RE 9-10 SPECIES Millions of different species exist on the earth. These millions of species, which have evolved over billions of years, are the result of two distinct but simultaneously occurring processes: the processes of speciation and extinction. One of the processes that affects the number of species on earth is speciation, which results when one species diverges into two distinct species as a result of disparate natural selection in separate environments. Geographic isolation is one common mechanism that fosters speciation; speciation as a result of geographic isolation occurs when two populations of a species become separated for long periods of time into areas with different environmental condition. After the two populations are separated, they evolve independently; if this divergence continues long enough, members f the two distinct populations eventually become so different genetically that they are two distinct species rather than one. The process of speciation may occur within hundreds of years for organisms that reproduce rapidly, but for most species the process of speciation can take thousands to millions of years. One example of speciation is the early fox, which over time evolved into two distinct species, the gray fox and the arctic fox. The early fox separated into populations which evolved differently in response to very different environments as the populations moved in different directions, one to colder northern climates and the other to warmer southern climates. The northern population adapted to cold weather by developing heavier fur, shorter ears, noses, and legs, and white fur to camouflage itself in the snow. The southern population adapted to warmer weather by developing lighter fur and longer ears, noses, and legs and keeping its darker fur for better camouflage protection. Another of the processes that affects the number of species on earth is extinction, which refers to the situation in which a species ceases to exist. When environmental conditions change, a species needs to adapt to the new environmental conditions, or it may become extinct. Extinction of a species is not a rare occurrence but is instead a rather commonplace one: it has, in fact, been estimated that more than 99 percent of the species that have ever existed have become extinct. Extinction may occur when a species fails to adapt to evolving environmental conditions in a limited area, a process known as background extinction. In contrast, a broader and more abrupt extinction, known as mass extinction, may come about as a result of a catastrophic event or global climatic change. When such a catastrophic event or global climatic change occurs, some species are able to adapt to the new environment, while those that are unable to adapt become extinct. From geological and fossil evidence, it appears that at least five great mass extinctions have occurred; the last mass extinction occurred approximately 65 million years ago, when the dinosaurs became extinct after 140 million years of existence on earth, marking the end of the Mesozoic Era and the beginning of the Cenozoic Era. The fact that millions of species are in existence today is evidence that speciation has clearly kept well ahead of extinction. In spite of the fact that there have been numerous periods of mass extinction, there is clear evidence that periods of mass extinction have been followed by periods of dramatic increases in new species to fill the void created by the mass extinctions, though it may take 10 million years or more following a mass extinction for biological diversity to be rebuilt through speciation. When the dinosaurs disappeared 65 million years ago, for example, the evolution and speciation of mammals increased spectacularly over the millions of years that ensued.