Encelia resinifera and E. virginensis
| Encelia resinifera | Encelia virginensis | |
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| Fig. 1. Photos of the leaves and flowers of Encelia resinifera var. resinifera (left) and those of Encelia virginensis (right). The color and hairiness of the leaves is the main separator of the species. The leaf of E. resinifera is green in color, and it has mostly rough scabrous hairs and sometimes strigose hairs. The leaf of E. virginensis is gray or gray-green in color with sparsely canescent and strigose hairs.
The flowers of E. resinifera generally have a wider disk compared to the width of the flower as a whole, although there is much overlap in the ratio. E. resinifera has a ratio of disk to head width of 0.35 to 0.63, with half of the measured heads having a ratio of 0.45 or greater. E. virginensis has a ratio of 0.34 to 0.44. The ratio of the head of E. resinifera shown here is 0.44, the median value for measured heads of that species. The ratio of the head of E. virginensis shown here is 0.38, the median value for measured heads of that species. Click on the pictures to go to the original iNat obs, which gives the information as to who took the photos. | ||
Introduction, including How To Distinguish These Species
History of the names for these two species
Introduction, including How To Distinguish These Species
Encelia resinifera and E. virginensis are very similar, but different, species which are both thought to be ancient hybrids between E. actoni and E. frutescens; see Diversification, disparification and hybridization in the desert shrubs Encelia by Singhai et al 2021! Their DNA results (using only a small part of the entire genome) nestle both species together.
That's worth saying again. Hybridization of the same two species at two separate times resulted in two closely-related, but different, species!
This is not as surprising as it seems. For one thing, one species may have resulted from pollen from E. actoni on the ovary of E. frutescens, and the other species may have resulted from pollen from E. frutescens on the ovary of E. actoni.
Also, hybrids go through a process of change to their genome with successive generations until the genome stabilizes; see Eukaryote hybrid genomes by Runemark et al 2019. This process may result in different end points, even when the hybrid starts in the same way. The two hybrids therefore might have differed significantly almost immediately, and then their separate evolution would increase their differences.
As expected of species of ancient hybrid origin, both species occur in disjunct habitats from their parent species. I.e., the combination of the genes from the two parents allowed them to adapt to different environments, with the result that the ranges of the two species are different from the ranges of the parents. For example, E. actoni does not occur in the range of E. resinifera, showing that E. resinifera is its own species now, having spread far from its origin within the range of E. actoni.
It is striking that the main difference between these two species is that E. resinifera has a leaf like one of its parents, E. frutescens, and E. virginensis has a leaf like the other parent, E. actoni; see Fig. 1.
While there is considerable variation in the leaf of each species, the color and hair difference holds up to distinguish the two species. I've made two pages showing the variation in the leaves of each species, which can be opened side by side on a computer to show the differences:
The flowers of E. resinifera generally have a wider disk compared to the width of the flower as a whole, although there is much overlap in the ratio. E. resinifera has a ratio of disk to head width of 0.35 to 0.63, with half of the measured heads having a ratio of 0.45 or greater. E. virginensis has a ratio of 0.34 to 0.44. Thus flowers with a ratio definitely larger than 0.45 are highly likely to be E. resinifera. Flowers with a ratio of 0.45 or below could be either species.
History of the names for these two species
Both of these two species for many years were considered varieties of E. frutescens. Keck in 1958 wrote that E. virginensis should be its own species, and it was restored to species status in the Munz and Keck 1959 California Flora, albeit with the inclusion of E. actoni as a subspecies under E. virginensis.
E. resinifera:
1907: E. frutescens f. radiata
1913: E. frutescens var. resinosa
1998: E. resinifera var. resinifera; E. resinifera var. tenuifolia
In 1906, Brandegee described a species he called E. resinosa from Hidalgo, Mexico, but that apparently was a different taxon from E. resinifera.
E. virginensis:
1904: E. virginensis
1907: E. frutescens f. virginensis
1913: E. frutescens var. virginensis
Geographic Distributions
The geographic distribution of the two species from vouchers is shown in Figs. 2 and 3.
| Encelia resinifera | Encelia virginensis | |
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| Fig. 2. Map of Encelia resinifera (left) and E. virginensis (right) from vouchers from SEINet retrieved in June 2026. The color differences denote different names which are synonyms or subtaxa of the species name. The distributions are somewhat disjunct, with only E. resinifera found in Utah and Arizona east of the longitude of St. George, Utah, and with mostly only E. virginensis found west of Las Vegas and south of Flagstaff. However, both species are found together in a significant area.
