The adaptive radiation of Darwin's finches in the Galapagos archipelago stands as a model of species multiplication. The radiation began two to three million years ago, and resulted in 14 species being derived from the original colonizing species. This system is highly suitable for investigating the causes of speciation because closely related species occur sympatrically in several combinations and in environments with relatively little anthropogenic disturbance. The role of natural selection and adaptation to feeding niches in the allopatric phase of speciation has been demonstrated repeatedly. In the sympatric phase of speciation, differences in song and morphology act as a premating barrier to gene exchange. This form of reproductive isolation evolves at least partly as a passive consequence or byproduct of adaptive divergence in beak morphology. Song characteristics diverge in allopatry, largely independent of beak morphology and for a variety of reasons, not all of which are well understood. The barrier to gene exchange in sympatry is not completely effective, however; species hybridize rarely, and under some circumstances the hybrids are surprisingly fit. These results challenge some current notions of species. For example, the ground finch species Geospiza scandens Gould and G. fortis Gould on the island of Daphne Major have lost morphological diagnosability, as a result of introgressive hybridization, while retaining vocal diagnosability. Speciation is a process of divergence, and therefore these two populations are currently despeciating. With a change in climatic conditions they are expected to respeciate. Such merge-and-diverge dynamics may occur frequently in hybrid zones and in relatively young radiations in habitats subject to strong environmental fluctuations.