The book's main idea is that phenotype should not be limited to biological processes such as protein biosynthesis or tissue growth, but extended to include all effects that a gene has on its environment, inside or outside the body of the individual organism.

Dawkins considers The Extended Phenotype to be a sequel to The Selfish Gene (1976) aimed at professional biologists,[1] and as his principal contribution to evolutionary theory.

[2] The central thesis of The Extended Phenotype, and of its predecessor by the same author, The Selfish Gene, is that individual organisms are not the true units of natural selection.

One well-known example of this second type of extended phenotype is the suicidal drowning of crickets infected by hairworm, a behaviour that is essential to the parasite's reproductive cycle.

The relevant adaptation lies in the cuckoo producing eggs and chicks that resemble sufficiently those of the host species so that they are not immediately ejected from the nest.

[2]In developing this argument, Dawkins aims to strengthen the case for a gene-centric view of the evolution of life forms, to the point where it is recognized that the organism itself needs to be explained.

The reviewer states that no "satisfactory answer is given" to this question in the book, though Dawkins suggests that replicators that "interact favorably to create 'vehicles' (organisms) may be at an advantage over those that do not (Chapter 14)."

[7] Subsequent proponents expand the theory and posit that many organisms within an ecosystem can alter the selective pressures on all of them by modifying their environment in various ways.