Description of the potential for existing adaptations to limit within a given lineage the evolution of subsequent adaptations.

The trouble is that natural selection has difficulty moving a population "backwards", that is, to a lower fitness. If development of a given adaptation requires some degree of "undoing", then that may not be achievable, at least by natural selection alone (though, alternatively, might be reachable via genetic drift).

Populations thus are often limited in terms of their ability to evolve in absolute terms, that is, in the sense that no mutation could possible move the lineage down some particularly evolutionary path.

Note that one way around this dilemma is horizontal gene transfer, where rather than modifying existing adaptations, an organism instead may be able to simply acquire new adaptations, essentially intact, from another lineage within which those adaptions had evolved. Alternatively, and as happens quite often, an organisms may simply acquire another organism whole, along with that organism's adaptations. See, for instance, endosymbiotic theory.