Rough-skinned newts produce a strong toxin called TTX as a defense mechanism against predators. Garter snakes can tolerate a very limited amount of the toxin. However, several populations of garter snakes have recently shown

an increased tolerance for TTX. The different populations of snakes have independently acquired a gene variation

that allows them to better block the effects of the toxin.

Acquired resistance

Explanation:

The phenomenon is called acquired resistance because the organism did not have tolerance before and its previous species is susceptible to that toxic substances, it acquire this tolerance recently due to changes in the genetic makeup of that organism. So those characteristics which can be acquire due to outer environment so that phenomenon is known as acquired resistance.