It is likely that some vouchers are misdetermined, such as the ones of "E. virginensis" in southeastern Utah, since the Utah flora gives only E. resinifera in that location. In fact, examination of the two vouchers with photos shows that the leaves are green, not gray, indicating they are indeed E. resinifera, not E. virginensis. For the full interactive voucher map from SEINet, see E. resinifera and E. virginensis. See Fig. 3 for a map with both species together. Click on the maps here for larger versions. | ||
Fig. 3. Map of Encelia resinifera and E. virginensis from vouchers from SEINet retrieved in June 2026 (small symbols with label "V") and from 46 iNat observations of the two species reviewed by the author (large symbols with label "TJC"). The area shown is the same as the areas shown in Fig. 2; refer to those maps to see landmarks near a given location.
E. resinifera Subspecies
There are two subspecies of E. resinifera. Ssp. tenuifolia has lengths of leaf blades and ray flowers at least three times their widths, as shown in Fig. 4. Ssp. resinifera has leaf blades that usually have roughly the same width as length, and always have lengths less than three times their widths, and ray flowers with lengths less than three times their widths, as shown in Fig. 1.
Ssp. tenuifolia has a very narrow distribution only in the Grand Canyon "following the 1200 m (4000 foot) contour". It intergrades with ssp. resinifera at the east end of the Canyon. All other locations have only ssp. resinifera.
There are only three iNat obs of plants that have the narrow leaves of ssp. tenuifolia, but the ray flowers of those plants are not narrow. That may indicate that they are intergrades, or may just mean that ssp. tenuifolia doesn't always have narrow ray flowers.
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| Fig. 4. Ray flowers and leaves of Encelia resinifera ssp. tenuifolia. Click on the photos to go to the type specimen or the iNat observation. | ||
I discovered something interesting when I went through all the iNat obs of Encelia in the Moab area to determine them. In this area, of these two species, only E. resinifera exists. Prior to September 2024, most of the iNat obs were correctly determined as E. resinifera, with no iNat obs determined as E. virginensis, almost always by identifiers who knew the species there. But after that time, there were 51 obs determined as E. virginensis, and only 37 obs determined as E. resinifera. The reason is almost surely that during that period, the iNat Computer Vision Model suggested only E. virginensis for those observations, with no mention of E. resinifera, probably because E. resinifera was not in the Computer Vision Model (and still isn't), and doesn't appear in the "expected nearby" species.
The key to separate these species in the Curtis Clark paper defining E. resinifera is:
1. Leaves silvery pubescent, broad2'. Leaf pubescence of long, soft hairs; short, broad, coarse hairs; and glands .... E. virginensis
1'. Leaves green with sparse, coarse hairs
3'. Rays present
6. Leaves broadly ovate, rays broad .... E. resinifera ssp. resinifera
6. Leaves lanceolate or linear, rays narrow .... E. resinifera ssp. tenuifolia
The Flora of North America key to separate these species is:
7 Leaves scabrous to strigose (not canescent) .... E. resinifera7' (modified by 8) Leaves sparsely canescent and strigose ... E. virginensis
The Utah Flora key to separate the species is:
1. Herbage strigose, also with some pustular-based hairs; involucral bracts abruptly caudate-acuminate, strigose; plants of Washington County ... E. virginensis1' Herbage hispid with pustular-based hairs; involucral bracts gradually attenuate, more or less glandular (sometimes strigose); plants of the Colorado Basin ...E. resinifera
The Utah flora also says for E. resinifera:
There is a cline of glandularity in leaves from definitely glandular in the southern portion of the range in Utah to no glands at all in the northern material.Type of E. resinifera from Little Colorado River "near Winslow, Arizona". Type of E. virginensis from "Valley of the Virgin, Nevada".
Analysis showing that E. virginensis is consistent with a hybrid between E. actoni and E. frutescens:
Curtis Clark measured eight characteristics of 119 plants of the E. actoni, E. frutescens, E. virginensis, and F1 hybrids of the first two species. He found that E. virginensis came out intermediate between the two parents in a PCA using those characteristics, and that it was indistinguishable from the F1 hybrid. A plot of the mean achene length and width also found intermediacy. Internal transcribed spacers of nuclear rDNA found that the parent species differed by nine bases, and every sampled E. virginensis had "either the E. actoni base, the E. frutescens base, or a polymorphism consisting of both bases." See Phylogeny and Adaptation in the Encelia Alliance.