Selection and Adaptation

Natural Selection

Charles Darwin was a naturalist who, from 1831 to 1836, traveled around the world on the sailing ship the HMS Beagle. Throughout the trip he collected fossils and made detailed observations about the types of plants and animals he saw, as well as the habitats in which they lived.

Darwin’s travels took him to a group of small islands off the west coast of South America known as the Galapagos Islands. Although the individual islands are relatively close to one another, they have different climates and different species of plants. Darwin observed that some islands were hot and dry with few plants, while others received more rainfall and had a larger variety of plants. Darwin collected information about many species of organisms. In particular, he made extensive observations of giant tortoises that lived on the islands. Darwin noticed that the shells of the tortoises were different on each island. For example, one type of tortoise, the saddle back tortoise, had a long neck and, a curved shell, while a tortoise on a different island, the domed tortoise, had a shorter neck and a shell that looked like a dome.

In addition, Darwin observed 13 different species of finches, a type of small bird. Each species of finch had a different-shaped beak. The shape of the beak was related to the type of food available on the island where the bird lived.

Finches

These are some of the finches Darwin observed on the Galapagos Islands. Notice how their beaks are different. The shape of the beak is related to the food available where each finch lives.

Darwin’s Theory

After returning to his home in England, Darwin spent many years trying to explain the observations he had made. In 1859, Darwin presented his theory called natural selection in a book titled On the Origin of Species by Means of Natural Selection. It is based on the idea that most organisms produce more offspring than are able to survive. The offspring must compete with each other for food, water, and space. Some offspring have variations, or traits, that will make them better able to survive. These variations can be inherited and are known as favorable variations. Organisms that inherit favorable variations are said to be more “fit” than others.

Those organisms that are fit are more likely to live to reproduce (this is sometimes known as survival of the fittest). When organisms survive to reproduce, they pass the favorable variations on to their offspring. Over time, the favorable variations are found in more and more members of the species. In this way, the traits of a species evolve. Sometimes the changes are so great that a new species is formed. This evolutionary process by which new biological species arise is called speciation. Natural selection can be used to explain changes in most species over time.

Natural Selection

Artificial selection is the process by which humans breed other animals and plants for particular traits. In his book, Darwin discussed how selective breeding produces change over time, using examples of selective breeding of pigeons, cats, cattle, and dogs. Use of selective breeding to produce plant and animal strains is common in agriculture. Darwin used selective-breeding changes to support his theory of natural selection.

Adaptations

The theory of natural selection explains what Darwin observed about tortoises and finches. Darwin suggested, for example, that all the tortoises evolved from a common ancestor. The domed tortoise, which generally has a short neck and a shell that does not allow it to stretch its neck easily, was found on an island with plenty of plants. It didn’t need to stretch its neck to reach food, so it thrived on this island.

The saddleback tortoise lived in a dry habitat with fewer plants. It needed to stretch to reach taller plants. In this environment, tortoises with long necks and shells that were open around the legs and neck were favored by natural selection. As a result, they were more likely to survive and reproduce.

Over many generations, the adaptations-a saddleback shell and a long neck-increased within this species. An adaptation is any trait that helps an organism survive and reproduce in its environment. Eventually only saddleback tortoises could be found on the dry, sparsely vegetated islands. The finches could be explained in a similar way. A single species of finch, which came from nearby South America, originally inhabited the islands.

Each island provided a different sour.ce of food. Some birds needed a strong beak to crack open seeds. Others needed a narrow beak to reach into plants for food. Natural selection favored those birds with the variation that made them better able to obtain food on their island.

Adaptations

Think about Science

Directions: Answer the following questions.

  1. Why might farmers use artificial selection when they plant crops from one year to the next?

There are many examples of adaptations that help organisms survive in their environment. An example is something or someone that represents a whole group. A polar bear has thick fur and a layer of fat to keep it warm in its cold surroundings. The only heat a polar bear gives off comes from its breath.

Another adaptation the polar bear has is its large feet, which help the bear paddle through water and spread out the bear’s weight so it can walk safely on ice while hunting seals.

Plants, too, have adaptations to help them survive. The saguaro cactus lives in the very dry environment of the desert. This plant can reach heights of up to 12 m and can live to be 200 years old. The cactus has a thick, waxy stem with ribs that can expand to store water. The roots of the cactus do not grow deep into the soil. This makes it possible for the cactus to take in as much water as it can when it rains. The plant then saves some water for periods when there is no rain.

