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‘Adaptation,’ a central concept in evolutionary biology, represents the degree of suitedness between organisms’ characteristics and their environments. It is the cause of differences in organisms’ performances of biological tasks that lead throughout their life cycles to reproductive success, the distinct concept of ‘fitness.’ Organisms’ characteristics need not be adaptive but may be ‘neutral’ (i.e., they have no consequences for fitness or may be subject to various constraints). Misperceptions of adaptation may result from confusions of terminology. The assumption that all features of organisms are adaptive is common but misleading. Future study of adaptation will include diverse approaches and greater rigor of conceptual analysis.
- Adaptations Identity and Its Distinction from Fitness
- Basic Definitions of Adaptation
- Elaborations of the Basic Concept
- The Distinction between Adaptation and Fitness
- The Roles of Adaptation and of Fitness in Darwin’s Argument for Natural Selection
- Alternatives to Adaptation in Evolution
- Misdefinitions of Adaptation or Misconceptions of Its Role
- Adaptationism and Its Drawbacks
- The Future Study of Adaptation
The adaptation, or adaptedness, of organisms to their environments is a central concept in evolutionary biology. It is both a striking phenomenon needing explanation and a basic feature of the mechanisms underlying the patterns of evolutionary stasis and change alike. The organism–environment interaction that the adaptation concept embodies is the causal driver of the process of evolution by natural selection. Its nature, role in the evolutionary concept structure, and limitations must all be understood if a clear view of evolution is to be possible. In particular, adaptations distinctness from and relation to the concept of fitness must be seen clearly. Only thus can evolution by natural selection be properly understood.
Adaptations Identity and Its Distinction from Fitness
If no concept is more central to evolution by natural selection than adaptation, then also none has been more debated. All the basic features of its definition are found in the work of Darwin, but progress in unfolding its full scope and implications continue even at present. Biological evolution, as distinct from cultural evolution (though often interwoven with it; e.g., Cavalli-Sforza and Feldman, 1981), is manifested as change in the genetic composition of populations over time. Therefore, some genetic terminology is needed at the outset. A ‘gene’ is a functionally coherent sequence of bases in nucleic acid, usually DNA (except for some viruses), determining or influencing some biological structure and/or function. An ‘allele’ is one possible sequence (a variant) of a gene, determining one alternative state of gene action. Many organisms, including most animals, carry two copies of each gene (and are thus termed ‘diploid’). ‘Genotype’ refers to the whole heritable composition of a creature, whether viewed gene by gene (e.g., carrying two copies of the same allele, hence a ‘homozygote,’ or one copy each of two different alleles, hence a ‘heterozygote’) or more broadly up to the whole ‘genome’ which includes all genes. ‘Phenotype’ refers to the expressed structure and function of the organism as it develops via interactions of its genotype with the environment in which development takes place. Present understanding of the complexities of the evolutionary process requires this terminology to avoid ambiguity and confusion.
Basic Definitions of Adaptation
As a general concept, adaptation or adaptedness is best defined as the extent of matching or suitedness between the heritable features (heritable functional phenotypes) of organisms and the environments in which they occur. In other words, adaptation comprises genotype–phenotype–environment interactions. It finds direct expression in the effectiveness with which organisms perform essential biological tasks (osmoregulation, locomotion, capturing food, evading predators, etc.) in their environments. As such, its states are in principle quantitatively measurable, or at least orderable, rather than only qualitatively organized. This general definition is found in many parts of Darwin’s writing, as in the introduction to his Origin of Species (1859/1872) where a woodpecker appears equipped “with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees.” Here, the phenotypic states of these morphological characters, modified as compared to simpler forms found in other birds, are related to their functional performance effects in acquiring food resources that other birds, lacking those specific adaptive phenotypic states, cannot reach.
