The bones in the forelimbs of terrestrial and some aquatic vertebrates are remarkably similar because they have all evolved from the forelimbs of a common ancestor. This is an example of homologous structures.

years. In evolutionary terms, dolphins are as distant from fish as are mice or humans. But they have evolved streamlined bodies that closely resemble the bodies of fish, sharks, and even extinct dinosaurs known as ichthyosaurs. These kinds of evidence from many different fields of biology allow evolutionary biologists to discern whether physical and behavioral similarities are the product of common descent or are independent responses to similar environmental challenges.

Evolution accounts for the geographic distribution of many plants and animals.

The diversity of life is almost unimaginable. Many millions of species live on, in, and above the Earth, each occupying its own ecological setting or niche. Some species, such as humans, dogs, and rats, can live in a wide range of environments. Others are extremely specialized. One species of a fungus grows exclusively on the rear portion of the covering on the wings of a single species of beetle that is found only in some caves in southern France. The larvae of the fly Drosophila carcinophila can develop only in specialized grooves beneath the flaps of the third pair of oral appendages of a land crab that is found solely on certain islands in the Caribbean.

The volcanic birth of the Hawaiian Islands in the Pacific Ocean over 2,000 miles from the nearest continent allowed one or a small number of windblown drosophilid flies such as the example pictured to evolve into more than 500 species in the islands’ specialized environments. This rampant speciation was made possible in part because many of the environments in which they evolved were largely free of insect competitors and predators.

The occurrence of biological evolution both explains this diversity and accounts for its distribution. Consider,



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