Cicindela formosa (the big sand tiger beetle) is a not uncommon species that occurs across much of North America east of the Rocky Mountains in deep, dry, open sand habitats. It is absent in Appalachia and much of the Interior Highlands, understandable given the rarity of deep sand habitats on these elevated landforms; however, its absence across much of the southeastern coastal plain as well as south and west Texas, despite the widespread presence of apparently suitable habitat, is not easily explained. In Missouri, dry sand habitats are rather limited, occurring primarily along the Missouri and Mississippi Rivers, a few of the larger Ozark rivers, and along Crowley’s Ridge and the Blodgett Terrace in the Mississippi River Alluvial Plain. The individual in these photos was seen last weekend at Sand Prairie Conservation Area (on the Blodgett Terrace), where I also recorded it earlier this year. Despite its relative commoness, I always get a little excited whenever I find this species – it’s a big, chunky thing with bold markings and sufficient habitat specificity to keep it from being too pedestrian (unlike Cicindela repanda and C. punctulata, which usually evoke only a groan – okay, maybe western forms of the latter, with their gorgeous suffusion of green and blue excite me a little bit). Cicindela formosa populations to the west are even more brilliantly colored and localized – it’s a handsome species, indeed! Adults are powerful fliers that terminate their long escape flights with a comical tumble or two across the sand before ending up on their feet. Normally a difficult species to get close to, cool temps and overcast skies on this morning resulted in a cooperative subject and excellent lighting for this series of photos.
Missouri populations are assignable to the eastern subspecies generosa – mostly, that is. There is a population known from the Ozarks, along the beautifully pristine Current River, that exhibits tendencies towards the bright coppery-red dorsal and metallic purple ventral coloration of the nominate subspecies found further west. I’ve also located another population in the northeastern Ozarks on “sand” flats – not true sand, but expansive dumpings of pulverized limestone tailings from former lead mining operations – that shows a similar intergrading with nominotypical characters. The occurrence of these populations near typical generosa populations and disjunct from nominotypical populations several hundred kilometers to the west, coupled with the existence of a broad intergrade zone between the two forms along both sides of the Missouri River through Nebraska, Iowa, and the Dakotas, raises interesting questions about the validity of a subspecific distinction for generosa. Additional subspecies have been described from eastern Texas and adjacent areas of Arkansas and Louisiana (pigmentosignata), southeastern New Mexico (rutilovirens), and southwestern Saskatchewan (gibsoni). Each of these populations is at the edge of the nominate subspecies’ range of distribution and exhibits consistent differentiation in multiple characters – primarily color and maculation – from nominotypical populations. As a result, the case for according subspecific status to these populations is more convincing despite the occurrence of intergrades along narrow zones of contact with nominotypical populations. A truly allopatric population center occurs in northwestern Colorado and southeastern Utah – separated from nominotypical populations to the east by a distance of 230 km. The Colorado population strongly resembles and has thus been assigned to subspecies gibsoni. However, it is hard to imagine a mechanism by which the Colorado and Saskatchewan populations – with over 1,000 km separating them – derived from a common ancestry. A more likely scenario is independent adaptation to similar conditions in their respective habitats. Differences in coloration of the larval head capsule between these two populations lend support to this idea, which if true should qualify the two populations for consideration as distinct subspecies despite the similarity in their appearance. Interestingly, the Utah population resembles nominotypical forms further east, although intergrades with the adjacent Colorado population do occur along a narrow zone of contact.
The subspecies concept has been hotly debated for many decades now. E. O. Wilson and W. L. Brown (1953), in their seminal paper, The subspecies concept and its taxonomic application, questioned the validity of many subspecies on the basis that they failed to exhibit concordance across multiple characters and argued that subspecies that interbreed were not “real taxa” because the flow of genes and characters between them prevented divergence. This restrictive concept essentially limited subspecies to populations that showed significant divergence from their relatives but relied upon external mechanisms (i.e., allopatry) rather than internal (i.e., genetic) for reproductive isolation. Many of North America’s described tiger beetle subspecies would not meet these criteria, since there often exist zones of contact where intergrades (a result of gene flow within hybrid zones) do occur. Ernst Mayr took a more pragmatic approach in Animal Species and Evolution (1963), defining subspecies as “an aggregate of local populations of a species inhabiting a geographic subdivision of the range of the species, and differing taxonomically from other populations of the species” – in other words, subspecies are taxonomic units and not evolutionary units. Viewing subspecies as strictly taxonomic units is more convenient, since the presence of hybrid zones does not invalidate a subspecies as long as it retains its taxonomic distinctiveness. I acknowledge that taxonomic subspecies units are useful – named subspecies provide a convenient shorthand for discussing geographical variation within species and stimulate interest in their study and characterization. Also, as emphasized by cicindelid experts D. L. Pearson et al. (2006), the application of formalized subspecies names for distinctive, local populations makes conservation policy decisions more palatable to policians and legislators, thus enhancing the potential for protection. However, I also agree with O’Neill (1982) that the subspecies concept must be connected to an evolutionary unit to be truly meaningful, and the recent application of molecular techniques is now providing a genetic basis for assessing subspecies validity. Interestingly, some such studies have shown near complete blockage of gene flow across hybrid zones, even when hybridization is frequent, providing genetic evidence of “real taxa” that nevertheless interbreed at their boundaries (Mallet 2007). It would be interesting to apply molecular techniques to populations of generosa, nominotypical formosa, and the Missouri intergrades to understand their degree of genetic divergence, the presence of which could convince me that their status as distinct subspecies should be maintained.