provided by Smithsonian Contributions to Zoology
HOLOTYPE.—Male; Mexico, Hidalgo, near Jacala, 2–3 July 1965, O.S. Flint, Jr., and M.A. Ortiz, USNM Type 101312.
DESCRIPTION.—A multivoltine skipper (adults from February, July, August, and September; see complete dates in legend of Figures 58–65) occurring at middle elevations in the Sierra Madre Oriental (see localities and altitudes in legend of Figures 58–65 and see Figure 90). Nudum segments usually 19 or 20, but ranging from 17 to 21 (Table 3). Size (mean male fore wing length 22 mm; Table 3) and facies (Figures 58–73) as in populations of A. cellus inhabiting the same general region, but genitalia (Figures 74–84) distinct (at least in males): tegumen relatively long, sometimes nearly as long as uncus (much longer than in cellus); uncus prongs parallel and together (rather than divergent) except at tips, which turn slightly outward; uncus prongs in characteristic contact at a point just cephalad of their turned out tips; in dorsal view, uncus prongs not notably tapered toward tip (as they are in cellus); saccus no longer than tegumen (very much shorter than in cellus); valvae lower distally than proximally, with the posterior dorsal process (which is finely dentate) broader than the anterior dorsal process; valvae less asymmetric than in cellus; aedeagus shorter than in cellus. (Compare male genitalia of siermadror in Figures 74–84 with those of cellus in Figures 20–36 and, especially, Figures 85–89, which show directly comparable views of a cellus male that is almost sympatric with the siermadror male in Figures 80–84.)
HABITAT.—The known specimens of A. siermadror (eight males) all come from pine-oak forest varying in aspect from mesic and open to dry and scrubby. It is likely, then, that A. siermadror coexists with A. cellus. Though sympatry has yet to be established, the two species have always been found close to one another (see Figure 90).
Most of my systematic research over the past quarter-century has involved detailed studies at and around the species level, especially in skipper butterflies. In this connection, I have become ever more deeply enamored of male and female genitalia. Without understanding exactly why they are so good, I enjoy using them and thinking about them.
I argued early on and again recently (Burns, 1964, 1983) that genitalic differences between well-differentiated subspecies or sister species may be nonadaptive. Such differences, which are clearly genetic, often strike me as spin-offs of something like Mayr's (1954) “genetic revolution” (rapid genetic change leading quickly, in turn, to more extensive genetic reorganization in small, isolated, and therefore inbred, founder populations). Though complete mechanisms for genetically but harmoniously shaking up a miniisolate remain elusive (awaiting, say, far better comprehension of the behavior of DNAs and some of their molecular contacts), they probably involve rather more than selection (Burns, 1983:355). Gould and Lewontin (1979) and Gould (1980) vigorously doubt that selection is the supreme perpetrator. To say that genitalic differentiation may be nonadaptive is not to deny that male and female adjust to one another morphologically when accidental shifts in either's genitalic form so demand (Burns, 1983:341), but such coevolution is secondary.
Although individual variation in genitalia is ubiquitous and often generous, if one learns to recognize it and allow for it, the genitalia will usually look remarkably constant over the geographic range of a differentiate. Between sister differentiates one may then detect “constant” genitalic differences, sometimes so wonderfully gross as not to be missed by anyone, but sometimes exceedingly small. Because I have seen even the most trivial genitalic differences hold, even when the sister differentiates are allopatric and the range of each is discontinuous as well as extensive (Burns, 1964, 1983), I infer that these differences arise by chance in a tiny population, where they become fixed, and thereafter get carried wherever the differentiate goes, which may be “down/The lubricated gullet of extinction” (Burns, 1975b) but may be far and wide. I have trouble imagining a modest genitalic difference being selectively favored in a large population, gradually replacing what has come before, and finally spreading uniformly through the entire species population in all its far-flung and fragmented glory.
I do not see genitalia varying geographically as though in direct response to geographically varying selection. Whenever I have encountered geographic variation in genitalia and analyzed it in detail, I have had to conclude that genitalic differences originated in geographic isolation and variation resulted from secondary contact and hybridization between differentiates (Burns, 1964 and unpublished). Autochton cellus is no exception. By contrast, what I do find varying geographically and, at times, in parallel ways that must be ascribed to selection (e.g., Burns, 1964:204–205) are color-pattern, size, and voltinism. Differentiation in these kinds of “external” characters can be superimposed on a differentiate whose genitalia (among other things) tend to remain constant and true—till spun in the roulette of “genetic revolution” once more under special circumstances. For years I have felt, intuitively, that most differentiation happens in the speciation process, which is allopatric, promoted by founders, rapid, random, and, on occasion, highly productive (Burns, 1975b). I write much of this with a nod to what I like to call “punctrockers,” those paleontologists who have shaken phyletic gradualism and evolutionary theory generally with notions of punctuated equilibria and hierarchy (Eldredge, 1971; Eldredge and Gould, 1972; Stanley, 1975, 1979; Gould and Eldredge, 1977; Gould, 1980; Eldredge, 1982 ).
Geographic variation in the genitalia of A. cellus supports the pattern noted above. In each sex, despite considerable variation, there is essentially one kind of genitalia in the eastern United States and another from the Southwest (Arizona, New Mexico, Colorado) through Mexico to northern Central America. Variably intermediate genitalia occur together with the Southwestern/Mexican kind in Texas, which is geographically intermediate; and faintly intermediate genitalia (in males, at least) occur in adjacent Nuevo León. (For details, see sections on Male and Female Genitalia and Figures 19–57 and 85–89.) Readers with a gradualist bent may tend to see clinal variation in the genitalia from about the city of Washington to Arizona. I thought I did, myself, before analyzing numerous genitalia and properly appreciating and segregating individual variation.
Geographic variation in what seems a trifling color character (the extent of yellow dorsally at the apex and along the costa of the hindwing) corresponds with geographic variation in genitalia, except that the shift from one mode of expression to the other is abrupt, Texas populations resembling those in the eastern United States (see section on Facies and Figures 3–18).
None of the critical geographically varying characters (male genitalia, female genitalia, and facies) breaks in exactly the same way, though all change across the same area where populations are few and far between. This entire region is one of enormous biotic stress and transition. Two of the plants that A. cellus eats presently occur on opposite sides of it but not in it (see section on Larval Foodplants).
From all this I infer that A. cellus gave rise to a miniisolate that underwent a genetic minirevolution resulting in a mini- (but genitalic) differentiate that expanded its range. When these mother and daughter (so-called sister) differentiates made contact, they were compatible and hybridized. Their contact later weakened, apparently dissolving in very recent time; but evidence of extensive interbreeding between them remains. Because the distribution of A. cellus is much more extended, complex, and discontinuous from the southwestern United States to El Salvador than it is in the eastern United States, because the sister species of A. cellus (A. siermadror) is Mexican (Nuevo León to Hidalgo), and because all other species currently placed in Autochton variously occur between extreme southern Arizona and southern Brazil and Argentina (Evans, 1952), the mother differentiate within A. cellus is almost certainly the one now ranging from the Southwest through Mexico to Central America. Somewhere off its northeastern frontier, I presume, it founded a miniisolate that became the daughter differentiate now inhabiting the eastern United States.
- bibliographic citation
- Burns, John M. 1984. "Evolutionary Differentiation: Differentiating Gold-Banded Skippers--Autochton cellus and More (Lepidoptera: Hesperiidae: Pyrginae)." Smithsonian Contributions to Zoology. 1-38. https://doi.org/10.5479/si.00810282.405