Evolution

C

Gradual Change

Speciation may occur even when no isolating mechanism is present. In this case, a new species may form through the slow modification of a single group of organisms into an entirely new group. The evolving population gradually changes over the course of generations until the organisms at the end of the line appear very different from the first. Foraminifera, a tiny species of marine animals that live in the Indian Ocean, demonstrate this process, known as vertical or phyletic evolution. From about 10 million to 6 million years ago, the species remained relatively unchanged. These organisms then began a slow and gradual change, lasting about 600,000 years, that left them so unlike their ancestors that biologists consider them an entirely new species.

VI

Patterns of Descent

Whatever the cause of their reproductive isolation, independently evolving populations tend to adhere to general patterns of evolutionary descent. Most often, environmental factors determine the pattern followed. A gradually cooling climate, for example, may result in a population of foxes developing progressively thicker coats over successive generations. This pattern of gradual evolutionary change occurs in a population of interbreeding organisms evolving together. When two or more populations diverge, they may evolve to be less alike or more alike, depending on the conditions of their divergence.

A

Divergent Evolution

In the pattern known as divergent evolution, after two segments of a population diverge, each group follows an independent and gradual process of evolutionary change, leading them to grow increasingly different from each other over time. Over the course of many generations, the two segments of the population look less and less like each other and their ancestor species. For example, when the Colorado River formed the Grand Canyon, a geographic barrier developed between two populations of antelope-squirrels. The groups diverged, resulting in two distinct species of antelope-squirrel that have different physical characteristics. On the south rim of the canyon is Harris’s antelope-squirrel, while just across the river on the north rim is the smaller, white-tailed antelope-squirrel.

B

Adaptive Radiation

Sometimes divergence occurs simultaneously among a number of populations of a single species. In this process, known as adaptive radiation, members of the species quickly disperse to take advantage of the many different types of habitat niches—that is, the different ways of obtaining food and shelter in their environment. Such specialization ultimately results in a number of genetically distinct but similar-looking species. This commonly occurs when a species colonizes a new habitat in which it has little or no competition. For example, a flock of one species of bird may arrive on some sparsely populated islands. Finding little or no competition, the birds may evolve rapidly into a number of species, each adapted to one of the available niches. Charles Darwin noted an instance of adaptive radiation on his visit to the Galápagos Islands off the coast of South America. He surmised that one species of finch colonized the islands thousands of years ago and gave rise to the 14 species of finchlike birds that exist there now. Darwin observed that the greatest differences in their appearance lay in the shapes of the bills, adapted for their mode of eating. Some species possessed large beaks for cracking seeds. Others had smaller beaks for eating vegetation, and still others featured long, thin beaks for eating insects.

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C

Convergent Evolution

Sometimes distantly related species evolve in ways that make them appear more closely related. This pattern, known as convergent evolution, occurs when members of distantly related species occupy similar ecological niches. Natural selection favors similar adaptations in each population.

Some of the best examples of convergent evolution are the marsupial mammals of Australia and their placental mammal counterparts on other continents. About 50 million years ago the Australian continent separated from the rest of the Earth’s continents. Biologists speculate that few if any placental mammals had migrated to Australia by the time the continents split. They also surmise that neither marsupial mammals, nor their placental counterparts were capable of crossing the ocean after the landmasses drifted apart. As a result, the animals evolved entirely independently. Yet many of the marsupial mammals in Australia strongly resemble many of the placental mammals found on other continents.

For example, the marsupial mole of Australia looks very much like the placental moles found on other continents, yet these animals have evolved entirely independent of one another. The explanation for the moles’ similar appearances lies in the principles of convergent evolution. Both species evolved to exploit similar ecological niches—in this case, the realm just beneath the surface of the ground. Over the course of millions of generations in both marsupial and placental moles, natural selection favored adaptations suited for a life of burrowing: tube-shaped bodies, broad, shovel-like feet, and short, silky fur that sheds dirt or sand easily. The most striking difference between placental moles and marsupial moles is the color of their fur. Placental moles are usually dark brown or gray, a coloration that enables them to blend in with the soil in their habitat. Marsupial moles burrow in the golden or reddish sand of Australia, so natural selection produced golden or golden-red fur.

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