Evolution

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Evolution is the study of how modern organisms have descended from the earliest life-forms and of the genetic, structural, and functional modifications of a population that occur from generation to generation. The ability of a population of organisms to respond to change in their environment and survive and reproduce by developing the characteristics or modifications necessary for survival is known as adaptation. Understanding how life evolves is a central concept in biology. The incredible diversity of living organisms that exists and their adaptations to their environment are the direct result of evolution that has occurred over very long periods of time.

Many scientists have contributed to our modern-day understanding of evolution. One of the early pioneers of evolution was the French biologist Jean Baptiste de Lamarck. Working in the early 1800s, Lamarck compared fossilized forms of invertebrate organisms and noted that younger fossils showed advanced structural characteristics compared with older fossils. Lamarck suggested that younger fossils displayed adaptations, or modifications of structures, that were indicative of an increase in an organism's complexity. He proposed that these modifications arose from the use or disuse of body parts. He also hypothesized that certain structural features developed in response to an organism's environment and that many of these acquired characteristics could be inherited by offspring.

One classic example of such modifications suggested by Lamarck involves the long neck of giraffes. He proposed that the earliest giraffes were relatively short-necked animals that were forced to stretch their necks to eat their desired food, leaves that grew at the tops of trees. Over time, such stretching would produce animals with small increases in the length of their necks. This trait would be passed on to future offspring, which would also show increased neck length as they searched for food. Over many generations, these adaptive changes would produce giraffes with a greatly increased neck length compared with their early ancestors. At the time, however, a biological basis for Lamarck's hypotheses remained unclear.

The English naturalist Charles Darwin is generally recognized as the father of evolution. Many of Darwin's now-famous hypotheses were developed as a result of his 1831 voyage on HMS Beagle, a ship commissioned by the British government. In this historic voyage, the Beagle traveled to many coastal areas of South America. The location that resulted in Darwin's most important observations was the Galapagos Islands in the South Pacific Ocean, off the northwestern coast of South America. Darwin studied populations of Galapagos tortoises and observed their feeding habits regarding the fruit of prickly pear cactuses. Darwin noticed differences in tortoise variety and prickly pear cactus growth that appeared on the various islands of the Galápagos. For example, he observed that on islands without tortoises, prickly pear cactuses grew fruit very close to and, in some cases, on top of the ground. But on islands with tortoises, the prickly pear cactuses showed tall trunks that elevated the fruits of the cactus out of the reach of ground-dwelling tortoises. These observations led Darwin to consider interrelationships between tortoises and their food source. He also observed that many of the different islands of the Galapagos contained distinctly different varieties of finches. It was from Darwin's observations of finches that many of his pioneering theories in evolution by natural selection were developed. Most of these theories continue to provide the basis for our current-day understanding and study of evolution.

Several years after Darwin's initial voyage to the Galápagos Islands, he developed several hypotheses that he felt explained how diverse species of tortoises and finches may have developed on the different islands. He realized that although the populations of these...
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