mockingbirds on one island be different from that of a closely related mockingbird on an island only 30 miles away? And why were the various types of animals on these islands related, but distinct from, the animals in Ecuador, whereas those on the otherwise very similar islands off the coast of Africa were related to the animals in Africa instead?

Darwin could not see how these observations could be explained by the prevailing view of his time: that each species had been independently created, with the species that were best suited to each location on the earth being created at each particular site. It looked instead as though species could evolve from one into another over time, with each being confined to the particular geographical region where its ancestors happened to be—particularly if isolated by major barriers to migration, such as vast expanses of ocean.

But how could one species turn into another over the course of time? In constructing his hypothesis of how this occurred, Darwin was struck by several other observations that he and others before him had made.

  1. People who bred domesticated animals and plants for commercial or recreational use had found and exploited a great deal of variation among the progeny of their crosses. Pigeon breeders, for example, had observed wide differences in colors, beaks, necks, feet, and tails of the offspring from a single mating pair. They routinely enhanced their stocks for desired traits—for example, selectively breeding those animals that shared a particular type of beak. Through such artificial selection, pigeon fanciers had been able to create many different-looking pigeons, known as breeds. A similar type of artificial selection  

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