might have developed new mating behaviors. At the same time, this population could acquire chromosomal inversions that would distinguish it from the original population of flies.

At some point, the daughter population would have diverged so greatly from the original population that its members generally would not recognize courtship signals or body markings from the original population. As a result, individuals from the isolated population would no longer mate and produce offspring with flies from the original population. If, as time progressed, physical and behavioral traits became sufficiently different between the two populations so that individuals from the two populations could no longer produce viable or fertile offspring, the daughter population would be considered a new species.

This speciation process has occurred many, many times among the drosophilid flies of Hawaii. Sometimes it happened when a small group of flies, or just a fertilized female, was carried, was blown, or flew from one part of an island to another. Other times it occurred when a fly journeyed or was transported from one island to another. Because of their volcanic origins, the Hawaiian islands have different ages, with the younger islands to the southeast. As each new island rose from the waves, small populations of flies or single fertilized females made their way from older islands to newer ones, where their descendants could become increasingly distinct from the ancestral species.

The Hawaiian islands also have many kinds of environments, from lowland rainforests to dry upland forests. Different drosophilid species have evolved adaptations that enable them to thrive in different types of forests.

Also, on the mainland, a new drosophilid species would face competition from other insects already living in an area or from predators. But because the Hawaiian islands are isolated in the middle of the Pacific, new species had far fewer competitors or predators when they appeared. On their island paradise, the drosophilids have flourished.

Biologists call the evolution of many new species from a single ancestral species an adaptive radiation. The adaptive radiation of the Hawaiian drosophilids is one of the most dramatic examples of evolution found anywhere in the world.

Review Questions

  1. What characteristics of the Hawaiian islands might have led to the enormous diversity of drosophilid species in the Hawaiian islands?

  2. How might the diversity of mating behaviors be related to the diversity of drosophilid species in the Hawaiian islands?

  3. What evidence suggests that all of the drosophilid species in the Hawaiian islands have descended from a single fertilized female fly that colonized the islands millions of years ago?

  4. How could individual flies spread from island to island?

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