Adaptation also refers to the process of successively descended, modified phenotypes becoming more suited, ‘better adapted,’ to environments via natural selection on variation in those phenotypes. Paleontology finds strong evidence for improvement of adaptation over time, as in the escalation of predator and prey attack and defense morphologies in marine invertebrates (Vermeij, 1987). Real-time studies have shown adaptive improvement directly, such as in the evolution of a bacterial stock in novel culture conditions over periods of 104 generations: a stock at an early stage in the process, if samples are frozen for later reactivation, is found to be inferior in performance to its better adapted descendants sampled late in the experimental history (Lenski and Travisano, 1994).
This evolutionary refinement of an adaptive state has led to debate over when a phenotypic feature may be called ‘an’ adaptation and when it may not be (i.e., how far it has been specifically selected for its current functional state). Given that adaptive states differ quantitatively, any viable phenotype has some level of adaptation, and this debate loses urgency. Recognizing a phenotype as ‘an’ adaptation only if it is the best available at a given time (as argued by Reeve and Sherman, 1993) would require continuous revision as newer alternatives arise, and it offers no compensating advantage.
Elaborations of the Basic Concept
Gould and Vrba (1982) extended and refined definitions of adaptation in useful ways. In their terminology, ‘aptation’ describes the primary, historically unmodified relation of suitedness between phenotype and environment – that of any newly arisen variant, positive or negative, in its functional effects. They regarded ‘adaptation’ as the successive refinement of phenotypic suitedness by selection of newer variants, and coined the term ‘exaptation’ for the coopting of a phenotypic feature by selection for a new function, as in the modification of skull–jaw joint bones toward the ossicles of vertebrate ears (e.g., Romer, 1955). The exaptation–adaptation distinction poses problems of discrimination (how much change under a new selection pressure is needed before a phenotype of exaptive origin is recognizable as presently adaptive? See Reeve and Sherman, 1993) and also emphasizes that we are dealing with quantitative scales of variation, not alternate qualitative categories. Often the Gould–Vrba terms are not used unless the distinctions are pertinent to the issue at hand, and otherwise ‘adaptation’ is used as a generally inclusive term.
Another important extension of the adaptation concept is the work of Laland et al. (1996, 1999) on ‘niche construction.’ This term refers to the active modification of environments by organisms in ways favorable to their own function and fitness, as emphasized by Lewontin (1983). It occurs in diverse ways in different groups: for example, bacteria may release protease enzyme catalysts into their surroundings to aid in foraging upon potential food items, while among multicellular animals beaver lodges and dams are a dramatic case of such activities (aside from the obvious capabilities of humans in this direction). Evolutionary models incorporating niche-constructive feedbacks on organism–environment interactions may have very distinct properties from those not including such active forms of adaptation (Laland et al., 1996, 1999).
The Distinction between Adaptation and Fitness
Alternative states of adaptation are the causes of evolutionary changes through their differences in genotype–phenotype– environment interactions and hence performances of these phenotypes. These performances, minute by minute to year by year, cumulatively alter how long individuals live and how much they reproduce. In short, adaptive differences among phenotypes alter their demographic parameters: survivorship (¼ lx of demography, where x denotes time intervals) and male mating success or female fecundity (¼ mx of demography). These parameters are components of what, since the advent of mathematical population genetics, has been termed ‘fitness’ or ‘Darwinian fitness’ (though Darwin did not use the word in this way): the reproductive success of whole populations or of specific genotypes. Adaptation and fitness, then, are serially related concepts, but are in no sense the same.
In evolutionary genetics, fitness is usually measured as the net replacement rate of organisms, whether an average value for a whole population or more specific average values for particular genotypes. It is defined in ‘absolute’ terms as R ¼ Slxmx (e.g., Roughgarden, 1979) under simple demographic conditions of nonoverlapping generations and homogeneous reproductive periods (as, e.g., in annual plants or many insects). For complex demography in age-structured populations, the most similar expression is l (the leading eigenvalue of the demographic ‘Leslie matrix’), a number that summarizes complex interactions of age-specific survivorships and fecundities (e.g., Charlesworth, 1994; McGraw and Caswell, 1996). The concept of fitness is the same among these cases; what varies is the measure of fitness as is proper to each case. If either R or l, as appropriate, is compared among genotypes by taking the ratio of each value to that of a chosen standard genotype, there result ‘relative’ genotypic fitnesses, whose value for the standard genotype is 1. Most evolutionarygenetic models use relative fitnesses for symbolic or numeric convenience.
