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In my book series, there are several species of dragons, the common ancestor of which ultimately evolved from pterosaurs. One of these dragon species, the Arturian dragon, however, is the only species of dragon that reproduces asexually. It does this by vomiting up eggs whenever prey density is high and dragon density is low (it has no conscious control over this however). How would I evolutionarily justify a pterosaur descendant switching to asexual reproduction all of a sudden?

Other info to note about Arturian dragons:

  • Anywhere from 14-18 feet long (3-6 feet of which is the tail)

  • Anywhere from 900-1,300 lbs in weight

  • Lives in forests, grasslands, and mountains

  • Photoshop render I made of one (this one is a baby but the body proportions scale up regardless): enter image description here

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Lots of vertebrates reproduce asexually, with this being the preferred or only form of reproduction for many lizard species. You can find out more about this on the Wikipedia article about parthenogenesis:

Most reptiles of the squamatan order (lizards and snakes) reproduce sexually, but parthenogenesis has been observed to occur naturally in certain species of whiptails, some geckos, rock lizards, Komodo dragons and snakes. Some of these like the mourning gecko Lepidodactylus lugubris, Indo-Pacific house gecko Hemidactylus garnotii, the hybrid whiptails Cnemidophorus, Caucasian rock lizards Darevskia, and the brahminy blindsnake, Indotyphlops braminus are unisexual and obligately parthenogenetic.

As for how that came to be:

Parthenogenesis has been studied extensively in the New Mexico whiptail in the genus Aspidoscelis of which 15 species reproduce exclusively by parthenogenesis. These lizards live in the dry and sometimes harsh climate of the southwestern United States and northern Mexico. All these asexual species appear to have arisen through the hybridization of two or three of the sexual species in the genus leading to polyploid individuals. The mechanism by which the mixing of chromosomes from two or three species can lead to parthenogenetic reproduction is unknown. Recently, a hybrid parthenogenetic whiptail lizard was bred in the laboratory from a cross between an asexual and a sexual whiptail. Because multiple hybridization events can occur, individual parthenogenetic whiptail species can consist of multiple independent asexual lineages. Within lineages, there is very little genetic diversity, but different lineages may have quite different genotypes.

If your dragons only reproduce asexually when their population is low and prey is abundant, switching to sexual reproduction in other situations, then the mechanism above might not be adequate. One hypothesis I have for you is that females are able to produce both haploid and diploid eggs, but this is controlled by hormones and the trigger is the surplus in food combined with lack of contact with members of the same species. Asexual reproduction may always happen, but it will be rare (say, less than a birth in every 10,000) if there are many available mates or little available food.

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    $\begingroup$ Do the readers know that the rest of the dragon species reproduce sexually? Does it matter to the story? This is the reason that I try not to give readers too many details of the worlds I've built, so that I can change some of the underlying details without confusing the readers. $\endgroup$
    – NomadMaker
    Commented Oct 1, 2020 at 1:59
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It does this by vomiting up eggs whenever prey density is high and dragon density is low

The lowest dragon density is 1 per territorial area. When there are 2 you are practically halving the resources pro capite, with further reduction once the hatchlings are born.

Thus it is convenient, resource wise, to not have to involve another dragon in the business of reproduction.

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  • $\begingroup$ Wild cats have the same issue, and rather than reproduce asexually, each male’s territory simply overlaps 3-5 females’. $\endgroup$
    – StephenS
    Commented Sep 30, 2020 at 19:18
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The common ancestor of almost all life is basically a single celled critter that divided and divided. Basically a genetic machine, theorized to be a self replicating molecule that grew from a chemical soup of common materials.

One can't really go however from a creature that lays eggs after being inseminated to 'budding', you'd have to say they were around billions of years longer with say the amphibian tendency to sometimes change their gender, and maybe they fertilized their eggs like sea creatures :P maybe they lay their eggs in water so there is a soft membrane for the spermatoa to penetrate :P

Otherwise you could always say they're magic :P

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