How Does One Decide Whether A Distinctive Population Is A Different Species?

Anolis sagrei nelsoni. Photo by Steve Busack.

Bob Powell raises an important point in a recent comment, concluding “So, at what point does a differentiated isolate cease to be a subspecies and become a species?” In other words, how do we decide whether to recognize a distinctive population as a different species? I’m not talking about the situation where somebody goes out and comprehensively samples a clade and then uses the latest fancy-pants statistical wizardry to decide how to “delimit” taxa into one or multiple species. Rather, I’m going old school, focusing simply on the situation in which one has a population that is distinctive from other populations. Should we recognize that population as a different species? What kind of evidence is sufficient?

The context is the population discussed last week, Anolis sagrei nelsoni, from the remote Swan Islands 90 miles off the coast of Honduras. This isolated population is morphologically distinctive in many ways from other sagrei. It’s larger, has more lamellae on its toes, and, at least in the photograph above, has a very dark dewlap. Very likely, when Randy McCranie is done examining their scalation, he’ll document  other peculiarities. Is this enough to decide to recognize Anolis nelsoni? Certainly, in recent years the same approach has been taken to recognize a number of other Central American anole populations as distinct species.

One problem with this approach is that nelsoni probably lies phylogenetically somewhere within what is currently recognized as A. sagrei (just my guess, but the only phylogeographic study on sagrei to date placed Belize populations in the middle of sagrei). Hence, raising nelsoni to species status would render the rest of sagrei paraphyletic, which bothers some people more than others.

Another option is to name the population (or in this case, retain the population) as a subspecies. I recently semi-seriously suggested to a collaborator that we name two subspecies for populations of a species found on two islands. He just laughed.

The answer, of course, is to conduct a detailed systematic study of the entire A. sagrei clade, using both molecular and morphological data. The problem is that this is a huge undertaking. Even Al Schwartz didn’t tackle variation in sagrei! So, it may be a while before this gets done. What do we do in the meantime? Is it A. sagrei nelsoni or A. nelsoni? Good question, Bob!

Update on Expedition to Swan Islands

While we’re on the topic of A. sagrei nelsoni, I thought I’d fill you in on the star-crossed trip to the Swans, announced with such hullabaloo last week. Here’s what happened. The team assembled in the coastal Honduran town of La Ceiba on December 3. Our pilot came by to say that all was in order and that he’d pick up us after breakfast the next morning. Dawn duly arrived, and just as I was polishing off my breakfast burrito, Randy comes in and says that the pilot has called and that there’s a problem with the paperwork that was discovered when he went in to file his flight plan, and that we’d be a little delayed.

Here’s what happened: the pilot had talked with the naval authorities in the capital, Tegucigalpa (the only occupants of the Swan Islands are a small Honduran naval garrison). They had okayed the trip out there and said so in an email. They said they would send a piece of paper to the pilot, but it never arrived. The pilot didn’t think anything of it, because he had the email documenting that everything was set.

Apparently not. No paper, no flight. Email immaterial. And thus the scrambling began. The naval authorities in Tegucigalpa wouldn’t answer their phone. December 4 came and went. Maybe tomorrow. Friends of friends were enlisted back in Tegus (as it is called, herpetologically enough) to call lawyers, police chiefs, military brass to bring pressure for the permit. December 5 dawned. Still no permit, nor any clear indication if and when it might materialize. At that point, I bailed. Having to return shortly to Cambridge for my teaching commitments, it didn’t make sense to hang around on the off-chance that I’d get maybe a day in the Swan Islands (much as it killed me to give up my chance to visit there). Instead I went to Utila, in quest of the two endemic anoles from that tiny island (more on that later).

Meanwhile, Randy, Alexis and Leo braved it out in La Ceiba (and did a little anolizing when they could). The last I heard was an email last night, saying that everything looked good for a 6 a.m. departure this morning. And today…nothing. So, I’m hoping the gang has arrived in the Swans (where there is no internet, or much of anything else) and is madly collecting data and observing endemics. We’ll hopefully have a report in a week or so when they return. Stay tuned.

About Jonathan Losos

Professor and Curator of Herpetology at the Museum of Comparative Zoology at Harvard University. I've spent my entire professional career studying anoles and have discovered that the more I learn about anoles, the more I realize I don't know.

3 thoughts on “How Does One Decide Whether A Distinctive Population Is A Different Species?

  1. It often helps to make a table comparing the subspecies or populations on distinct charaters such as scale counts, size, color, etc. Then, see what level of difference exists for described species. Color is otten used by us to reconize a form but highly variable (often just reflects a match with background). These are old school ways to describe a species. Inject use of genetic analyses and you have to decide how much weight will be placed on these differences. Many new species have both criteria (morphology and genetics). I thought the concept of subspecies was in disfavor or not used anymore. A subspecies can just be a color variant. My rule of thumb is that I believe a new description based on who describes it! In large part, it is a professional call by an expert on the group. Today, one struggles to accept splits of species or new descriptions mostly based on genetic analyses, but that is the trend. There is no one answer here, either.

  2. I understand your frustration, Jonathon. Just barely over a year ago I arrived in Pedernales, Dominican Republic with my eyes fixated on spending a day/night on Isla Alto Velo with goal of collecting fresh material of the endemic anole (A. altavelensis) as well as exemplars of Aristeliger expectatus and Sphaerodactylus altavelensis to complement my extensive sampling of these species.

    We talked with a number of fishermen about boats, the price kept rising, and the terms became increasingly sketchy. The best offer was around $400 but would mean I’d be dropped off on the remote island and picked up the following morning. Although the price tag also included another day/night trip to Isla Beata, I concluded that I was already several sketchy boat rides deep in my thesis and probably running low on luck. I mean, the last time I attempted to go to Isla Beata (a comparatively safer destination) the engine failed and the “captain” attempted to rebuild the carburetor on the water while we braced the boat against razor-edged diente de perro limestone.

    Considering the Honduran approach to low-lying, small aircraft of “Shoot now, don’t ask questions later,” I reckon you made the correct decision. (

  3. Is nelsoni absolutely distinct, or just modally? The problem you will hit, even if absolutely distinct, is the contrast of real evolution versus phylogenetic philosophy. Real speciation is typically paraphyletic: the ancestral species does not usually change just because an isolate evolves off into something new. The novel isolate may be utterly distinct and fully reproductively isolated….

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