Usage of the terms adaptation and fitness has changed dramatically since Darwin. He, Wallace, and other early evolutionists used ‘fitness’ as a synonym for ‘adaptation,’ and by ‘survival’ they often referred not to the demographers’ life cycle variable lx but to ‘persistence over long time periods.’ Spencers phrase ‘survival of the fittest,’ translated, meant ‘the persistence through time of the best adapted.’ Darwin had (necessarily) a clear view of the concept that evolutionary biologists now denote by the term ‘Darwinian fitness,’ but he represented it by versions of a stock phrase (for which he had no summary term), “the best chance of surviving and of procreating,” in the Origin of Species (1859/1872). Failure to recognize these usage changes, and thus blurring of the sharp distinction between adaptation as cause and fitness as within-generation result, has led to much confusion in later literature, including mistaken claims of an alleged circularity of evolutionary reasoning.
The Roles of Adaptation and of Fitness in Darwin’s Argument for Natural Selection
From the inceptions of both Darwin’s and Wallace’s ideas of natural selection, differences in adaptation among heritable variants played the central, causal role in the process. Darwin formalized his argument in Chapter 4 of Origin, especially in its first paragraph and its concluding summary, in such a way that it can be cast as a verbal theorem – as Depew and Weber (1995) make clear by judicious editing of Darwin’s summary. Here it may be reformulated in modern terms.
To begin the argument, there are three points ‘given’ by direct observation: (1) organisms vary in phenotype; (2) some of the variants are heritable; and (3) some of these heritable variants can perform their biological functions differently in a specific habitat (i.e., some are better adapted than others to that habitat). Then, Darwin’s Postulate is that the better adapted, and hence better performing, variants in a habitat will survive and/or reproduce more effectively over their life cycles (i.e., have higher fitness) than other variants. Demography shows that greater reproduction of variants will maintain or increase their frequencies in successive generations of a population. One thus concludes that when the Postulate holds, the best-adapted heritable phenotypes will persist and/or increase in frequency over time, realizing evolution by natural selection. This completes Darwin’s Theorem.
The distinction between differences in organisms’ adaptive performances, minute by minute to year by year, and resulting fitness differences among them over their life spans is simply the difference between cause and effect. Its recognition is essential to keep straight the logic of natural selection and to organize empirical studies of the process (Feder and Watt, 1992; Watt, 1994).
Alternatives to Adaptation in Evolution
Adaptation is not ubiquitous, and natural selection is not all-powerful. ‘Darwin’s Theorem,’ as summarized in this research paper, is empirically testable and indeed may not hold in some well-defined circumstances. Two main sources of limitation on the scope of adaptation are now considered.
As Darwin wrote in Chapter 4 of Origin, “Variations neither useful nor injurious would not be affected by natural selection.” The modern concept of neutrality (Kimura, 1983; Gillespie, 1991), which he thus described, is the null hypothesis for testing all causal evolutionary hypotheses. It occurs at each of the recursive stages of natural selection, as recognized by Feder and Watt (1992).
First, at the genotype/phenotype stage, genetic variants may differ in sequence but not in resulting function. For example, the ‘degeneracy’ of the genetic code often means that differences in DNA base sequence lead only to the same amino acid’s insertion into a given position of a protein molecule. Alternatively, at least in the case of some positions in proteins, substitution for one amino acid residue by a similar one may sometimes have little effect on the protein’s function.
Next, at the phenotype/performance stage, functional differences among variants may not lead to performance differences among them, as other phenotypic mechanisms constrain or suppress their potential effects. For example, in the physiological reaction pathway used by bacteria to digest milk sugar (lactose), a twofold range of natural genetic variation in a phenotypic parameter (the Vmax/Km ratio) is observed for each of the protein catalysts, or enzymes, catalyzing the first two reactions. When these variants’ resulting performances were measured under steady-state growth conditions, variants of the first enzyme in the pathway showed sizable, reproducible differences, but no such effects were seen among variants of the second enzyme despite the similar size of their phenotypic differences – due to system constraints related to the position of the reactions in the pathway, analyzable by the theory of metabolic organization (Watt and Dean, 2000).
