Up Close and Personal with Natural Selection in Action: The Tale of Two Islands of the Galapagos

Marine iguanas are one of the better examples of adaptation in the Galapagos. All photos credit: Collin Dunn

Each of the islands in the Galapagos is incredibly different. From landscape to ecosystem to the endemic species that can only be found in that one tiny spot, the diversity of life and living systems is truly amazing. It's one thing to say that -- "Sure," you think to yourself, "Different islands, different stuff, I get it" -- but really a totally different experience to see it for yourself. The smallest change, whether it's the few nautical miles in between two islands, or the way an individual island's orientation affected it's geological history, has a huge impact that you can see from the top of the food chain on down.

Still, I think you really have to see it to believe it. Here's a quick tour -- a small glimpse into the myriad examples of natural selection in action -- of two islands we visited on the same day.

Evidence of it's volcanic history is prevalent, but Isabela is one of the greener islands.

Isabela, at Urbina Bay

Isabela is the largest island in the Galapagos, with a land area of about 4600 square kilometers (almost 1800 square miles). It was formed by five active volcanoes, and one, Volcano Wolf, is the highest point in the Galapagos.

Because of these factors (and more), it's a very distinct, unique place. It's one of the few islands that has lots of vegetation, everywhere from the shore inward; this is owed to it's age. It's been around longer than many other islands, which has given the island time for two things: Break down the volcanic rock into soil; and allow species to evolve that can thrive in the harsh, rocky land.

Yellow cordia is one of the many, many yellow or white-flowered plants on Isabela.

A whopping 95 percent of the native flowers on the island are either white or yellow. They've evolved those features because there aren't very many insects on the island, so plants don't have to work so hard to compete for their attention, so they can focus their efforts on growing and not so much on the flowers themselves. One of our group members was wearing a white shirt, and she was very popular with the big Galapagos bumblebees that we saw (and who subsequently followed us around the trek on the island).

Galapagos cotton is a smaller, less thirsty version of its more well-known cousin.

The Galapagos cotton also grows here, but has evolved to require little water to grow -- Isabela, like many islands in the Galapagos, doesn't have much freshwater, despite being surrounded by ocean. It's smaller that the cotton we're used to seeing in the U.S., but has attracted the attention of cotton growers around the world for potential hybridization with more water-depending versions of the crop.

The land iguana has a pretty long, and very interesting, history here.

Isabela is also home to land iguanas, who have their own pretty unique story. Iguanas arrived on the islands several million years ago, courtesy of a floating raft of vegetation and the southerly Panama current, from somewhere around the Amazon basin. When they bumped up against Isabela, the island was still relatively young, and the abundance of hard volcanic rock (and lack of soil) meant there wasn't much to eat on land. So they evolved the ability to swim, stay under water for quite a time, and eat and digest seaweed, algae, and other sea-bound plant life (more on them below).

Untroubled by unnatural influences (way before any humans plodded about the islands, or sailed in the adjacent waters), the iguanas lived a largely aquatic life, until about 2 million years ago, when Isabela's rock started to soften into soil, and plants started to find a way to live there. As plant life on land increased, so did the iguana's ability to live there, and, just like that, the formerly marine iguanas were on land. Their colors changed from dark grey and black to beige, orange, and red, and they got bigger, as they no longer had a reason to be svelte in the water.

We didn't see any marine iguanas at Urbina Bay, in Isabela, and, though it lies just a but to the west, we didn't see any land iguanas on Fernandina, later that day. Fascinating.

Fernandina has one of the larger Marine iguana populations in the Galapagos.

Fernandina, at Espinosa Point

Contrary to our experience on Isabela, Fernandina is sparsely vegetated, and by mostly mangroves in a ring around the island. 40 percent on the island is vegetated, while 60 percent is lava. And, while Isabela is mostly free of invasive wild goats, many invasive rats still remain (targeting the eggs and babies of both iguanas and tortoises); Fernandina is what is referred to around here as a "pristine island" -- there aren't any known invasive plants, animals, viruses, etc. There also aren't any humans that live there.

Sally Lightfoot crabs and Marine iguanas live happily together on Fernandina island.

The island is home to Galapagos hawks, Pacific Green sea turtles, Sally Lightfoot crabs, and Marine iguanas (among many more). The iguanas are one of the best examples of adaptation in all the islands. Like I mentioned above, they learned to eat algae, learned to dive in the water (the colder temperatures carry more nutrients, which makes a better home for the algaes they like to eat), and learned to stay down there for 40 to 60 minutes; their heart rate slows to about 25 percent it's normal rate, at about 7 or 8 beats per minute. They aren't as active as they are on land, and don't move quite as fast, but can search for and feed on algae the whole time they're down there.

The iguanas used to use their kidneys to filter the salt from the seawater they drank, but that proved a bit too biologically challenging. So they developed a gland in their necks, and they use that to sort of spit out the salt they don't need in their bodies; if you ever see a video of a Marine iguana doing something that looks like spitting, it's actually that salt-dispensing maneuver. This in-body desalinator technically makes them both a land and marine animal, but they only feed under water.

It's pretty easy to see where pāhoehoe (on the left) starts and ʻaʻā (on the right) ends.

Fernandina's relative young age also makes it easy to see how it was formed. Two types of lava flows -- ʻaʻā and pāhoehoe -- came together in several places on the island. Pāhoehoe (say it with me: pa hoy hoy) is the dominant type. It rolls out from underground at about 1600 degrees Celsius, and covers about 70 percent on the island. ʻAʻā (ah ah) is much cooler as lava, so it loses any head-to-head battles with the pāhoehoe, and only covers about 30 percent of the island. Pāhoehoe dries in a pretty smooth, matte finish; ʻaʻā is sharp, chunky, and looks crusty up close.

Perhaps in another few million years, the dividing lines will be less distinct, and the relatively young lava of today will be replaced by volcanic soils and home to a host of plants that aren't anywhere to be found today. What those will be is anyone's guess, but that's sort of the beauty of natural selection: That it will find the best, most effective way to move forward.

More on the Galapagos
5 Things Everyone Should Know About the Galapagos: An Introduction
What is Really Being Done to Save the Galapagos?
Are the Galapagos Islands Ready for More Tourism?
Should The Galapagos Be Taken Off The Endangered Sites List?
Weird and Wonderful Galapagos Wildlife Worth Saving (Slideshow)

Tags: Animals | Conservation | Galapagos

Best of TreeHugger