Sky Islands: one of Earth's great evolution laboratories

Let's think of places on Earth where scientists have done great big natural "experiments" on evolution.

Looking east from near the top
of Mount Lemmon, January 2021

Here are some that ought to come to mind (in no particular order other than the first):

1. The Galapagos Islands, with their famous finches and their less-famous tomatoes and all their otherworldly animals, probably belong at the top of the list. Mr. Darwin found inspiration there, but the greatest experiments began more than a century later. I can think of few more inspiring stories of great science done by great people than the lifetime-long work of Rosemary and Peter Grant. If you haven't yet read The Beak of the Finch, get thee to a library or a bookstore.

2. The Caribbean islands, and especially the Bahamas, are a fruitful laboratory for the study of the (rapid) evolution of lizards called anoles. Jonathan Losos was a major figure in launching and leading that subfield, and his 2017 book Improbable Destinies is a great lay-level exploration of evolution and a resounding rebuttal to the random/luck/contingency views associated with Stephen Jay Gould.

3. The Hawaiian Islands are home to hundreds of species of fruit fly (many of them are biology's adored Drosophila) and zero species of ant. These hundreds of species have all evolved in the last 25 million years! Check out the laboratory of Cassandra Extavour at Harvard for a glimpse into the latest research on the evolution of Hawaiian fruit flies.

4. The Great Lakes of East Africa (including Victoria and Tanganyika) hosted one of the most rapid and spectacular adaptive radiations known to humans. Hundreds (likely thousands) of species of cichlid fish live in these lakes, and all of them were "born" in a blink of evolutionary time.

5. The streams of Trinidad are home to guppies, an unremarkable fact until you learn about one of the best-known experiments in the history of evolutionary biology. Over many years, research teams led by John Endler and David Reznick used this natural laboratory to study natural selection (and other topics) in the wild.

There are surely more. But I'm here to tell you about the one that literally surrounds us here in Tucson.

Rincon Mountains,
another Sky Island near Tucson

The 55 Sky Islands of southern Arizona and northern Sonora are also called the Madrean Archipelago. Note first the emphasis on islands: these are patches of forest ecology perched on top of mountains that rise from deserts. To earn the title of "Sky Island", a mountain must be at least 3000 feet (915 m) tall and host a forest biome (an oak woodland, to be precise). Very near to where I live in Tucson, one of the most impressive Sky Islands rises to 9160 feet (2800 m) from a desert starting point of about 2600 feet (800 m). This is Mount Lemmon, the tallest peak in the Santa Catalina Mountains and the backdrop of Tucson itself.

It's a 45-minute drive from our home to the top of this mountain. That journey traverses eight biomes, beginning with desert scrub and ending at mixed conifer forest. It typically involves a drop of 30 degrees F (from, say 100 to 70) or 17 C (38 to 21 C). Rainfall differs by almost threefold.

The Santa Catalina Mountains
seen from east Tucson

The Sky Islands are beautiful and interesting, so much so that I recorded a set of little videos about a journey to Mount Lemmon last fall. But I want to focus on two other little things here.

First, the Sky Islands are... islands. That means that populations on these islands are relatively close to each other but isolated from each other by the "ocean" of desert that separates islands. Populations of pine trees, burrowing mammals, or flightless insects are almost completely isolated and are therefore developing and evolving separately. This is what makes any set of islands (like some of those mentioned above) a laboratory to study rapid evolution. So, there are ongoing research programs (like the Moore Lab at the University of Arizona, headed by Prof Wendy Moore) looking at evolution of populations on the various Sky Islands.

Second, the Sky Islands can help us see a simple error we might make when we think about natural history, and this error is relevant in my ongoing discussion of how we tend to think that evolution is a lot harder than it is. To reveal this potential error, just consider this question: how did the forests, with their plants and animals (and fungi and bacteria) get to the top of those mountains? We've established that the islands are isolated by desert. It's not hard to understand why a pine forest is happy at 9000 feet/3000 m but that's not the question. How did it get there?

The phrasing of the question contributes to its apparent mystery, because by asking it that way I have directed our attention to an active process, maybe some kind of improbable migration between one founder mountain and the others, or maybe the gradual accumulation of quasi-miracles like a pine cone being dropped by an eagle or something like that. The question tempts you to assume that the mountain rose up in the middle of a desert, and somehow a forest then grew there.

That's not what happened. The real story is great but it's much less extraordinary: the whole region was previously forested. Then the climate changed, and the region got hotter and drier, and the forests were "trapped" on top of mountains, where they'd always been, but now surrounded by deserts that weren't there before. It's just like the way Great Britain became an island.

Maybe the Sky Islands can help us avoid that error when we consider things like proteins in a vast library of possibility. Asking "how did it get there" is potentially misleading.

But mostly I hope to write more about the Sky Islands, to visit them more often, and perhaps to get involved in a research project. They're very inspiring!