At the stage of performance/fitness, performance differences may not lead to corresponding fitness differences (e.g., if improved performance has less fitness impact above a threshold value of habitat conditions). For example, performance differences among feeding phenotypes (bill sizes and geometries) of Darwin’s finches have little fitness impact when food is plentiful in wet seasons, but have much more impact when it is scarce in dry seasons (Grant, 1986).
Finally, at the stage of fitness/genotype, which completes the natural-selective recursion, small population size can allow random genetic drift to override fitness differences, as in the loss from small mouse populations of developmental (‘t-system’) mutant alleles that should be in frequency equilibrium between haploid gametic selection favoring them and recessive lethality at the diploid developing-phenotypic stage of the life cycle (Lewontin and Dunn, 1960). Because the usual statistical null hypothesis is that no treatment effect exists between groups compared, any adaptive hypothesis of difference between heritable phenotypes is ipso facto evaluated against neutrality by statistical testing.
Further, there is a subtler neutral hypothesis, that of association or ‘hitchhiking’: variants that seem to differ in fitness at a gene under study may be functionally neutral but genetically linked to an unobserved gene whose variants are the real targets of selection. But ‘hitchhiking’ predicts that fitness differences seen among variants will not follow from any functional differences among them, so it is rejected when prediction from function to fitness is accurate and successful.
Indeed, where substantive adaptive difference exists among genetic variants in natural populations, neutral null hypotheses may be rejected by testing at any of these levels, from phenotypic function to its predictable fitness consequences and the persistence or increase of the favored genotypes. This has been done, for example, for natural variants of an energy-processing enzyme in the ‘sulfur’ butterflies, Colias (Watt, 1992). The explicit test of adaptive hypotheses against neutral nulls gives important rigor to experimental study of natural selection in the wild (Endler, 1986).
Gould (1980, 1989) emphasized that many features of organisms may not result from natural selection, but rather from various forms of constraint due to unbreakable geometric or physical properties of the universe at large or of the materials from which organisms are built, or other, more local biological limitations or conflicts of action. Functional or geometric constraints may play a major role in the form or function of organisms (e.g., in snail shells’ form) (Gould, 1989), or in the negative interactions in combinations of individually positive amino acid changes, which constrain the paths of positive evolution of antibiotic resistance in bacteria (Weinreich et al., 2006).
Selection among phenotypic alternatives at one time may entail diverse predispositions or constraints at later times. In one such case, the tetrapodal nature of all land-dwelling vertebrate animals (the bipedality of birds, kangaroos, or hominid primates is secondary) follows from the historical constraint that their ancestors, lobe-finned fish, swam with two pairs of oar-like ventral fins having enough structural strength ab initio that they could be exaptively (i.e., not through natural selection) modified into early legs (e.g., Gould, 1980; Cowen, 1995). In a more pervasive case, the evolved rules of diploid, neo-Mendelian genetics constrain many evolutionary paths. For example, if a heterozygous genotype is the best adapted, and hence most fit, in a population, it can rise to high frequency in that population but cannot become the only genotype present because it does not ‘breed true.’
Conflicts among different aspects of natural selection may constrain the precision of adaptation in diverse ways. As a case in point, adjustment of insects’ thermoregulatory phenotypes may be held short of maximal or ‘optimal’ matching to average conditions in cold, but highly variable, habitats, because such ‘averagely optimized’ phenotypes would overheat drastically in uncommon but recurrent warm conditions (Kingsolver and Watt, 1984). This illustrates the general point that environmental variance may sharply constrain adaptation to environmental means.
