A comprehensive evidentiary regime that would encompass adaptations, exaptations, spandrels, and constraints requires both a standard to be satisfied and a predesignated default presumption to be maintained before the standard is satisfied. Andrews et al. focus principally on the former component. Some thoughts are here offered on the latter.

Specifying exact selection pressures for identifying adaptations is unnecessary. Novel behaviors are not spandrels since they can only develop because of prior functions. An adaptationist approach has a high prior probability, whereas spandrel hypotheses attempt to prove a negative. The concept of maladaptive spandrel is criticized. The utility of dysfunctional states for identification of adaptations gone wrong is emphasized.

Andrews et al. attempt to clarify the standards for determining whether traits are adaptations. The authors argue that tests of adaptationist hypotheses best proceed by assessing the consistency of the traits with the proposed standards. Critical tests of such standards must assess inconsistency – hypotheses must be falsifiable. To fully understand trait evolution, we must consider both adaptive and nonadaptive hypotheses.

The authors outline research strategies that may identify the possible adaptive value of a trait. But this does not solve the problem of how to decide which characteristics of living organisms require an adaptive explanation. I suggest that knowledge of the ontogenetic and phylogenetic construction of a trait facilitates the identification of features that may have been acted on by natural selection.

Andrews et al. present a clear discussion of the various criteria needed to identify adaptations. However, they also imply a history of the debate between adaptationists and their critics that is incomplete. The history implied is one of only genuine scientific disagreement. This neglects the role of nonscientific motives and strawman arguments on behalf of the critics of adaptationists.

Homology can provide strong evidence against exapted learning mechanism (ELM) explanations for psychological and behavioral traits. Homologous traits are constructed by commonly inherited developmental mechanisms. As such, demonstration of homology for a trait argues for its construction by an inherited rather than an exapted developmental process. We conclude that comparative evidence can play an important evidentiary role within evolutionary psychology.

Although antiadaptationist authors encourage us to consider alternatives to adaptationist positions, the alternatives offered do not necessarily relieve us of the burdens of adaptationist explanations. Even if something is an exaptation, it may be derived from an adaptation. If it is a byproduct, it is a byproduct of an adaptation. Even the ELM, the hypothetical exapted learning mechanism, is an evolved learning mechanism, though used outside its natural domain.

Adaptationist explanations for well-known phenomena are of limited value to psychological science. To be truly useful, evolutionary psychological research programs must produce novel discoveries about contemporary cognitive and behavioral processes. The concept of special design is especially useful. Even if special design cannot be convincingly demonstrated, rigorous attempts to meet this evidentiary standard can produce novel scientific contributions.

Andrews et al. argue for adaptationism in cognitive research. However, the problem of evolvability brings into question the number of genes required for the evolution of cognitive mechanisms. Are there enough? Also, greater consideration should be given to alternative vicarious selection processes, which may produce cognitive mechanisms. Finally, identifying constraints with optimality arguments is more difficult than the authors think.

The authors present a balanced critique of the adaptation/exaptation debate and specify some of the hard evidentiary criteria that are needed to advance our understanding of human evolution. Investigators must build more “special design” criteria into their theorizing and research. By documenting that certain traits meet these rigorous criteria, the evolutionary sciences will ultimately rest on a firmer theoretical foundation.

The target article adopts an adaptationist research strategy that, while logically coherent, suffers from various limitations, including problems in reconstructing past selective environments, ambiguity in how narrowly to define adaptive problems or selection pressures, and an overemphasis on specialization in evolved psychological mechanisms. To remedy these problems, I support a more flexible approach involving phenotypic adaptation and cultural evolution.

Rather than starting with traits and speculating whether selective forces drove evolution in past environments, we propose starting with a candidate gene associated with a trait and testing first for patterns of selection at the DNA level. This can provide limitations on the number of traits to be evaluated subsequently by adaptationism as described by Andrews et al.

The expression exapted as is offered as a substitute for the target article's exaptation for and exaptation to on the grounds that exapted as is less likely to foster the pernicious intuition that natural selection designs for future consequences.

Andrews et al.'s analysis suffers from a series of conceptual confusions they inherit from Gould's work. Their proposal that adaptations can be distinguished from exaptations essentially by specific design criteria fails because exaptations are often maintained and secondarily adapted by natural selection and therefore, over evolutionary time, can come to have similar levels of design specificity to adaptations.

Although Gould and Lewontin's (1979) paper stimulated an extraordinary response, the current study of adaptation is – and should be – more than a defense against their criticisms. Adaptations are studied by biologists in new and exciting ways, including experimental manipulations of populations in the field and laboratory, comparative analyses of taxa with known evolutionary relationships, and quantitative genetics. These techniques go beyond ascertaining whether or not a trait is an adaptation.

In our target article, we discussed the standards of evidence that could be used to identify adaptations, and argued that building an empirical case that certain features of a trait are best explained by exaptation, spandrel, or constraint requires the consideration, testing, and rejection of adaptationist hypotheses. We are grateful to the 31 commentators for their thoughtful insights. They raised important issues, including the meaning of “exaptation”; whether Gould and Lewontin's critique of adaptationism was primarily epistemological or ontological; the necessity, sufficiency, or utility of design evidence, phylogenetic analyses, homology, and molecular genetics in distinguishing exaptations from adaptations; whether adaptationists accept adaptationist hypotheses too quickly; and the real utility of adaptationism to human behavioral science. We organize our response along the major points of the target article, in some situations defending our original claims and in others modifying them. While debate on these issues will undoubtedly continue, we are cautiously optimistic that the main points of the target article (as modified by our response) will help move the debate in a positive direction.