Natural Selection in the Galápagos (Riya Johnson)

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Introduction

When you hear the term “survival of the fittest,” what do you think? You may know that according to the theory of natural selection, nature favors organisms that are well-adapted to their environments, so these organisms survive longer. However, a misconception about this term is that evolutionary success is determined by the ability to survive when, in reality, it is determined by the ability to reproduce. Imagine two sea lions in the Galápagos. One lives to be 10 years old and produces zero offspring while the other lives to be seven and produces two. Which is more successful in an evolutionary sense? The latter. Although the former lived longer, its genes died with it, for it did not pass them on to any offspring.

Selective Pressures

Predation

After I better understood natural selection, I began observing this mechanism of evolution in the Galápagos. Natural selection is driven by selective pressures, limitations in a given environment that cause some members of a population to thrive and others to flounder. A common selective pressure is predation. However, in the Galápagos, many animals do not have predators. To clarify, when animals such as tortoises and crabs are babies, they are vulnerable to predators, but once they mature and develop harder shells, they can live independently without fearing predation. Likewise, iguanas evolved from a South American ancestor that used the spines along its back as protection. Once the iguanas migrated to the Galápagos, they did not need this defense mechanism due to the lack of predators and competition, so their spines became softer and smaller.

Food

Nevertheless, Galápagos fauna suffer daily due to the lack of food and the difficult conditions. One of the primary causes of the food deficit is introduced species that compete with endemic ones for food. For example, farmers introduced goats to the islands, but these animals disrupt the equilibrium of the ecosystem. Since they eat all the food, there is none left for endemic animals. Some islands have eradicated the goats, but there were still some on San Cristóbal Island when we visited.

Food is such a significant limitation that only 30% of sea lion babies fully mature. Additionally, there is a small population of about 25 penguins on Floreana Island, and in a lagoon on Rabida Island where little food is available, only three-to-four of flamingos’ eggs hatch.

Adaptations to the Food Deficit

Nazca Boobies

Due to this selective pressure and the low chance of survival, animals have increased the number of offspring they produce. Nazca boobies lay a maximum of two eggs at a time, but inevitably, only one baby survives, a fact known as mandatory siblicide. They produce the second egg in case the first one does not hatch.

Sea Lions

Female sea lions exhibit delayed implantation, which they likely inherited from their ancestors, for it is imprinted in their genes. Mating occurs at the same time each year, but sperm float in females’ uteri for about three months until fertilization occurs, and the gestation period is nine months. Females give birth in the spring, when food is more abundant, and become receptive at the end of the summer, when males are usually stronger and more competitive. Additionally, females almost always have one baby at a time because producing an egg requires a lot of energy. However, two weeks after delivery, they have another baby due to the low survival rate.

Albatrosses

When I saw a baby albatross, I noticed that it had trouble walking. I soon learned that albatrosses’ parents feed them so much that, in around October to November, babies can weigh up to 2 lbs more than their parents. This behavior has developed because parents leave their babies unfed for extended periods of time while foraging for food.

Tortoises

Tortoises display other adaptations to the food scarcity. Two species of tortoises in the Galápagos are the dome shell and saddleback. Dome shells live at very high elevations with lush greenery. Their shells are adaptations to wet areas, where there is more food and it is located closer to the ground. In contrast, saddlebacks live in lowlands, where the islands are flat. They have developed longer necks and limbs to reach vegetation in desert areas (as well as to walk on the rocks).

Evolution Gone Wrong

Evidently, natural selection favors genetic adaptations (such as sea lions’ delayed implantation and tortoises’ body shapes) as well as behavioral adaptations (such as higher offspring production and albatrosses’ preemptive feeding) because they help organisms withstand the selective pressure of food scarcity.

Yet evolution can also go wrong. Birds have oil glands, known as preening glands, to make their feathers waterproof. Seabirds, such as boobies, use their glands much more, for they dive frequently. However, frigate birds cannot dive because, for some reason, they are the only seabirds to lose their preening glands. Thus, frigate birds developed a survival tactic: skillfully picking food from the water’s surface and attacking other birds for it. No process is perfect, but the fact that frigate birds adapted after losing their oil glands shows that natural selection can correct its mistakes!

Sources

  1. Thank you to our Abercrombie & Kent guide in the Galápagos for sharing the information upon which this post is based.
  2. Charles Darwin Research Station