I'm building a world for a novel and would like to have a single-sex, sapient species. The world is based on the general physics and scientific nature of our world but where more than one sapient species has evolved. What could be the major occurrences that would bring about an evolutionary change from a 2 sexed to single sexed species in such a setting? Preferably in a reptilian creature.
Thanks for any thoughts.
Evolution through the intermediary of partial sexual reproduction
You specifically ask for an evolutionary reason - rather than asking about a species that was always single-gendered.
This has precedents in nature, for example with the ubiquitous aphid Myzus persicae
This small insect has an option of reproducing sexually or asexually.
In the warmer months, and throughout the year in warmer climates, the ... aphid reproduces asexually Myzus persicae From Wikipedia, the free encyclopedia
Note that aphids generally have a very biased sex ratio - roughly 1 male to 20 females.
If you really want to take a deep dive into the subject then here is where to look!
The whiptail lizard family (to name just one) has several groups where only females exist. They do not have sexual reproduction, but instead reproduce by parthenogenesis. Other species (like the Komodo dragon) are capable of parthenogenesis if no males are available.
There are several ways these species can occur. Some cases involve a hybrid lizard (similar to a mule) caused when two related species mate. The female can still ovulate and produce viable offspring by parthenogenesis, but don't have any males (probably because male hybrids are non-viable). In other cases, conditions or disease may have wiped out all the males.
Some of these species engage in mating behaviors with other females of the species, and while there is not genetic exchange, mating behaviors appear to increase fertility (possibly by altering hormone levels). In other cases, the females are triggered to ovulate by mating with members of other related reptile species. Again, no genetic exchange takes place. Sometimes, they just don't have sex.
Long-term, these species are considered a dead end because they lack a method of exchanging genetic variation. In the short term, however, these lizards can exploit resources and increase their numbers rapidly because every member of the species is female and all members can produce offspring (as opposed to half of the members being male and not directly producing offspring). They don't need to engage in risky, energetic or competitive mating behaviors to produce offspring either.
Gender and sexual reproduction aren't necessarily linked.
Evolution is favoured by sexual reproduction because genetic character mixing is faster, but you could have your creatures be eterohermaphrodites (they have both sets of sexual organs and can inseminate one another, cannot inseminate themselves).
Extreme Dimorphism
Your species has two options for reproduction.
Female makes unfertilised eggs. These only contain genetic info from the mother. Perhaps they are her clones. Perhaps she scrambles the genome to activate and deactivate different bits of DNA and get different young. In any case the offspring are all female.
Female makes eggs which are fertilized by a male. These have genetic info from both parents. The young can be male or female.
Style 2 is historically better because it leads to more genetic variation and faster evolution. It also creates both males and females. But in the modern era people only do Style 1. This leads to males dying out.
The reason is males are much smaller and stupider than the females. It is debatable whether they are sapient at all.
In the hunter-gatherer era it was useful to have a lot of males around. The reason for this will depend on the exact biology and I leave it to your imagination. They eat less than a female -- perhaps they act as cheap lookouts. Perhaps they can fly? Perhaps they have more acute senses?
In any case, in the civilized era, males are no longer needed and people see them as a burden. A male in your family is too stupid to work a job, or leave the house on its own. But it is smart enough to tear up the furniture unless you walk it four hours a day.
This leads to Style 1 becoming the norm and males dying out.
They are hermaphrodites
They reproduce sexually. A scenario of a meeting of equal and equivalent partners would have each be both male and female, each fathering the child that the other carries. Under some circumstances an individual could be both father and mother: still sexual because it would use meiotically produced sperm and eggs, but one individual.
One could spin this in a way where at some prior state there would be a single male that would compete for the right to be a father for multiple offspring. The society has progressed to the point that such a system is considered a primitive throwback.
Have two genetic contributors allows for more genetic variety in a short time. Asexual production produces what are basically clones, identical copies of the single parent. there will be some evolution due to mutations, but at a much slower rate than sexual production.
An advanced specie will develop the ability to alter their genes. This will initially be done avoid genetic defects, but will quickly become produce enhanced off spring. This will result in even faster evolution than sexual production.
Selecting sex based on genes will be an easy change. Since it is far easier for Females to give birth, females will be preferred and number of male children will plummet and possibly disappear. if artificial wombs are developed this may change instead to favor males instead of females.
While the choice of male or female is up for debate, the most likely outcome will be a society of all one or the other.
The OP asked about "species". Most answers refer to animal species, and the answers contain interesting examples. Plants are also characterized by species, and are also sexually, asexually, and "self-sexually" reproducing organisms.
Many plants have flowers with both male and female elements, permitting self-fertilization while also permitting other-fertilization. One can ascribe utility and "fitness" in that if the population density is low than reproduction will occur through self-fertilization, but if population density is high other-fertilization will be more likely.
Further speculation suggests that if the population density is low, the incidence of predators and pathogens specific to the plant species is also low, so the loss of sexual reproduction is less of a liability. When the density is high, there is a greater chance of an opportunistic specialist foe, and the benefits of sexual reproduction are higher.
Many reptiles have evolved the ability for the females to, after mating, store their partners' sperm for use later. From there, some females may develop the ability to produce their own sperm in this system, becoming able to reproduce asexually, obviating some of the need for males, and then later develop the ability to impregnate each other, obviating the need for males entirely.
In real life temperature effects the gender of some species. If it's hotter there are more males, if it's cooler there are more females, or vice versa.
It could be that some form of environmental contamination, such as pollution or a disease interferes with this process, causing species to have attributes from both sexes, rather than one. In essence, children could be born as fully functional hermaphrodites.
