Hood Racer: An Allopatric Sibling

Explanation: This beautiful, unassuming serpent has much to teach about taxonomy and evolution.

This is the only snake resident on the tiny island of Española (also called Hood Island), in the remarkable archipelago of the Galapagos Islands, made famous by the explorations of Charles Darwin. The Galapagos are known for their "Darwin's finches," that set of 13 species known, from Charles Darwin's research, for having evolved a diversity of bill shapes, body sizes, and nesting behaviors, from an original immigrant species.

However, less commonly known is the diversity of other life forms that have also evolved on those islands ... including this fine serpent.

This is known variously as the Hood racer, Española snake, and other names (not surprisingly because Española Island is also known as Hood Island, but that's another story).

There are three species of terrestrial snakes native to the Galapagos Islands (plus one marine sea snake), and all belong to family Colubridae. Hood (Española) Island -- along with San Cristobal (Chatham) and Floreana (Charles) Islands -- holds only one species, the one pictured this week ... and a number of other islands of the chain hold the other two species.

So if you see a snake on Hood (Española) Island, just by distribution alone you can be sure it's a Hood Racer.

Usually.

However, it's important, as a naturalist, not to always jump to conclusions about species identification based just on distribution alone.

I say this because the three species of snakes on these islands are very difficult to tell apart, being best distinguished by scale counts, that is, the number of scales on various parts of the body (much like various species of garter snakes in western North America) ...

... such as: the size, shape and number of scales between the nasal openings on the head, the number of scales along the upper and lower "lips," the number of body scales counted around the neck, mid-body, and tail, and the number of scales covering the anal opening.

You pretty much have to have the snake in hand to determine all this!

Further, the taxonomy (the scientific naming) of this snake complex is ... well, complex.

This snake has been identified as belonging to genus Philodryas. And also Pseudalsophis. In Philodryas, it is named as subspecies Philodryas biserialis hoodensis. But in Pseudalsophis it is named as a full, separate species, Pseudalsophis hoodensis. Some sources even use Philodryas hoodensis as the full species. Confused yet?

What is interesting is that Hood Racer (by any other name) had somehow made it to its current location of its three islands -- and then diverged in morphology (body form, including the scale counts mentioned above) into the species (subspecies?) we know today. Its two very similar "sibling species" cousins, on the other islands, also diverged, in isolation ("allopatry," as ecologists call it).

And chances are strong that these snakes continue to change from each other over time.

So here is a fine example of a complex taxonomic (scientific naming) knot ... resulting from allopatric (separated) sibling (similar) species ... as yet another great instance of Darwin's evolutionary theory come to life, on these amazing islands that continue to provide surprises.

EPOW - Ecology Picture of the Week