Charles Darwin and The Galapagos Islands: Birthplace of Evolutionary Theory

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Darwin found 13 varieties of finches in the Galapagos. The differences in their beaks attests to the fact beak size and shape was an adaptation to their diet. Image of John Gould, in the public domain (taken over 100 years ago).

Darwin found 13 varieties of finches in the Galapagos. Image by John Gould.

Charles Darwin’s birthday is today – his evolutionary theory is one of the cornerstones of anthropology: the study of humankind. It was in the Galapagos Islands, with their myriad natural anomalies, that Darwin developed what he called the theory of “natural selection,” which forms the basis of evolutionary theory, in all the sciences.

Darwin and Natural Selection

Darwin’s theory of natural selection rests on phenomena that are subject to direct observation and experimentation: In every species, there is a range of variation of specific traits (hair and skin color, for example.)

Not every individual in any given species reproduces. In a nutshell, Darwin’s theory states that those who do reproduce pass down their specific traits to their offspring. If a certain trait is more advantageous in a specific environment, then the carriers of that trait are more likely to survive into adulthood and to generate offspring than those who do not have that trait.

Cumulatively, over generations, the traits that are advantageous for a species will proliferate. Darwin originally termed this process “descent with modification.” By the late 1860s, he started to call it “survival of the fittest,” a term that is very familiar to the contemporary ear.

Darwin’s Finches

Perhaps the most significant evidence of natural selection that Darwin found on the Galapagos Islands was in the anatomy of the finches, whose variety piqued his curiosity. Darwin discovered that Galapagos finches have thirteen different kinds of beaks.   Through careful observation and consultation with an ornithologist, Darwin began to understand that the difference in the size and shape of the finches’ beaks was due to their adaptation to a particular environmental niche.

Specifically, the different kinds of beaks are adapted to differences in the food supply. For example, in niches where insects are abundant, the finches have developed long, thin beaks which they insert into tree bark in order to extricate their prey. Whereas, in areas where nuts are abundant, the finches’ beaks are shorter and sturdier, allowing the birds to crack the nuts open more easily.

Adaptive Radiation

Adaptive radiation is another example of natural selection at work. Adaptive radiation refers to the process in which a species evolves rapidly to exploit an uninhabited environmental niche. This principle explains some of the variety in physical traits of human beings, even though we are all members of the same species.

Galapagos Marine Iguanas

One of the most interesting examples of adaptive radiation is the case of the Galapagos marine iguanas. The only marine iguanas in the world, they are on almost every island of the archipelago, and number between 200,000 and 300,000.

Anthropologists believe that these iguanas arrived on the Galapagos by “island-hopping” from Central American jungles: that is, they floated on rafts of vegetation that drifted through the ocean, eventually ending up 600 miles from the coastline.

The Galapagos marine iguanas are the only marine iguanas known to exist. They are black, like the volcanic rocks on the coastline which they inhabit. Image courtesy of Leslie Cohen, the author.

The Galapagos marine iguanas are the only marine iguanas known to exist. They are black, like the volcanic rocks on the coastline which they inhabit. Image courtesy of Leslie Cohen, the author. All rights reserved.

Adaptation to their Specific Environment

The food supply is radically different on the Galapagos than in the jungles of Central America, and the only vegetation that the iguanas found on the Galapagos was seaweed and algae, which became the mainstay of their diet.

Adaptation to the Cold

As members of the reptile family, the marine iguanas are cold-blooded. This poses a problem, as their main sources of food are located deep in the ocean. And the water in the Galapagos archipelago is particularly cold, as the Humboldt current feeds into it.

The marine iguanas have developed several ways of coping with the cold. At night, they huddle together for warmth. More importantly, they have adapted their skin color: it is almost the same shade of dusty black as the volcanic rocks which they inhabit. Their dark coloration allows them soak up the sun’s heat. Maintaining their body heat is essential when they enter the cold ocean water to feed.

A Unique Adaptation to Danger in the Environment

Covered by sharp spikes all along their spines, the marine iguanas have few natural predators. A far greater threat is the extreme fluctuation in weather conditions, which recurs every 3-7 years. The El Nino current causes a radical warming of the ocean, which kills off the green and red algae that the iguanas thrive on, replacing it with a brown variety that they are unable to digest. Consequently, they die off in great numbers: 70% – 90% of their population may starve to death.

Tortoises with a dome-shaped carapace feed off wild grasses on the ground. Image courtesy of Leslie Cohen, the author.

Tortoises with a dome-shaped carapace feed off wild grasses on the ground. Image courtesy of Leslie Cohen, all rights reserved.

In the face of famine, the marine iguanas have evolved in yet another way: their bodies shrink in size. They are able to reabsorb part of their bony skeleton by digesting it,  so that their bodies can become as much as 20% shorter during the lean seasons.  Amazingly, the marine iguanas are able to grow back to their original size as their food supply increases. Adult iguanas can shrink and grow as often as necessary. Scientists believe that this ability to reabsorb  and regrow bone is unique to the Galapagos marine iguanas.

Tortoise Shells

Perhaps the most famous of the Galapagos animals is the tortoise, which gave its name to the archipelago. Darwin observed two different kinds of giant tortoises, distinguished by the shapes of their carapaces (shells).

One variety of tortoise has a dome-shaped carapace, while the other has a saddle-back shaped shell. As with the finches and the iguanas, scientists agree that the shape of the tortoise shells is an adaptation to  their diet. Tortoises are herbivores, subsisting on cactus, grass and fruits. Those who live in environmental niches where low-lying grasses are abundant have the dome-shaped carapace. Those living in regions where grasses are scarce and the diet necessitates grazing from trees have developed the saddle-shaped carapace which allows them to stretch their necks upward to feed.

Saddle-back tortoises can lift their heads to feed off vegetation from bushes and  trees. Images courtesy of Leslie Cohen, the author.

Saddle-back tortoises can lift their heads to feed off vegetation from bushes and trees. Images courtesy of Leslie Cohen, all rights reserved.

Ongoing Research in Evolution

Anthropologists view the Galapagos as a microcosm of evolution. The diversity of species found there, as well as their differences from animals of the same kind living in different areas of the world, is particularly enlightening.  Galapagos ecosystems provide a vast body of material for the study of evolution, with worldwide implications and applications. The Galapagos Conservancy regularly hosts groups of scientists from many different disciplines, and serves to protect the delicate environment of the Galapagos as well as to research it.

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© Copyright 2014 Leslie Cohen, All rights Reserved. Written For: Decoded Science
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