Property of a given genotype to produce different phenotypes depending on different environmental conditions, thereby enhancing organisms’ fitness.
The relationship between organism and environment is highly dynamical and can be thought of as a trade-off between the demands imposed by the environment and the organism’s adjustments to those demands. On the one hand, the environments change; on the other hand, organisms fit the requirements posed by the new conditions “in the struggle for life.” Phenotypic plasticity, the capacity of a genotype to give rise to different phenotypes in response to different environmental conditions, is crucial if organisms are to adapt to new environments. Adaptive plasticity is simply the phenotypic plasticity that enhances the organisms’ fitness.
Although plasticity has been known for over a century, until recently it was taken to be uninteresting or even irrelevant. Currently, however, it arouses great interest; proof of this is the fact that while <10 papers were published per year before 1983, about 1300 papers were published in 2013 (Forsman 2015: 282). This exponential growth was promoted by the recent questioning of the traditional evolutionary framework (see below).
This entry will show why adaptive phenotypic plasticity is currently of such importance and will introduce its main characteristics. In addition, it will illustrate this property with the most paradigmatic instance of plastic phenotype, i.e., learning, the evolutionary consequences of which are emphasized by the so-called Baldwin Effect.
What Is Adaptive Plasticity?
Phenotypic plasticity, which comprises very different phenomena (Bateson and Gluckman 2011; Pigliucci 2001), is the property of a specific genotype to give rise to different phenotypes in the face of different environmental circumstances. This property is one of the two main adaptive mechanisms possessed by organisms, the other being population-level allele frequency change. However, as pointed out by Sultan (2017: 4), the emergence of favorable new genetic variants is rare and random, whereas plasticity has the great advantage that it provides the organism with immediate adaptive variation and in many individuals at the same time.
The key tool for the analysis of phenotypic plasticity is the notion of norm of reaction, defined as “the set of phenotypes produced by a given genotype in a specified range of developmental circumstances” (Sultan 2015: 21). However, it should be noted that the two notions are not equivalent: the norm of reaction is a function describing the genotype-specific relationship between environments and phenotypes, whereas plasticity is an attribute of the norm (Pigliucci 2001: 7).
Plastic responses to environmentally induced factors presuppose cue-response systems (see Sultan 2015), in which an organism perceives some aspects of the environment as information and is able to respond to the cue through specific phenotypic effects. The cues, which cover a huge typology, may be anticipatory (an environmental change is predictable through reliable cues) or immediate (direct environmental influences) (Sultan 2015).
As regards the kinds of plasticity, a distinction is widely acknowledged (see Sultan 2015 and references) between adaptive and inevitable plasticity. The former encompasses phenotypic responses to the environment that are functionally adaptive, whereas the latter characterizes the factors that limit physiological or developmental processes (for instance, organisms developing in environments with scant nutrients experience only limited growth). Another distinction worth considering (partially related to the former) is that which exists between active (anticipatory phenotypic changes in response to environmental cues) and passive plasticity (direct influence exerted by the environment on chemical or physiological processes) (for discussion, see Forsman 2015).
Unfortunately, the exact relationship between phenotypic plasticity and adaptive evolution remains unclear. Obviously, any instance of phenotypic plasticity that increases fitness in response to new environments or new environmental conditions is beneficial for the organism, in relation to nonplastic beings. However, this situation cannot be automatically conflated with an adaptation. Furthermore, it should be remembered that the notion of adaptation is poorly understood (Pigliucci 2001: 160). It is, therefore, extremely difficult to draw the line between adaptive and nonadaptive plasticity.
Plasticity provides organisms with an obvious advantage to successfully cope with new environments or environmental conditions, thus reducing the threat of extinction; consequently, it allows “a better phenotype-environment match across multiple environments than would be possible by producing a single phenotype in all environments” (DeWitt et al. 1998: 77–78). In contrast, if the environments remain constant over time, the aforementioned benefit does not apply.
However, it is also obvious that plastic organisms cannot adapt to any environmental conditions, and consequently no organism exhibits a perfect or infinite plasticity (DeWitt et al. 1998: 78). The lack of such a “Darwinian monster” (Pigliucci 2001: 174), i.e., an organism that copes perfectly with any environment, shows that (the evolution of) plasticity depends on the existence of associated costs. In this sense, DeWitt et al. (1998: 78) establish the distinction between costs and limits: “A cost of plasticity is indicated in a focal environment when a plastic organism exhibits lower fitness while producing the same mean trait value as a fixed organism. In contrast, a limit of plasticity is evident when facultative development cannot produce a trait mean as near the optimum as can fixed development.” These scholars make reference to five costs (maintenance costs, production costs, information acquisition costs, developmental instability, and genetic costs) and four limits (information reliability limit, lag time limit, developmental range limit, and the epiphenotype problem; for discussion, see DeWitt et al. 1998). While acknowledging the theoretical value of those costs and limits, Pigliucci (2001: 175ss.) argues, however, that those constraints on plasticity can hardly be distinguished from an empirical perspective.
