Specialized associations are surprisingly common among animal species, particularly among parasites that must complete development on a single individual host. In such associations, the host constitutes the entire environment of the parasite. Detailed studies of tropical plant-feeding insect species have demonstrated that more than half are extreme specialists, feeding on only one or a few closely related species of plants. Surveys of coral-reef shrimps symbiotic with sponges yield a similar figure. Moreover, phylogenetic analyses reveal that association with plants has strongly enhanced the evolutionary diversification of insects. Hence, the tendency of organisms to form specialized interactions with other species has been a major generator of Earth's biological diversity.
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