There already are:
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.
- Hermaphroditic species obviously are all the same gender. But for your question, I assume them to be two genders in one, rather than one gender. Otherwise, hermaphrodites are a viable answer to your question. There can be issues with this.
- Some species are capable of transforming from one gender to another. This is technically called being sequential hermaphroditic. For some fish, if the dominant male dies, the alpha female can transform into a male. There would not need to be a genetically separate gender for this, but there would still be two genders (so I assumed this didn't meet your needs). But there could be prolonged periods where there were no males, and select females transform, mate, and (possibly) die afterwards. If this were in response to stressful conditions and combined with parthenogenesis, you would have a species that could rapidly expand in good times and still generate diversity when conditions got tough.