty, not because sex is inherently “better,” but because the genetically diverse, often dormant offspring it produces are more likely to include some individuals suited to whatever unpredictable conditions lie ahead.

Stress-Driven Reproduction in Fungi

Fungi offer a particularly clear illustration of how environmental stress can redirect reproductive development. Many fungal species grow vegetatively when nutrients are plentiful, spreading through their substrate as a network of filaments. When conditions deteriorate, especially under nutrient limitation, they actively switch toward producing spores, either asexually or through sexual reproduction.¹⁵

The logic here mirrors the Daphnia case in broad strokes: grow and spread when resources allow, then shift to a reproductive mode that generates survival structures when things get tough. Sexual spores in fungi are typically more durable than their asexual counterparts and carry novel genetic combinations that may be better suited to the changed environment. Asexual spores, meanwhile, allow rapid local dispersal without the metabolic expense of finding a mating partner. By keeping both options available and deploying them based on real-time environmental feedback, fungi can respond nimbly to everything from drought to nutrient depletion to competition with other microbes.

Vertebrates That Can Go It Alone

For a long time, the ability to switch between sexual and asexual reproduction was considered a feature of invertebrates and plants. Vertebrate reproduction seemed too complex, too reliant on fertilization. That view has been overturned in recent decades by discoveries of facultative parthenogenesis in birds, reptiles, and fish.

Pitviper snakes in North America provided some of the first evidence from wild vertebrates. Researchers genotyped pregnant female copperheads and cottonmouths and their offspring, finding litters that had no genetic contribution from any male, the first documented cases of facultative parthenogenesis in wild-caught vertebrate mothers.¹⁶ These females had access to males, so the switch to parthenogenesis was not simply a last resort driven by isolation. The reasons are still debated, but the finding confirmed that even vertebrates with fully functional sexual reproduction can occasionally bypass it.

Among sharks and rays, most documented cases of parthenogenesis come from captive animals, where long-term observation is easier. A smooth-hound shark in an Italian aquarium produced pups without ever having contact with a male, and the births recurred over multiple years, suggesting it was not a fluke.⁶ Whether facultative parthenogenesis occurs regularly in wild shark populations remains an open question, but the capacity clearly exists across multiple elasmobranch families.

Jellyfish and the Alternation of Body Plans

Not all organisms that use both reproductive modes make a seasonal switch within a single body form. Some alternate between entirely different life stages, each with its own reproductive strategy. Moon jellyfish illustrate this vividly. The familiar bell-shaped medusa that drifts through the ocean reproduces sexually, releasing eggs and sperm into the water. The resulting larvae settle onto a surface and develop into tiny polyps, a sedentary stage that looks nothing like the adult jellyfish. These polyps can reproduce asexually by budding off clones of themselves, and periodically they undergo a process called strobilation, in which a single polyp segments into a stack of disk-shaped juvenile jellyfish that detach and swim away.¹⁷

In this system, asexual reproduction lets the polyp stage persist and multiply on the seafloor for months or even years, building up numbers without the need for a mate. Sexual reproduction in the medusa stage generates genetic diversity and produces larvae that can disperse to new habitats. The two modes are not alternatives so much as sequential parts of a single life cycle, each tuned to a different ecological role.

The Genomic Cost of Doing Without Sex

Asexual reproduction is fast and efficient, but it comes with a hidden genomic price. Without the gene shuffling that sex provides, harmful mutations can accumulate over generations with no way to purge them. Over time, this can drag down the fitness of an asexual lineage, a process sometimes called Muller’s ratchet. Sex counteracts this by recombining chromosomes, allowing natural selection to act more effectively on both beneficial and harmful mutations.

In plants that can reproduce both ways, asexual lineages may buffer the effects of mutation accumulation through extra copies of their genome, but they can also purge the accumulated load by reverting to occasional sexual reproduction.¹⁸ The option to do both gives these species an escape valve: they can enjoy the short-term advantages of cloning while periodically cleaning up their genomes through sex.

But the form that asexual reproduction takes matters a great deal. Some organisms reproduce through a process called automixis, in which eggs undergo a modified version of the cell divisions normally associated with sex, then restore a full set of chromosomes without fertilization. Depending on exactly how that restoration works, offspring can end up with significantly less genetic variation than their mother. Research on Australian stick insects showed that their particular form of automixis resulted in substantial loss of genetic diversity at marker genes, which could translate into reduced fitness at genes that matter for survival.¹⁹ Automictic reproduction, despite being technically “asexual,” can expose hidden harmful gene variants that would normally be masked by a second copy from a mate.²⁰

The Role of Bacterial Hijackers

Not every organism that reproduces asexually chose that path, evolutionarily speaking. In many arthropods, the transition to parthenogenesis was not driven by the organism’s own adaptive needs but by intracellular bacteria that manipulate host reproduction for their own benefit. A large-scale survey of arthropod species found that out of 139 species where the cause of parthenogenesis had been investigated, endosymbiotic bacteria were suggested as the cause in about three-quarters of cases, and convincingly demonstrated in roughly 42% of those examined.²¹ Bacteria like Wolbachia can alter host reproduction in ways that favor their own transmission, including inducing parthenogenesis, killing male embryos, or converting genetic males into functional females. In species where these infections are not fixed in the population, some lineages may reproduce asexually while others retain sexual reproduction, creating a patchwork of reproductive strategies within a single species.

When Both Options Backfire

It would be tempting to assume that having two reproductive modes is always better than one. But recent work on parasitoid wasps suggests the combination does not always deliver the best of both worlds. In the wasp Lysiphlebus fabarum, asexual lineages were tested to see whether they could also reproduce sexually when given the opportunity. Some could: females from all seven asexual lines tested mated with males and produced sexually generated offspring. But these facultatively sexual females did not gain the short-term output advantages of parthenogenesis or the genetic diversity advantages of sex. Instead, they suffered high rates of reproductive failure associated with sex and lower productivity associated with their asexual reproduction. The researcher described it bluntly: they got the worst of both worlds.²²

This finding is a useful corrective to overly tidy evolutionary narratives. Maintaining the machinery for two reproductive modes is not free. The cellular mechanisms for sexual reproduction and parthenogenesis can interfere with each other, and an organism that does both may do neither as well as a specialist. Facultative sex is most likely to be favored where the environmental pressures that reward each mode are strong, predictable, and temporally separated. When conditions do not clearly favor one mode over the other, organisms that try to keep both options open may pay a real cost for their flexibility.

Why One Explanation Is Never Enough

If you step back from all of these individual cases, a pattern emerges: no single factor explains why organisms maintain dual reproductive strategies. Parasite defense matters in the New Zealand snails. Reproductive assurance matters for the mayflies. Seasonal dormancy matters for Daphnia. Genomic maintenance matters for plants. Environmental bet-hedging matters for pond-dwelling crustaceans. Bacterial manipulation matters for many arthropods. And in some systems, the advantages are slim enough that the whole arrangement might be on its way out.

What unifies these explanations is that sex and asexual reproduction are good at different things, and environments rarely stay stable enough for one strategy to dominate forever. Asexual reproduction excels at rapid colonization and exploitation of favorable conditions. Sex excels at generating the variation needed to cope with parasites, changing climates, and the slow erosion of genomic quality. An organism that can deploy both is, in effect, carrying two tools instead of one. The cost of maintaining that second tool is real, and some organisms seem to pay it poorly. But across the breadth of life, the pattern is persistent: wherever environments fluctuate, mates are sometimes scarce, parasites are aggressive, or winters need surviving, the organisms that can switch between reproductive modes tend to stick around.