Fig. 3

A diagram showing the processes of exaptation (shifts in function), collage (assembling existing elements into new functional combinations), scaffolding (loss of a component that was formerly required for the assembly of a complex arrangements of parts), and direct adaptation by natural selection (including secondary adaptation) in the evolution of a complex feature. The complex organ (J) includes many parts, all of which must be present for the organ to carry out its current function. Although it can now carry out this function only when all of its components are present, an organ such as this can evolve through intermediates, all of which have some function—though not necessarily the function of the final complex organ (J). At an early stage, two simple structures, each already present and performing its own distinct function (A), come together into a combined structure (B) that is capable of carrying out a function that neither component could before. At first, the combined structure (B) may perform the new function rather poorly, but if it nonetheless confers some advantage over alternatives lacking the structure, then the components may be modified by natural selection favoring improvements in this new function (C). Later, a third component (D), which was also already present and serving its own function, is co-opted and becomes associated with the simple two-part organ to form a three-part organ (E) capable of performing yet another new function (though, again, not the one currently filled by J). Once again, the various components may become modified due to selection favoring improvements to this new function (F). Later, additional components (G)—which in this case are themselves built of combined, perhaps duplicated, components—become associated with the modified three-part organ (F) to form a complex but still not irreducibly complex organ (H) that takes on the function of the final complex organ, albeit not very effectively. This complex organ is, once again, modified for its new function and most of the components become more closely integrated (I). However, one component that has become structurally unnecessary is lost (e.g., because it is costly to produce and mutations that lead to lower investment in its production are advantageous), leaving behind an irreducibly complex organ (J) whose ability to carry out its current function is contingent on the presence of all its component parts