The most recent morphological and genetic analysis indicates that if the Fisher were to be included within Martes, this would render the genus paraphyletic due to the Fisher coming out as basal to a Gulo + Martes clade. As such it is becoming more common to class this species as Pekania pennanti
This article on the research into the cladistics of the mustelids in general provides more information on this as well as other interesting topics.
