[2] In 1938, Arthur H. Howell elevated the subspecies to full species status, on the basis of slightly larger skull proportions and the absence or reduction of the third upper premolar.

A. insularis males and females are slightly larger and darker compared to A. leucurus and display missing or vestigial upper premolar.

[8][9][10] An upper premolar is generally variable among the family Sciuridae but is significantly different as seen in A. insularis compared to other Ammospermophilus species.

[14] Glacial expansion and retreat during the Pleistocene resulted in isolation of the genus Ammospermophilus leading to divergence into its three general groups.

Southern Baja California A. leucurus species diverged approximately 2.5 million years ago somewhere between the end of the Pliocene and beginning of Pleistocene era.