Detecting cryptic species in sympatry and allopatry: analysis of hidden diversity inPolyommatus(Agrodiaetus) butterflies (Lepidoptera: Lycaenidae)

The taxonomic challenge posed by cryptic species (two or more distinct species classified as a single species) has been recognized for nearly 300 years, but the advent of relatively inexpensive and rapid DNA sequencing has given biologists a new tool for detecting and differentiating morphologically similar species. Here, we synthesize the literature on cryptic and sibling species and discuss trends in their discovery. However, a lack of systematic studies leaves many questions open, such as whether cryptic species are more common in particular habitats, latitudes or taxonomic groups. The discovery of cryptic species is likely to be non-random with regard to taxon and biome and, hence, could have profound implications for evolutionary theory, biogeography and conservation planning.

Although central to much biological research, the identification of species is often difficult. The use of DNA barcodes, short DNA sequences from a standardized region of the genome, has recently been proposed as a tool to facilitate species identification and discovery. However, the effectiveness of DNA barcoding for identifying specimens in species-rich tropical biotas is unknown. Here we show that cytochrome c oxidase I DNA barcodes effectively discriminate among species in three Lepidoptera families from Area de Conservación Guanacaste in northwestern Costa Rica. We found that 97.9% of the 521 species recognized by prior taxonomic work possess distinctive cytochrome c oxidase I barcodes and that the few instances of interspecific sequence overlap involve very similar species. We also found two or more barcode clusters within each of 13 supposedly single species. Covariation between these clusters and morphological and/or ecological traits indicates overlooked species complexes. If these results are general, DNA barcoding will significantly aid species identification and discovery in tropical settings.

Species numbers are increasing rapidly. This is due mostly to taxonomic inflation, where known subspecies are raised to species as a result in a change in species concept, rather than to new discoveries. Yet macroecologists and conservation biologists depend heavily on species lists, treating them as accurate and stable measures of biodiversity. Deciding on a standardized, universal species list might ameliorate the mismatch between taxonomy and the uses to which it is put. However, taxonomic uncertainty is ultimately due to the evolutionary nature of species, and is unlikely to be solved completely by standardization. For the moment, at least, users must acknowledge the limitations of taxonomic species and avoid unrealistic expectations of species lists.