Systematics

There are five currently recognized subspecies of L. amethystinus:

L. a. amethystinus (Swainson 1827)

Occurs in western, central, and eastern Mexico from southern Nayarit, southern Nuevo León, and southern Tamaulipas to south to Veracruz and eastern Oaxaca.

L. a. margaritae (Salvin and Godman 1889)

Occurs in southwestern Mexico in Michoacan, Guerrero, and western Oaxaca.

This subspecies differs the most from the other four subspecies: it is "much darker; adult male with throat metallic violet (violet to royal blue) instead of reddish purple" (Ridgway 1911: 499). Possibly represents a distinct species, as it has been collected at a site near where typical rose- or pink-throated populations are known (Binford 1989).

L. a. circumventris (Phillips 1966)

Occurs in southern Mexico in southwestern Oaxaca. Very similar to nominate amethystinus, and doubtfully distinct (Binford 1989).

L. a. salvini (Ridgway 1908)

Occurs in the highlands of southern Mexico, Guatemala, and El Salvador.

L. a. nobilis (Griscom 1932)

Occurs in the highlands of Honduras.

"Strikingly different from salvini Ridgway of western Guatemala, its nearest relative; smaller; adult male with upper parts more bronzy green, the rump purple not bronze, the upper tail-coverts purplish black; whole auricular region white - a patch, not a line; suborbital area velvety black; under parts uniform dark smoke-gray with no brownish tinge, and metallic bronze overtone of other races confined to a narrow line just below the reddish purple throat, and consequently wanting on breast, flanks, and sides; female more bronzy above, much darker below, the throat duller and darker cinnamon, the rest of under parts differing from salvini just as in the male" (Griscom 1932: 58).

The Amethyst–throated Hummingbird is similar to the Blue-throated Hummingbird (L. clemenciae), and some authors suggest that these two are sister species (e.g., Züchner 1999). Phylogenetic analysis of DNA sequence data (from both mitochondrial and nuclear genes) did not resolve the relationship of clemenciae to other Lampornis: different analyses of the data suggeted that clemenciae was "either basal to all Lampornis ...  or as sister to L. amethystinus" (Garcia-Moreno et al. 2006).