The Galápagos Islands and their Tortoise Residents
The Galápagos Islands are an archipelago consisting of sixteen volcanic islands located 600 miles west of Ecuador in the Pacific Ocean. They formed about 4 million years ago when a series of underwater volcanoes erupted, spewing up magma that cooled to form the cone-shaped islands. When the islands first formed they were devoid of life, but over time animal and plant species colonized them, producing the ecological communities that exist there today.
Colonization of the islands took place over time by several modes of transportation. Most of the reptiles and mammals that made it there (without the help of humans) probably rafted there on pieces of driftwood or vegetation. Plant seeds may have floated there by sea, drifted in the wind, or arrived in the guts of birds that could fly there. Transportation by any of these methods was certainly difficult. Only the strongest of seabirds could have made the flight, and anything that rafted or drifted there had to survive for several weeks on the open ocean.
One example of a creature that made the journey to the Galápagos is the tortoise. Tortoises from the mainland of South America (most likely relatives of the Chaco tortoise Geochelone chilensis) drifted to the Galápagos Islands and colonized almost every island in the archipelago. Each island has a unique habitat because of differing climate patterns, topography (number and size of volcanoes, other terrain anomalies) and species compositions. Some islands are dry and desert-like while others have extremely wet and lush highlands. At the time of colonization, the tortoises on each island were similar. However, over time, the tortoises with traits favoring survival and reproduction on their island were selected for (natural selection) and each island’s population of tortoises evolved into a new species so that fourteen species lived on fourteen different islands.
The Galápagos tortoises underwent severe population declines when humans colonized the islands. Not only were the tortoises killed for food and collected for pets and scientific specimens, but humans introduced rats, dogs, goats, and disease that all contributed to their demise. Only about 5-10% of the original population remains today. There are currently eleven different species, and they are distributed among nine islands. On a positive note, the Galápagos Islands are now a wildlife preserve and the tortoises are making a comeback.
Natural Selection as a Mechanism for Evolution
While evolution may be intimidating to some, the concept boils down to a simple truth: populations of organisms change gradually over time in response to genetic and environmental factors, such that species become differentiated from their ancestors. As a result, the diversity of life on the planet has differed throughout Earth’s history. Accepted almost universally by scientists today, the idea was a novel one in the mid-19th century. Although several mechanisms for evolution exist (random mutation, genetic drift, etc.), natural selection is by far the easiest to observe. Developed by the naturalist Charles Darwin in the decades following his expeditions to the Galápagos, the process of natural selection can be broken down into the following five basic steps, easily recalled using the acronym “VISTA”:
- Variation: Individuals in a population often have unique mixtures of traits, be it size, color, ability to ward off disease, or talent at attracting a healthy mate.
- Inheritance: Some of these traits are encoded in one’s DNA, and therefore passed from parents to offspring.
- Selection: If some parents have traits that better help them survive and reproduce, their variations will be the ones passed on to future generations.
- Time: As generations of successful offspring are reared, advantageous traits will become more common.
- Adaptation: As a result of this evolution (change over time in a population), some feature of the species makes it better suited to inhabit its environment.
As you teach natural selection to your students, keep this acronym in mind. Use the sequential nature of the steps to clarify the process or reverse misconceptions. It is important to remind the students that organisms don’t change or evolve during their lifetime. Evolution occurs over generations due to the inheritance of DNA passed on from parent to offspring.
Tortoise Specimens on Exhibit at the California Academy of Sciences Island Name, Resident Tortoise
Santiago Island, Santiago Tortoise: Geochelone darwini
Rábida Island, Rábida Tortoise: Geochelone wallacei
Santa Cruz Island, Santa Cruz Tortoise: Geochelone porteri
Española Island, Española Tortoise: Geochelone hoodensis
Isabela Island, Wolf Volcano, Volcán Wolf Tortoise: Geochelone becki