#rana

This is just a Frog™, right?

Wrong. These four frogs belong to four different families (respectively, Dicroglossidae: Limnonectes paramacrodon, Ranidae: Rana temporaria, Mantellidae: Aglyptodactylus madagascariensis, and Pyxicephalidae: Amietia tenuoplicata) from across the world (respectively, I took these photos in Borneo, Denmark, Madagascar, and Tanzania).

These represent just some of the huge number of genera across more than half a dozen families that have independently produced species that look like this:

In Europe: Ranidae: Rana. In North America: Ranidae: Boreorana/Lithobates. In South America: Leptodactylidae: Leptodactylus. In Africa: Pyxicephalidae: Amietia or Arthroleptidae: Arthroleptis. In Madagascar: Mantellidae: Aglyptodactylus. In mainland Asia: any of a dozen or more genera of the family Ranidae; or Ranixalidae: Indirana. In Australia: Pelodryadidae: Rhyaconastes. and more, besides!

Why? Well… that is kind of a fundamental question in evolutionary biology. Why, with the infinite theoretical possibilities of evolutionary processes, do we see the same kind of anatomies evolving again and again? Does this mean that evolution is in some way predictable?

The answer is complex (there are good books on this topic!), but basically, yes: similar pressures can (but do not always) lead to similar solutions. I think this may apply to frogs to an exceptional degree, in part because their body plans are so simple to start with, that the tools available for tinkering are pretty streamlined and refined. These are just one stark example, but you can do this with whole ecological assemblages of frogs in some cases. Madagascar is especially excellent for this, because the island was only naturally colonised by frogs five times, and yet you have practically every main 'type' of frog there, independently derived from one of these colonising lineages.

Yet, each of these generic brown frogs is also subtly different—and their skeletons reveal pretty fundamental differences unique to their lineages. Not to mention their totally distinct tadpoles!! And that is part of the key, too: although some parts of evolution can be predictable, no organism can separate itself from its own evolutionary past. There is always a degree of contingency involved.

I find these frogs to be a really elegant demonstration of this principle. And also, a nice demonstration of why it is fucking impossible to ID frog pictures you guys send me without a more or less precise location.

Common frog (Rana temporaria) tadpoles mass together in England

An overzealous male Grass Frog (Rana temporaria) amplexes a male Edible Frog (Pelophylax kl. esculentus), both family Ranidae, Stuttgart, Germany

Amplexus is the mating embrace of frogs.

The Grass frog is also called the Common Frog.

The Edible Frog is also called the Common Water Frog.

The Edible Frog is now understood to be the fertile hybrid of the Pool Frog (Pelophylax lessonae) and the Marsh Frog (Pelophylax ridibundus).