 Some mechanisms that generate variation in DNA are invoked in particular conditions (e.g., stress conditions), and produce variation that is non-random in location and/or pattern (e.g., Levy &amp; Feldman 2004; reviewed in Jablonka &amp; Lamb 2005: chap. 3). However, it is widely perceived that most genetic variations are the result of non-directed processes that are not responsive to specific inducing conditions.