There has been some recent discussion on this forum about why some genera have huge numbers of species and what even constitutes a species.
Some iNat users may have come across the concept of a “ring species”. This is the paradoxical situation where each population within a distribution can breed with its neighbors and yet there are some places where “end” populations coexist and cannot interbreed. Generally there has to be some kind of geographic obstacle that restricts population interactions.
One classic example of this in the western United States is the Ensatina (Ensatina eschscholtzii). Ensatinas are a type od salamander that lives in California’s Coast Ranges and Sierra Nevada, stretching north through Oregon and Washington to southern British Columbia, and south into the northern portion of Baja California. Across this range, herpetologists have identified seven subspecies of Ensatina, each of which has distinct coloration.
One area Ensatinas do not live in is California’s Central Valley (sometimes called the Great Valley), as it is too dry and hot. As a result, as you move down the west coast from north to south, there distribution of Ensatinas splits into two arms at the southern end on the Klamath Range. One arm continues down the coast through San Francisco and Los Angeles all the way to Tijuana, Mexico. The other heads south-east to the western slopes of the Sierra Nevada. Each arm of the population represents a continuum of interbreeding populations forming an elongated oval “ring” of something like 19 identifiable populations from 7 subspecies all of which can interbreed with their neighbors.
But at the south end of the Central Valley, Yellow-blotched Ensatinas of ssp. croceater that live in the Transverse Ranges (marked brown in the first map below) cannot interbreed with neighboring Monterey Ensatinas of ssp. eschscholtzii (marked bright blue).
This illustrates how mutual fertility can be problematic as a criterion for defining the species concept. None of the subspecies can be elevated to species rank because each can interbreed with at least one other. And yet some of the subspecies definitely cannot interbreed even though they live right next to each other.
This paradox was first uncovered by Robert Stebbins in the late 1940s and has been the subject of extensive study since then. There’s a good overview at UC Berkeley’s Understanding Evolution site. Richard Dawkins gave it some prominence in his 2004 book The Ancestor’s Tale.
This made me wonder what this ring pattern looks like among iNat observations. iNat actually has two ways to map out this type of distribution and both of them illustrate the ring species concept pretty well.
One option is an iNat Taxa Map. There isn’t a user interface for this, so you need to build up the URL yourself, but it’s not too complex. You just add the various taxon IDs you want to map into a URL such as the following: https://www.inaturalist.org/taxa/map?taxa=120135,144104,154880,123959,123689,142202,123168,518535#5/41/-120 After your list of taxon IDs you can specify a magnification level (5), and a latitude and longitude (41/-120 to indicate that the map will be centered at 41 degrees north and 120 degrees west). This map shows observations of the seven subspecies of Ensatina plus one recognized hybrid form.
The second mapping option uses the experimental Compare tool. This provides an interface for iNat users to build maps that use different colored markers for each of several different search criteria (typically taxon IDs). Here’s the Compare map for the same seven Ensatina subspecies and the hybrid.
Advantages of the Compare tool include:
- An actual UI to build your map choices
- Supports other search criteria, not just taxa
- Supports reordering of groups, which also swaps marker colors
- Allows you to label each group of markers
Advantages of the Taxa Map include:
- Has an intelligible URL
- Not deemed “experimental”
- Can default the center point and zoom level
- Users can toggle on and off markers for GBIF observations
Either way, if you build something nice with one of these tools I recommend you keep a note of the URL (maybe share it here).
[Edited 12/4/2020 to better describe the sequence of radiation proposed by Stebbins.]