Kim Sterelny & Paul E. Griffiths, Sex and Death: an introduction to philosophy of Biology, Chicago, The University of Chicago Press, 1999, p. 73.
A somewhat simpler example of an extended phenotype, and one that allows a direct comparison with a standard adaptation, is house construction by caddis fly larvae. These larvae typically live on the bottom of streams, and glue together assorted debris to form a « house » in which they live. These houses plainly serve the same protective function for the larva that is served by a mollusk’s shell. The replicator/ interator distinction enables us to see the evolutionnary identity of house building by the caddis with shell secretion by a mollusk. The gene’s eye view allows us to treat like cases alike; to see the genes for both as building adaptations for physical protection. We cannot treat like cases alike under the received view, for the caddis house is no trait of the caddis.
Defenders of the received view might reply by relocating the phenotypic effect of the gene. They might suggest that the phenotypic effects of the parasite gene are the chimical signals that subvert the host’s behavior, rather than those subverted behaviors themselves. These chemical signals are features of the conventionnal parasite phenotype. The cadddis gene’s adaptive effect is house-building parasite phenotype – a behavioral trait of the larva – rather than the completed house, which is not.