Protection from Predators

It is a hot afternoon on the Serengeti. A hungry pride of female lions is watching a group of zebras. As the zebras move, their stripes blend together. It is difficult for the lions to pick out a single zebra because they cannot tell how many there are or exactly how far away they are. Unless one zebra strays from the crowd, the lions will go hungry-for now. Stripes on a zebra are an example, or illustration, of an adaptation. The mixture of stripes created when many zebras stand together is an adaptation that helps each zebra survive. Like a zebra’s stripes, many adaptations protect an organism from predators. Some adaptations help organisms hide from predators, while others, such as strong odors, encourage predators to stay away. Still other adaptations, such as an antelope’s fast speed, help prey escape from predators.

Camouflage

Some organisms blend in with their environment. For example, the color and slow movement of a stick insect might make it look like a twig on a tree. Camouflage is the ability to blend into the surroundings.

Mimicry

Mimicry is the ability of a harmless organism to look like a different, often more dangerous, organism. King snakes, which are harmless, and coral snakes, which are dangerous, have similar coloring. Because the king snake, shown on the left, looks similar to a coral snake, predators are less likely to try to consume it.

Snake

Protective Covering

Porcupines are one example of an animal with a protective covering. The pointy quills that cover its body protect it from predators. Some plants have sharp needles for the same reason.

Warning Colors

The bright blue poison arrow frog does not blend into its environment at all. In fact, its bright coloring invites predators to notice the frog. Like the poison arrow frog, many brightly colored organisms are poisonous, and the bright colors act as a warning to predators not to eat them.

Predator Adaptations

Like prey, predators have adaptations that help them survive. These include any traits that help them detect and catch their prey. Some predators, such as owls, have excellent vision, which helps them spot a tiny mouse among the leaves on the forest floor. Most species of owls have outstanding night vision as well. In addition, their feathers direct sound toward their highly sensitive ears, which helps them find prey that might otherwise stay hidden. Owls have sharp, powerful talons on their feet they use to grab and hold on to prey until it suffocates.

Female lions have a keen sense of smell to help them detect if prey is nearby. They also have excellent night vision, which they use to hunt for prey in the dark Female lions can reach speeds of about 50 kilometers per hour for short distances, which helps them catch their prey. Once they do, they have large, sharp teeth to hold and kill the prey.

Other animals, such as Burmese pythons, have heat sensors in their top lips. Along with a keen sense of smell, the sensors help them find prey, such as warm-blooded mammals. They also have jaws with a loose hinge. Once a snake has captured its prey, this loose hinge makes it possible for the snake to open its mouth wide enough to fit the prey inside.

Predator Adaptations

Think about Science

Directions: Answer the following questions.

  1. How could the adaptations of the giant tortoises of the Galapagos Islands be explained through natural selection?
  2. What adaptations might an animal that lives in or near the water, such as a crocodile, have to help it survive?
  3. What adaptations might birds have evolved to help them survive?

Speciation

The process by which a new species of organism arises from a parent population is called speciation. Speciation can occur in more than one way.

Allopatric Speciation

When populations of the same species become isolated from each other geographically, they can evolve distinct characteristics. During his time in the Galapagos Islands, Darwin noticed that finches varied from island to island. He later learned that each finch was a separate species that had evolved from a common ancestor. The birds had diversified to take advantage of different food sources on each island. This is called allopatric speciation. It is a process in which populations of organisms are impacted by natural selection and diversify rapidly into new forms when a change in an environment makes new resources available.

Sympatric Speciation

Sometimes new species arise from populations that are not geographically isolated. Even though the two groups have a range that overlap, they still evolve into distinct species. This is called sympatric speciation. The best-known examples of this are cichlid fishes in the lakes of East Africa. There are at least 800 species, all of which have diversified from a common ancestor. Each species occupies a distinct niche. In Lake Victoria, more than 300 species of cichlid fish from one parent species.

Adaptive Radiation

Darwin’s finches, discussed earlier, are examples of adaptive radiation. The 13 different species of finches all evolved from a common ancestor and have adapted different beaks so that each of them is able to consume a different type of food. The marsupials in present-day Australia also evolved from a common ancestor. Kangaroos evolved powerful legs that allow them to travel over vast areas in search of food. Koalas’ smaller size, centralized center of gravity, and climbing ability have allowed them to climb in order to reach the eucalyptus leaves they eat.

Cichlid Fishes

Think about Science

Directions: Answer the following questions.

  1. What is speciation?
  2. What is the main difference between allopatric speciation and sympatric speciation?