One of the classic examples of adaptive radiation under natural selection is the evolution of 15 closely related species of Darwin's finches (Passeriformes), whose primary diversity lies in the size and shape of their beaks. Since Charles Darwin and other members of the Beagle expedition collected these birds on the Galápagos Islands in 1835 and introduced them to science, they have been the subjects of intense research. Many biology textbooks use Darwin's finches to illustrate a variety of topics of evolutionary theory, such as speciation, natural selection and niche partitioning. Today, as this Theme Issue illustrates, Darwin's finches continue to be a very valuable source of biological discovery. Certain advantages of studying this group allow further breakthroughs in our understanding of changes in recent island biodiversity, mechanisms of speciation and hybridization, evolution of cognitive behaviours, principles of beak/jaw biomechanics as well as the underlying developmental genetic mechanisms in generating morphological diversity. Our objective was to bring together some of the key workers in the field of ecology and evolutionary biology who study Darwin's finches or whose studies were inspired by research on Darwin's finches. Insights provided by papers collected in this Theme Issue will be of interest to a wide audience.

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