"Prerequisite: Any natural weapon.
Ability Score: None
Benefit: The evolutionist chooses one of his natural weapons and increases its threat range or its critical multiplier by one. No single natural weapon can have a critical range higher than 18-20 or a multiplier higher than x4. The effects of this mutation do not stack with the Keen enchantment or the Improved Critical feat.
Further Mutations: -"

I'm confused is this capable of being taken multiple times or not? And if so on the same weapon? For example base natural attacks are generally 20/x2 could you say take it 4 times and end up with 18-20/x4?