Misdefinitions of Adaptation or Misconceptions of Its Role
Many misdefinitions of adaptation err by confusing it with fitness in one fashion or another. Much of this may originate in the usage changes, discussed earlier in this research paper, between the early Darwinians and the rise of evolutionary genetics, such that ‘fitness’ ceased to be a synonym of adaptation and came to mean instead the ‘best chance of surviving and of procreating’ (e.g., Darwin, 1859/1872, p. 63). This entirely distinct concept is, as noted above, the cumulative demographic effect of adaptation. Some writers on evolutionary topics have been confused by inattention to these usage changes, but others have erred through conscious disregard or blurring of the adaptation–fitness distinction.
For example, Michod (1999), despite early recognition of the separate nature of adaptation and fitness and of their antecedent–consequent relationship (Bernstein et al., 1983), sought to collapse these concepts into different ‘senses’ of the single term ‘fitness’ to be used in different contexts to refer to both ‘adaptive attributes’ and their consequences in reproductive success. Authors may choose terminology for their own uses within some limits, but this usage is at best an ill-advised source of confusion, and at worst a mistaken conflation of distinct concepts.
Another misconception was asserted by Lewontin (1983) during an otherwise important argument for studying ‘niche construction’ (cf. above; Laland et al., 1996, 1999). Arguing that the adaptation concept implies ‘passiveness’ of adapting organisms, he criticized it for allegedly implying that adaptation is like ‘filing a key to fit a preexisting lock.’ But no such passivity is really in evidence. In Darwin’s example above, the woodpecker’s feeding ‘strategy’ actively transforms its environment compared to that experienced by more generally feeding birds, using a resource that those other birds do not even perceive. Further, Darwin’s discussions (1859/1872, Chapter 4) of mutualisms between flowers and pollinators also show the constructive nature of those adaptations: pollinators obtain resource rewards from plants and spread their pollen during their foraging, and by supporting the reproduction of their food sources in this way, they increase their own future resource bases. Niche construction is thus an important form of adaptation, not distinct from or opposed to it.
Lewontin also misstated the role of adaptation in the evolutionary process, arguing that ‘three propositions’ – variation, heritability, and differential reproduction – alone were sufficient to explain natural selection, and that the adaptation concept was gratuitously introduced into the argument by Darwin for sociological reasons (e.g., Lewontin, 1984). This claim is wrong and has been widely critiqued (e.g., Hodge, 1987; Brandon, 1990; Watt, 1994; Depew and Weber, 1995). Adaptation is the one element that distinguishes natural selection from artificial selection or sexual selection. Without it, Lewontin’s three propositions are sufficient only to define ‘arbitrary’ selection, wherein we do not know the cause of any differential reproduction of heritable variants. But the adaptive cause is, indeed, central to evolutionary change resulting from natural selection.
Finally, a serious barrier to effective study of adaptation has been a claim by Mayr (1961, 1980) that ‘proximate causes’ (phenotypic mechanisms) have little to do with ‘ultimate causes’ (evolution by natural selection) in biology. Since adaptation comprises genotype–phenotype–environment interactions as the driver of natural selection, this ‘proximate–ultimate’ dichotomy distorts the basic nature of the evolutionary process and discourages the breadth of approach needed for study of evolution (Watt, 2000, 2013; West-Eberhard, 2003; Laland et al., 2011).
Adaptationism and Its Drawbacks
Rose and Lauder (1996) identify adaptationism as “a style of research . in which all features of organisms are viewed a priori as optimal features produced by natural selection specifically for current function.” Some, such as Parker and Maynard Smith (1990) or Reeve and Sherman (1993), hail the assumption of adaptiveness as a virtue, while others (e.g., Gould and Lewontin, 1979) indict it as a vice. The question is: is it helpful, or legitimate, to assume that adaptation is ubiquitous?
First, is it true that adaptiveness is often assumed in practice? The usual null hypothesis in statistical testing is that there is no ‘treatment effect.’ Thus, any statistical test of adaptive difference among character states assumes ab initio that there is no such difference (i.e., that the character states in question are neutral). Only if this null hypothesis can be rejected according to standard decision rules is an effect recognized. All the null models of population genetics itself, beginning with the single-gene Hardy–Weinberg distribution, start with neutral assumptions. Tests of the population genetic consequences of putatively adaptive differences in phenotypic mechanisms may or may not find departure from neutrality, but it is the routine ‘starting point.’
