Primula ludlowii - Species, Subspecies or Variety?

In 1934, Frank Ludlow and George Sherriff discovered Primula plants growing on a cliff on the Diwangiri-Trashigong Road in Bhutan. Collection #552 was named Primula sherriffae in honor of George Sherriff's Mother. The spelling of this has been corrected to P. sherriffiae, according to the IAPT code 60C. On the same cliff, they gathered collection #554 which was initially listed as P. sherriffae forma pygmaea, as the plants were much smaller than those collected under #552. A live plant of #554 was brought back by George Sherriff, which flowered in cultivation. Differences were noted between it and P. sherriffiae and in 1936, W. W. Smith described #554 as the new species P. ludlowii.

The characteristics which differ in P. ludlowii are size (both the plant and flowers are smaller), efarinose corolla, corolla tube is finely pilose on the inside and outside. The flowers are solitary (or two flowered) and the scape is almost obsolete. Additionally, it seems as though P. ludlowii has flowers which are darker purple in color.

P. ludlowii (L), P. sherriffiae (R) differ in a number of characteristics

So we have two related species growing on exactly the same cliff. That is suspicious in terms of whether we actually have two distinct species. This brings up the obvious - what determines whether a plant is a distinct species, a subspecies or a variety? Unfortunately scientists have failed to agree on a  precise definition of these terms so that, to some extent, personal opinion is a component of classifying species. Because there are different concepts of species (morphological, phenetic, phylogenetic, ecological, etc) that may result in different levels of "splitting" and "lumping", I think it is prudent in any comprehensive document on a Genus that the author states which concept he choses to follow and applies that consistently in his taxonomy or at least note where he deviates and why.

The following definitions are often cited: Species are a group of individuals that interbreed or could interbreed in nature and thus have a high level of genetic similarity, Subspecies are groups of individuals that could interbreed but do not because of factors such as geographic isolation, but are not distinctive enough to warrant species status, and Varieties are individuals which can breed but differ from others by some consistent minor noticeable characteristics. A species may evolve over time by one or several factors such as reproductive isolation, geographic isolation, and reduction of gene flow. Two species that live in the same location may have evolved from a single ancestor species, through a process called sympatric speciation. This process is not accepted by all authorities.

Certainly further study of these plants, including a survey of variation and genetic analysis, could be helpful in determining the status of P. ludlowii, but there will likely be no right or wrong answer - just a difference of opinion. See the species gallery for more images of both P. ludlowii and P. sherriffiae.