|In the Alpha, it was adjustable with Attribute Points. In the Beta, Evasion raised with each level. It's hypothesized that the amount the stat increases with each level depends on the discipline being used. Currently, only Blue Mage is able to directly modify their Evasion by using the ability Toad Oil, which increases evasion by 20%.
Historically, in another Square Enix game (Final Fantasy XI) evasion is a stat that is compared to an enemy attacker's accuracy to determine the chance that a character has of avoiding an attack. Evasion in that game is directly affected by a character's evasion skill and AGI. For every two points of AGI a character has, their evasion is raised by one point. There is currently no evidence that such a thing occurs in Final Fantasy XIV as well, however.