Mayr (1988) argued for testing all possible adaptive causes for phenotypes before turning to the explanation of chance (i.e., neutral) origins. But this argument depends on a historicist approach to evolution. If one can analyze a phenotype by testing among neutrality, constraint, or adaptation with present-day experiments, historicism is not needed. Even fossil structures absent in living relatives may be studied functionally by various means (Hickman, 1988). A historical approach may sometimes be indispensable, but it is not the only one available to evolutionary biology.
As Gould (1980) observed, assuming the ubiquity of adaptation discourages attention to structural or constrained alternative explanations of phenotypes. It is not enough merely to test one specifically adaptive hypothesis about some phenotype against neutrality; one should consider all feasible alternative hypotheses, including the constraint-based one that a phenotype does make a nonneutral difference to performance and thence fitness, but does so in a particular way because no other way is feasible or possible, rather than because it is ‘optimized.’
Indeed, the strongest objection to adaptationism may be that using the optimal adaptedness of a phenotypic feature as a null hypothesis (as sometimes suggested by adherents of this view) runs the serious risk of falling victim to ‘the perils of preconception.’ How can a scientist making such an attempt know that the phenotypic function has been correctly identified, or that an appropriate adaptive hypothesis has been arrived at, to begin with (cf. Gordon, 1992)?
As a cautionary illustration, the behavior of certain ‘laterally basking’ butterflies in orienting their wings perpendicular to sunlight was at first guessed to be an adaptation to minimize casting of shadows, hence to avoid predators’ attention. More careful study shows that it does not do so! Parallel orientation of the closed wings to the solar beam truly minimizes shadow. It was instead shown experimentally, with proper testing against neutral null hypotheses, that the perpendicular solar orientation is adaptive, but in relation to thermoregulatory absorption of sunlight (Watt, 1968).
Some users of adaptationist approaches do recognize these concerns, and they build optimizing models for testing in comparison to possible constraints or other alternative explanations (e.g., Houston and McNamara, 1999). Nonetheless, the intellectual hazards of assuming the adaptiveness of phenotypes outweigh the possible advantages. Certainly, studies of adaptive mechanisms in diverse organisms are routinely carried out, achieving results that are both rigorous and generalizable, without this assumption (e.g., Lauder, 1996; Watt and Dean, 2000).
The Future Study of Adaptation
Mechanistic studies of adaptation in the wild are increasing in diversity and effectiveness, as in the application of biomechanical approaches to the function of morphological adaptations (Lauder, 1996), or of molecular approaches to adaptation in metabolism and physiology (Watt and Dean, 2000). Diverse tools of modern ‘genomics’ and ‘proteomics’ may be used in the study of adaptation in complementary ways, either (1) focusing at first on natural variation in genes and processes of known function, and working upward through their genotype– phenotype–environment interactions to their performance and fitness effects in the wild (e.g., Wheat et al., 2006; Barrett and Hoekstra, 2011); or (2) screening organisms’ genomes broadly for genetic variation that is associable with biogeographic or other patterns of possibly adaptive biological specialization (e.g., Fournier-Level et al., 2011; Hancock et al., 2011), then working downward to identify those genes and variants’ roles in genotype–phenotype–environment interactions and their effects (e.g., Prasad et al., 2012).
At the same time, philosophical ground clearing may reduce misunderstanding or misapplication of the adaptation concept, and lead to better specific work as well as greater possibilities for general insight (Brandon, 1990; Lloyd, 1994; Watt, 2000). There has often been tension between the study of well-known ‘model’ systems, which can maximize experimental power, and the fascination with diversity that drives the study of evolution for many workers. Both have value for the study of adaptation, and the tension may be eased by the interplay of comparative and phylogenetic studies (Larson and Losos, 1996) with genetics-based experimental or manipulative study of organism–environment interactions and their demographic consequences in the wild. This synergism of diverse empirical and intellectual approaches holds great promise for the widening study of adaptation as a central feature of evolution by natural selection.
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