Taxonomic Delimitation in a Cryptic Species Complex of Ferns
You are studying a fern genus in the Western Ghats, India. Morphological measurements of 45 herbarium specimens show continuous variation in frond length (15–45 cm), sorus position, and rhizome scale color. Two named species, Pteris confusa and P. indica, are separated only by frond length and scale color, but the ranges overlap. You sequence the rbcL and trnL-F regions for 30 samples. The phylogeny shows two well-supported clades (bootstrap > 90%) that do not correspond to the morphological species. Clade A includes samples from high elevation (1200–1800 m) and Clade B from low elevation (300–800 m). However, two samples from mid-elevation (1000 m) fall into Clade A, and one into Clade B. Spore morphology shows no difference. You must decide whether to recognize one or two species and what additional evidence you need.
1. Evaluate the strength of the evidence for recognizing two species. What taxonomic decision would you make, and why?
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The molecular data show two well-supported clades, but they are not congruent with the current morphological species. The clades correlate with elevation, suggesting ecological differentiation or isolation by altitude. However, the presence of mid-elevation samples in both clades indicates that the genetic boundary is not absolute and may reflect admixture or incomplete lineage sorting. Morphological characters are continuous and overlapping, and spore morphology is identical, so there is no clear diagnostic character for either clade. Under the phylogenetic species concept, the two clades could be recognized as separate species if they are diagnosable and monophyletic. But under the biological species concept, reproductive isolation is unknown. A conservative decision is to recognize one variable species, P. confusa (priority), and treat elevation as ecotypic variation, until further evidence is available. Alternatively, if the clades are found to be reproductively isolated, they could be recognized as subspecies. The decision should be provisional and clearly stated.
2. What additional evidence would you gather to resolve the species boundary? Outline two specific analyses.
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First, perform population genetic analysis using microsatellites or SNPs on a larger sample (n > 100) across the elevation gradient. This would test whether the two clades are genetically distinct or connected by gene flow. Calculate FST and STRUCTURE admixture proportions; if FST > 0.25 and admixture is low, support for two species increases. Second, conduct common garden or reciprocal transplant experiments. Grow spores from both clades under uniform conditions and measure morphological traits and reproductive success. If traits remain distinct and hybrids have reduced fitness, reproductive isolation is supported. Additionally, examine the type specimens of both names to confirm which clade they belong to, and use scanning electron microscopy to check for subtle spore or scale differences. If no diagnostic characters are found, recommend synonymy. The final decision should integrate molecular, morphological, and ecological evidence, and be published with a clear taxonomic rationale.