Biogeography and distribution of species
Imagine if I told you that I was studying a species of fish that lived only in the Okefenokee Swamp and in the Everglades. That would be an interesting geographic distribution, and the field of Biogeography is all about studying the hows and whys of the distribution of species. So, let’s think about the biogeography of this fictitious species for a moment! Those two famous wetlands are hundreds of miles apart and, because we are considering a fish in this example, a prevailing question is: How can the fishes in those two swamps possibly maintain genetic connection? Clearly, the fish cannot travel overland between the sites in order to mate. This is a classic example of what we call allopatric populations. That is, two or more populations of the same species existing in different places. Now the question becomes: If they cannot mate to maintain genetic connection, then how can they be the same species. Now it is getting interesting!
A traditional view of species is that “if two individuals (or populations) can interbreed and produce viable young, then they are the same species.” That is overly simplistic, however, as we all know that hybridization between species can and does occur. So, the real point is not whether animals from the two populations can interbreed and produce viable offspring, but rather do they do so and do they do so frequently enough that they maintain genetic integrity across populations. When considering our fishes in the two swamps, we need to determine:
- Are they actually completely separated? (i.e., are there populations throughout Florida that remain undiscovered?). In our example, the answer to that is probably not so—but many areas of the world remain under-surveyed, so creatures like little swamp-fishes may not be well documented.
If they are actually totally geographically isolated, then we need to determine:
- How long have they been separated, and have the separate populations diverged in any manner of anatomy, behavior, or genetic (DNA sequences) feature that we can measure. In other words, are they actually different from one another?
I am not interested to see if a transplanted male from Georgia can produce young with a female from the Everglades. I am trying to discover the evolutionary history of these populations to determine if they actually do maintain connectivity, and if they have not done so, have they diverged in any way (or not!).
Our colleagues that study population genetics have calculated that it takes really very little genetic connectiveness to prevent allopatric populations from diverging from one another. In other words, a few instances of dispersal between the two populations, even if they are thousands of years apart, can keep them genetically connected. If this were not the case, every farm pond in Georgia would have its own species of bass and catfish! I flew into Kansas City in the summer of 1993 as the region was experiencing one the greatest floods in history. From the air, I could see nothing but water for many miles. Because I am an incessant nerd, my thoughts kept returning to “This is how populations fail to diverge!” A rare event was scrambling and allowing mixing between populations of aquatic creatures among what we would consider to be separate ponds, lakes, and stream drainages. One event like that in a million years is capable of preventing new species from forming.
For most of my career I’ve been studying a group of cool toads (well, I think they are cool anyway) that range from the southern U.S. to the very top of South America. They have neat bony crests atop their heads, so I refer to them as the crested toads (genus Incilius). Based on my work on them, I have concluded that there are 39 species. When I started working on the group in 1990, the world thought there were only 27 species. So, I’ve been busy discovering the underestimated actual diversity in the group and trying to understand their distributions from a biogeographic standpoint. One unassuming toad that lives only in Mexico (marbled toad, Incilius marmoreus) caught my attention because it appeared to have an allopatric population, as we had in our fictitious fish. I have been aware of this odd distributional pattern for a long time, and I finally got around to investigating it with my team of Ken Wang, a hard-working undergraduate at Georgia Tech, and Dr. Todd Pierson, a professor at Kennesaw State University and also a leading expert in genetic analyses of amphibians. Because I’ve had this project in mind for decades, through my various research trips to Mexico, I made a point to get tiny skin samples (for DNA) from marbled toads from all along their lengthy distribution along the Pacific Coast of Mexico and from the smaller, apparently isolated, population on the Caribbean Coast near the city of Veracruz in Veracruz State. Now, with my team in place, we started testing the hypothesis that the toads in these areas had not had any genetic contact for a very, very long time and, over that time in isolation, they have diverged in anatomy and/or genetics. This would result in the “discovery” of a new species, when we actually were already aware that they physically existed, we had simply misjudged their identity. Let’s get to work!
We reviewed every distribution record we could find, including historical specimens in natural history museums, citizen-science portals like iNaturalist, and field notes from naturalists like myself. Our efforts supported the idea that these populations appeared to be isolated from one another, with broad intervening areas where this species simply does not live (why? That’s another question!). We borrowed museum specimens and measured everything measurable on them and compared their anatomy carefully and determined that, except for a minor trend toward the Caribbean population being a bit smaller, they were indistinguishable.
The genetic analyses by Ken and Dr. Pierson revealed that the isolated population on the Caribbean Coast is actually more closely related to the toads on the southern end of the Pacific Coast than those southerly toads are related to their more northerly neighbors on the same coastline. Wow—those toads along the Pacific Coast don’t move around much and even though their distribution is continuous along the coast, there had been some sort of separation of them around 850,000 years ago and the legacy of that event (whatever it was) remains encoded in their genome. Meanwhile, the Caribbean population has only been isolated for about 330,000 years. That may seem like a long time, but it evidently has not been sufficiently long that the toads diverged, or evolved, in any significant manner.
I’ll be honest and tell you that I was surprised by the results. My working hypothesis, and thus the one we explicitly tested, was that the Caribbean population was in fact a separate species and simply no one, including myself, had examined it carefully. Nope. The data did not support my hypothesis, and we have a well-documented example of a species currently existing in two allopatric populations. I can’t predict the future, but for these toads I can imagine it playing out in one of three ways: 1) eventually these populations will diverge in some measurable manner; or 2) something will bring them back into contact, such as these populations expanding their range and re-establishing contact (because clearly at some point in history they have been in contact); or 3) very rare events, like a prodigal toad moving across the landscape, may keep them just barely connected for millennia to come.
I know I will never stop thinking about exactly how that weird population near Veracruz City got stuck way out there! We figured out what happened, but not how…….. Biogeography is a fascinating field and once you start thinking analytically about the distributions of species, you’ll never look at the maps in your field guide or the signs at Zoo Atlanta quite the same way again!
Reference: Wang, K., T. W. Pierson, and J. R. Mendelson III. 2026. Resolving the taxonomic status of the marbled toad (Bufonidae: Incilius marmoreus): 2RAD-based phylogeography including an isolated population in Veracruz, Mexico. Ichthyology and Herpetology 114: 227–239. https://doi.org/10.1643/h2024107
You can also get a copy of the original paper here:
Marbled toad photo: W.E. Duellman
Joe Mendelson, PhD, Director of Research
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