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Insect Evidence of the Evolutionary Process Gene Kritsky C harles Darwin never wrote a book devoted exclusively to insects. Yet his first contribution to science involved new records of beetles, and he used insect examples liberally in his books and papers. Indeed, insects illustrate all aspects of his concept of the origin of species by natural selection. Those points include population potential, variation within a species, limits to population growth resulting in a natural selection, population differentiation, and the origin of species. Population Potential All populations produce more individuals than can survive. Many pest populations illustrate this point, and the periodical cicadas provide a natural example of this requirement of the evolutionary process. 8 American Entomologist • Spring 2006 Variation within a Species No two individuals are exactly alike and much of that variation is inherited. Insect taxonomists will quickly point out that this is true with insects. The variations within Cicindela denverensis are clearly shown in this collection from Theodore Roosevelt National Park. Natural Selection Limits to population growth, when combined with natural variation, result in a differential survivorship. Darwin called this “natural selection.” A natural example can be found in the oviposition of periodical cicadas. As optimal oviposition sites become scarce, some cicadas lay their eggs in thin branches, damaging the vascular system of the tree and killing the tip of the branch. Approximately 50% of the cicadas’ eggs laid in these dying branches will perish, compared to the 15% mortality of eggs laid in branches that did not wither and die. Differentiation within a Species Natural selection differs over a species’ range and may produce changes in a species as its populations adapt to local conditions. Such clinal variation has been documented within many insects. The clinal variation of Cicindela tranquebarica from New Jersey to Virginia to Tennessee can be seen in this photograph. If species will change across their range because of changing selection pressures, species should also change through time as conditions change. Vestigial Structures Vestigial structures challenge the notion of design in nature. For example, if wings were designed for flight, then why do flightless birds have wings? If design fails for one structure, it fails as an explanation for all structures. To Darwin, vestigial structures were also evidence of common descent. Hemiptera do not possess labial palpi, but the vestigial buds of the lost labial palpi of enicocephalids show that hemipterans were once related to insects with fully developed labial palpi. American Entomologist • Volume 52, Number 1 9