Finally, a further point in question is the deep implications of phenotypic plasticity for evolutionary theory, as emphasized by West-Eberhard (2003). This scholar argues that phenotypic plasticity strongly influences adaptive evolution through phenotypic accommodation, which integrates developmental and evolutionary change without the need for genetic change to take place. According to West-Eberhard’s (2003) framework, the evolution of a new adaptive trait through natural selection begins with a genotypically or environmentally induced phenotypic change. The initial viability of the change is increased through adaptive plasticity (phenotypic accommodation), this step being followed by genetic accommodation through natural selection. Accordingly, “Genes are followers, not necessarily leaders, in phenotypic evolution” (West-Eberhard 2003: 158). In other words, genetic changes follow (i.e., do not necessarily precede) phenotypic change, in such a way that genetic novelties are not required in order for phenotypic novelty to evolve. Therefore, according to West-Eberhard (2003), phenotypic plasticity has a prominent role in facilitating and accelerating three main evolutionary processes: the origin of novelty, speciation, and macroevolution.
The following section illustrates the property of phenotypic plasticity with its most powerful instance, i.e., learning, by showing how learning capacity can give rise to new directions of evolutionary change. This aspect is known as the Baldwin Effect.
A Brief Overview of the Baldwin Effect
The Baldwin Effect (Baldwin 1896) is a (controversial) mechanism which speeds up natural selection (for discussion, see Weber and Depew 2003; Longa 2005). It can be broken down into two steps. The first one has to do with phenotypic plasticity, which enables an organism to develop new behaviors when exposed to new environmental conditions, or to adapt itself, through learning, to behaviors observed in the environment (for instance, by mimicking the phenotypic result of a mutation arising in another individual). Phenotypic plasticity thus provides the organism with fitness, which in turn enables it to stave off extinction. Therefore, the starting point of the process is a phenotypic change arising as a consequence of an ontogenetic adaptation made possible by the organism’s plasticity. However, for the Baldwin Effect to apply, a second and crucial step is required, namely the genetic assimilation of the phenotypic trait previously learned as a response to a given stimulus. In this step, the plastic learning mechanism for the phenotypic trait is replaced by a rigid mechanism based on heredity. It is in this sense that learning can guide evolution.
If the learning of a given phenotypic trait varies within a population, some individuals will be capable of learning the trait better than others. Natural selection will favor those who acquired the ability more easily (for example, on the basis of limited exposure to the stimulus), because those individuals will increase their fitness. Further down the evolutionary line, mutations will occur (in a non-Lamarckian sense) which will produce the same type of trait without the need for learning, the outcome being the reduction (or the deletion) of phenotypic plasticity for the trait. In other words, natural selection will tend to act on plasticity, by confirming and accelerating the evolutionary change which originally began via learning. It is for this reason that the Baldwin Effect speeds up evolution.
For a full understanding of the Baldwin Effect, it is also crucial to bear in mind that learning presupposes a trade-off between the costs and benefits associated with such a capacity. Although learning has undeniable advantages, it also has clear disadvantages. For example, it requires time, attention, and effort, and the learned behavior is exposed to dangerous contingent factors that might well be deleterious. Yet the disadvantages disappear if the learned behavior becomes inherited. The Baldwin Effect, therefore, reduces the costs of learning.
To sum up, the Baldwin Effect is another instance that shows, in agreement with West-Eberhard’s framework, that genes are followers, not leaders, in evolution.
The traditional evolutionary framework completely ignored phenotypic plasticity because this property did not fit in with the evolutionary assumptions of the Evolutionary Synthesis and the resulting Neo-Darwinist thought. The reason is clear: because evolution was taken to be a mere change in allelic frequencies, environmental conditions and phenotypic plasticity were put aside, for neither aspect was genetic and they were, therefore, considered to be irrelevant from an evolutionary point of view.
However, such traditional Neo-Darwinian assumptions are now being strongly challenged. In fact, there exists a wide range of extragenetic factors that do become inherited, many of them passing environmental information to the offspring (Sultan 2015) and producing nongenetic although inherited adaptations. This means that the genotype cannot dictate the adaptation to the environment (i.e., there cannot be a single predetermined developmental outcome); rather, the phenotype becomes actively generated through the developmental process (Sultan 2017). Accordingly, the environmentally induced phenotypic variation has a crucial role in creating the conditions that produce an adaptive genetic response (let us remember genes as followers, not as leaders). To sum up, the status of the environment greatly surpasses the role of a mere selective filter (Sultan 2015: 41). It is for this reason that phenotypic plasticity will presumably gain an increasingly important role in evolutionary biology in the years to come.
This entry has benefitted from a grant of the Spanish Government (Ministry of Economy, Industry and Competitiveness) (Ref. FFI2017-87699-P).
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