Some species of lizards do not have males. But reproduction still occurs producing all-female species.
Parthenogenesis is a Greek word which means “virgin birth”. It is the process of reproduction of viable offspring from virgin females without sex. This virgin birth or asexual reproduction will result in female offspring genetically identical to their mothers; except for rare genetic mutation, while sexual reproduction produces offspring not at all like their parents.
Many species can reproduce without engaging in sex. Lizards of the genus Cnemidophorus do not even have males in some species. It was once thought that females in these species had stored sperm for a long time giving the impression that they did not sexually reproduce. But when they are put in isolation, females can have clutches of eggs that weigh one third of its body weight without the presence of males. They were also thought to have both sexual organs in one individual but dissection of an asexual lizard proved otherwise.
An even more bizarre fact is that fertility in these lizards increase when they engage in what is called pseudocopulation, which can take place either in isolation or in nature. During pseudocopulation, one female takes the role of a male and mounts another female. This act produces a larger clutch of eggs comparing to asexual female that does not receive this kind of mounting.
This pseudosexual behaviour is correlated with the cycle of reproductive hormones. When a female produces eggs, the eggs on the ovary produce the hormone estrogen. During the considerable high level of estrogen, a female lizard will become more feminine-like and receptive to copulation – in the all-female species, by females and in the bisexual species, by males. After ovulation, the level of estrogen will drop and a high level of progesterone takes place. In many vertebrates, progesterone is often metabolized into the hormone testosterone. In this case, the high level of progesterone conditions the female to act male-like and have the urge to mount another female.
The asexual Cnemidophorus species lineages are thought to start when a male from a different species fell in love or confused a female from another species for its own and had an interspecific copulation. Their offspring will consist of at least one asexual daughter that will go on to reproduce more asexual offspring of her own.
There is however a downside to this parthenogenetic lifestyle. Sexual reproducers will get variable offspring that are might be more able to deal with environmental changes or development of new diseases. Even if one offspring is considerably weak, another one or two might survive having probably a combination of stronger traits from the parents. Asexual reproduction produces replicated genes that provide no means of eliminating mutational errors except through extinction of lineages. Inevitably, a low level of adaptability will not be in favour to the survival of asexual species.