This is a tricky issue, because second and third conditionals are in the subjunctive form. The subjunctive form for all English verbs is the same as the past tense (this rule is followed in several languages), except for "to be", which becomes "were". As a verb, "to be" tends to ignore many rules which other verbs follow. That's just how it rolls.
As to the growing use of "was" for the subjunctive form, speakers have basically become lazy and thoughtless.
This is my own solution to the problem (criticisms welcome):
- when you admit that the "if" part is completely fictional and unreal, use "were".
- if you consider the "if" part to be a real possibility, use "was".
An example: "If I were Noah, I would simply take a pair of each species onto the ark. This way, if Darwin was right, each represented species would evolve into a whole new family of creatures."