How Darwin’s Findings In Galapagos Contributed To His Theory Of Natural Selection

Charles Darwin Photograph c. 1881
Charles Darwin Photograph c. 1881

In 1835, not long before arriving at the Galapagos Islands, Charles Darwin wrote a letter to his friend and mentor John Stevens Henslow. “In a few days’ time,” he wrote, “the Beagle will sail for the Galapagos Islands. I look forward with joy and interest to this, both as being somewhat nearer to England and for the sake of having a good look at an active volcano.”

At this time, as his letter suggests, Darwin was more focused on geology than biology, his excitement piqued to a greater extent by a volcano than by the wildlife he was about to see during his five-week trip to the Galapagos. But as we now know, it was the wildlife of the Galapagos that would ultimately have the most profound impact on Darwin and his work, contributing greatly to his ideas of “descent with modification,” the “survival of the fittest” and the scientific theory of evolution.

For Darwin, the Galapagos Islands offered a kind of laboratory that would shape his future work. He saw how creatures on one island had become different from similar creatures on another isolated island, and how they had seemingly adapted to their different environments – ideas that challenged the existing notion of the stability of species.

Galapagos Giant Tortoise, Santa Cruz Island, Galapagos
Galapagos Giant Tortoise, Santa Cruz Island (Photo Credit: Annie Baggett)

After two weeks in the Galapagos, Nicholas 0. Lawson, the vice-governor of the archipelago, told Darwin that giant tortoises differed on each of the islands. For example, a tortoise with a rounded front to its shell came from a well-watered island with lush ground cover, whereas a tortoise from a drier island had a peak at the front of its shell, allowing it to better reach up to higher vegetation.

At the time, as Darwin freely admits, he didn’t pay sufficient attention to this information. He later wrote: “I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted.”

Galapagos Mockingbird, Genovesa Island
Galapagos Mockingbird, Genovesa Island

Darwin did, however, notice a strange difference among the mockingbirds of the Galapagos. The specimens he collected on Charles (Floreana Island) differed from those he had previously collected on Chatham (San Cristóbal Island). He then paid special attention to the mockingbirds of other islands, carefully separating his specimens for later examination. Darwin wrote: “These birds agree in general plumage, structure, and habits; so that the different species replace each other in the economy of the different islands. These species are not characterized by the markings on the plumage alone, but likewise by the size and form of the bill, and other differences.”

Marine Iguanas, Santiago Island, Galapagos
Marine Iguanas, Santiago Island

Marine iguanas also caught the attention of Darwin, both for their particular adaptations and for what Darwin considered their hideous appearance. Iguanas likely arrived on the Galapagos Islands after drifting across the ocean on rafts of vegetation. Over time, some then adapted to the marine environment. They learned how to swim to feed on seaweed and algae, becoming the only species of lizard to forage in the sea. Being cold-blooded, they also had to adapt to survive in the cool waters of the archipelago.

Darwin was intrigued by these adaptations, and the marine iguana’s “perfect powers of diving and swimming.” But he also found them quite repulsive. He later referred to the marine iguana as “a hideous-looking creature, of a dirty black colour, stupid and sluggish in its movements.” He called them “disgusting clumsy lizards” and frequently referred to them as “imps of darkness.” So while they played a role in Darwin’s theory of evolution, it’s safe to say he wasn’t a fan.

Darwin took a large collection of specimens from the Galapagos, including finches from the various islands. He had given these birds little consideration during his stay, and on his voyage home continued to think primarily of the mockingbirds and tortoises and the questions they raised about the supposed stability of species. But the finches would become a key part of his work once he returned to England.

Common Cactus Finch, Santa Cruz Island, Galapagos
Common Cactus Finch, Santa Cruz Island

Upon his return, Darwin presented many of his specimens to the Zoological Society of London. He gave his bird specimens, including the finches, to the famous English ornithologist John Gould, and his findings were pivotal.

Gould found that there were in fact thirteen species of ground finches brought from the Galapagos, each similar but with notably different beaks. The beaks of “Darwin’s finches,” as they later became known, would play an important role in his idea of the transmutation of species. They helped Darwin to understand — and ultimately prove — that the difference in the size and shape of the beaks was due to an adaptation to a particular environmental niche within the Galapagos archipelago. Ideas such as these, along with Darwin’s other findings in the Galapagos, would be central to his theory of natural selection.

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