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y hotter temperatures being one sex and cooler temperatures being the other. For others, the extreme temperatures are one sex and the middle temperature is the other. These temperature thresholds are known as Pattern I and Pattern II, respectively. The temperatures required for the specific sexes are known as the female promoting temperature and the male promoting temperature.[30] When the temperature stays near the threshold during the temperature sensitive period, the sex ratio is varied between the two sexes.[31] Some species set their temperature standards based on when an enzyme is created. These species that rely upon temperature for their sex determination do not have the SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature.[30] Some species such as the Nile Tilapia, Australian skink lizard, and Australian dragon lizard have sex determined by chromosomes, but this can later be switched by the temperature of incubation.[10] These species seem to be in a transitional state of evolution.
It is unknown how exactly temperature sex determination evolved.[32] It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season. However, at this stage it's mostly hypotheses.[32]
[edit] Other sex determination systemsWhile temperature dependent sex determination is relatively common, there are many other environmental systems. Some species, such as some snails, practice sex change: adults start out male, then become female (See also sex reversal). In tropical clown fish, the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses (Thalassoma bifasciatum) are the reverse. In the marine worm (Bonellia viridis), larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin. Some species, however, have no sex-determination system. Hermaphrodites include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and komodo dragon that can reproduce sexually and asexually, depending if a mate is available.[33] In the case of the boa constrictor, it can create both male and female offspring parthenogenetically.
In some arthropods, sex is determined by infection, as when bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.[34]
Other unusual systems (this section is still being researched):
Swordtail fish[10]
The Chironomus midge species
The Platypus has 10 sex chromosomes[18] but

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