Adaptationism, broadly speaking, is the explanation of biological traits as evolutionary solutions to design problems. Constraint, generally speaking, is the limitation placed on natural selection due to what is actually attainable from a selective point of view. It is a hotly disputed issue in biology that adaptationism is often taken too seriously, in particular in regard to the idea of constraint. Before diving into this issue, further explanation of both ideas is necessary.

    Adaptationsim explained further is the idea that natural selection has been the only important cause of most phenotypic traits in most species. In speaking of the phenotypic traits selected for or adapted, it is important to note that although natural selection selects the most optimal trait, it is not always (indeed, nearly never is) the best conceivable trait. The trait selected for via natural selection according to adaptationism is merely the best trait of what’s available. According to Elliot Sober, there are six major sayings that best explain the controversy of adaptationsim and anti-adaptationism. However only four that are important to the present inquiry. The first is that natural selection is the only natural process adequate to produce adaptational complexity. While this may be true, this would only tell us why a certain phenotypic trait was evolved, but would have no bearing on the optimality of the trait. That is, just because a trait was selected for via natural selection there is no assurance that the trait is optimal as opposed to harmful or even fatal. The second saying is that adaptationism is compatible with the existence of traits that initially evolve for one purpose but adapt to take on a different function. This principle is known as “exaption” or simply “opportunistic switching.” An example of exaption is the wings in certain flying insects; wings initially evolved as a temperature regulator, and later took on the function of flight. Exaption certainly does place natural selection in the driver’s seat, so to speak. In this way, natural selection is not only responsible for the initial evolution of a certain trait, but also the subsequent retention or switch of function of that trait.
    The third saying is that adaptationism is not compatible with the existence and importance of constraints that limit the power of natural selection. This saying encompasses the bulk of the controversy involving adaptationism. Most modern biologists (adaptationist or no) accept the force of constraint on natural selection. There are significant and necessary constraints, particularly regarding phylogeny and developmental biology. For example, the basic body structure and metabolic capabilities must remain unhindered with in order to give the organism even a chance of survival. If these basic structures are tinkered with by natural selection, the organism will most likely die. Furthermore, it is preposterous to deny the force of constraint on natural selection, for it would engender such abominations as a shark with an outboard motor attached to its underside. It seems even logical to posit the force of constraint in natural selection. The main argument against adaptationism is that, in emphasizing the power of natural selection, adaptationism must minimize the importance of constraint. However this criticism is actually a perversion of adaptationsim. In fact, many biologists who claim to be adaptationsists welcome the idea of constraint. To give an example of the necessity of constraint in adaptationism, imagine the evolution of leg size in zebras. Generally, the longer the leg, the faster the animal can run. However, the longer the leg, the more susceptible to fracture it is. Here there is a constraint on leg length that has to do with strength of the bones.

Therefore, adaptationists often consider constraint in the determination of optimality. So talk of constraint is actually a way of describing the traits that natural selection has available to act on. The question posed is not “will the optimal trait evolve?”, but rather “what does natural selection actually have to work with?”. This of course would be irrelevant if we were ensured that optimality would obtain every time a trait is selected for. However this is not the case due to the very rich assortment of variation choices available to natural selection.

    Further evidences for the necessity of constraint to adaptationsim include what is known as a “spin off” trait. This is a trait that evolves because of its correlation with another trait that is actually selected. This very idea assumes constraint. The spin off trait is constrained by the trait to which it corresponds (for example, chin size in humans is constrained by the size and structure of the mandible). Also what is known as phylogenic constraints involves a trait evolving against a background of other traits already in the population. In this case a trait evolves “on top of” or as a modification of an already present trait. Constraint is necessary to keep the underlying structures stable in order to allow for subsequent addition of traits.

    The fourth and final saying of adaptationsim is that adaptationsit thinking is an indispensible research tool and the only way to find if an organism is imperfectly adapted is to describe what it would be like if it were perfectly adapted. This is exactly right. This is why optimality models are so important, even if false. In order to know whether or not natural selection has controlled the evolution of a phenotypic trait we must know if the fittest available trait has evolved, and in order to know this we must know what the fittest trait is.

    In short, many argue that adaptaionsim is overly praised and that it conflicts with constraint. However, it is apparent that adaptationsim is indeed a powerful force in biology and that, in fact, requires constraint. Even if this is not accepted, it is crucial to note the difference in adaptationism as a way of conducting biology versus adaptationsim as a claim about nature. It is indisputable that adaptationsim is at least significant in the former role.