Eriogonoideae is a subfamily of the knotweed family, Polygonaceae, endemic to the New World, and is composed of 14 genera and perhaps 320 species. It differs primarily from the other members of Polygonaceae in lacking well-defined sheathing stipules or ochrea. The species of Eriogonoideae vary from tiny, fragile annuals to herbaceous perennials, low subshrubs or shrubs to large and often arborescent shrubs. The seemingly most primitive extant genus of the subfamily is Eriogonum (247 species), which is widespread in central North America. A series of genera are closely related to Eriogonum, and probably have evolved directly from Eriogonum. These genera are Oxytheca (9 species) of the western United States, and Chile and Argentina of South America; Dedeckera and Gilmania, both monotypic genera of the Death Valley region of California; Stenogonum (2 species) of the Colorado Plateau and adjacent areas of the Rocky Mountain West; Goodmania and Hollisteria, 2 monotypic genera of central and southern California; and Nemacaulis, a monotypic genus of the southwestern United States and northwestern Mexico. A second major complex of genera also probably evolved from Eriogonum. In this group, the most elementary genus is Chorizanthe (about 50 species), in which the extant perennial members of the genus are perhaps evolutionarily the oldest taxa of the subfamily. These perennials are restricted to Chile, while in the western United States and northwestern Mexico of North America, only annual species are found. Mucronea (2 species) of California and Centrostegia (4 species) of the southwestern United States and northwestern Mexico are clearly related to Chorizanthe. In a somewhat intermediate position between the Eriogonum complex and the Chorizanthe complex—but still more closely related to the latter than the former—is the genus Lastarriaea (2 species) found in California, Baja California, and Chile. All of these genera belong to the tribe Eriogoneae. A second tribe, Pterostegeae, contains only 2 discordant, monotypic genera: the shrubby perennial genus Harfordia of Baja California and the more widespread annual, Pterostegia, of the western United States. While time and evolution have obscured the relationships between Eriogoneae and Pterostegeae, the affiliations among the various genera of the tribes can be ascertained to some degree. The geographical center of origin of the subfamily may have been in a subtropical climate, with the differentiation of modern-day genera occurring in temperate, xeric regions of North America. The origin of Chorizanthe was an ancient development, with the migration of the primitive perennial members into South America in the Tertiary. The subsequent development of the annual habit, and migration of annual species of Eriogonoideae into South America has probably occurred in the Quaternary. The intermediate stages of evolutionary development of the genera and species of the subfamily occurred in a habitat similar to the pinyon-juniper woodlands of the Great Basin, while evolution of the more advanced genera and species has occurred in xeric grasslands, chapparral scrub, or xerophytic "hot desert" communities.
Reveal, James L.
"Distribution and phylogeny of Eriogonoideae (Polygonaceae),"
Great Basin Naturalist Memoirs: Vol. 2
, Article 11.
Available at: http://scholarsarchive.byu.edu/gbnm/vol2/iss1/11