What is a species?

[4 s @ 28.8 kb/s] Two color forms of T. sancheziae
These are both Teagueia sancheziae in spite of their different color. A red/yellow polymorphism is common in several of these new Teagueia species.

         The new Teagueia species are remarkably variable in flower color, leaf size and shape, and even leaf texture. It is quite a challenge to divide them up into good species. To do it right, we have to think carefully about the meaning of the “species concept” in biology. In the past, the concept of a species was oriented towards identification and classification, and forms which were visually very distinct from each other were usually classified as separate species. Forms which showed lots of variability were often all dumped into a single all-encompassing species. These decisions were made on the basis of intuition and individual judgement, and like all matters of individual taste, were subject to all kinds of fruitless debate among “splitters” and “lumpers”. However, in recent decades a more objective “species concept” has arisen in biology, and this concept removes some (though not all) of the arbitrariness of our taxonomic decisions. In modern biology a “species” is a population that has the potential to freely exchange genes, and evolves through time as a single unit. Thus two populations that live in the same place but don’t exchange genes are two distinct species, even if they seem very similar to us. On the other hand two plants that look very different may both belong to the same species, if there are lots of intermediate forms between those extremes in the population.

      This biological species concept is easiest to apply at a single site. I try to look at the populations and identify forms that seem to be distinct. Then I try to get a big enough sample to assess the variability of each of these forms. Sometimes two forms that at first seemed distinct are seen to be part of a single highly variable population. Sometimes two superficially distinct color forms turn out to be structurally identical, and the color forms turn out to be just a simple polymorphism (like the occasional white or semi-alba Cattleya in a population of lavender individuals). Other times a group of forms that seemed very similar prove to have subtle but consistent differences in multiple characters; these are distinct species.

       Things get more difficult when I try to decide whether two forms from different mountains belong to the same species. I first ask myself what would happen if the two forms were to grow together. Could they both share the same pollinator? I look at lip and column structure and size to help me decide this, and pay less attention to the parts of the flower that have less contact with the pollinator. If two forms from different mountains could not share the same pollinator, then they are not exchanging genes, nor could they potentially exchange genes in the future if their ranges were to someday overlap. These forms are thus two distinct species, each on their own evolutionary pathway. Unfortunately our knowledge of the pollination biology of Teagueia is almost zero, so much of this is guesswork. I try to be conservative and allow a lot of leeway for geographical variation, so I usually end up lumping similar forms from different mountains into the same species. Nevertheless my decisions, right or wrong, are testable by pollination experiments and perhaps by DNA analysis. This is the big advantage of the biological species concept. There are still gray areas, of course, especially during a speciation event (when a single species begins to diverge into two). We can only hope that those fuzzy moments in the evolutionary history of a species are rare, and we deal with them as best we can.