for example, the drosophilid flies of the Hawaiian Islands. More than 500 species of flies belonging to the genera Drosophila and the closely related Scaptomyza exist only in Hawaii. These Hawaiian species comprise about a quarter of all the species in these genera worldwide, and far more species than are found in a similar-sized area anywhere else on Earth. Why do so many different kinds of flies live exclusively in Hawaii?

The geological and biological history of Hawaii provides an answer. The Hawaiian Islands consist of the tops of mid-ocean volcanoes and have never been connected to any body of land. The islands formed as the Pacific tectonic plate moved over a “hot spot” where upwelling molten rock from the Earth’s interior heats the Earth’s crust. The newest islands are the tallest, while older islands progressively erode and eventually sink beneath the water. Thus, the oldest landmass in the chain, Kure Atoll, rose from the Pacific about 30 million years ago, while the youngest, the “Big Island” of Hawaii, is only about 500,000 years old and still has considerable ongoing volcanic activity.

All of the native plants and animals of the Hawaiian Islands — that is, those existing on the islands before the arrival of humans 1,200 to 1,600 years ago — are descended from organisms that made their way through the air or the water from the surrounding continents and from distant islands to the initially barren islands. In the case of the Hawaiian drosophilids, several lines of evidence, especially from DNA, indicate that all of the native Drosophila and Scaptomyza species are descended from a single ancestral species that colonized the islands millions of years ago.

These initial colonizers encountered conditions that were very favorable to rapid speciation. Individual species repeatedly served as ancestors for multiple other species as groups of flies occupied habitats with different elevations, precipitation, soils, and plants. In addition, small groups of flies — or in some cases perhaps a single fertilized female — periodically flew or were carried to other islands, where they gave rise to new species. Many new species were able to occupy ecological niches that on the continents already would have been filled by other species. For example, many Hawaiian drosophilids lay eggs in decaying leaves on the ground, an ecological niche that is filled by insects and other organisms on the continents but in the Hawaiian Islands was almost empty.

[Speciation: The evolutionary processes through which new species arise from existing species.]

The mammals that have lived in North and South America provide another good example of how evolution accounts for the distribution of species. These two continents were connected as part of a much larger landmass during the early evolution of the mammals. But the breakup of that landmass caused North and South America to separate, after which their respective mammals evolved in different directions. The mammals that evolved in South America include such modern-day groups as anteaters, sloths, opossums, and armadillos, according to the fossil record. In North America, horses, bats, wolves, and



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