Phenotypic Plasticity

What is phenotypic plasticity? How does it affect life history patterns?

My Answer:

Carl D. Schlichting of the University of Connecticut defined Phenotypic Plasticity as “the property of a genotype to produce different phenotypes when exposed to different environments. Phenotypic Plasticity can occur in the morphology and behavior of organisms.”

Thus defined, phenotypic plasticity recognizes individual organisms as complex “genetic and epigenetic” systems that respond according to the different stresses and constraints that figure in the dynamics of a continuously evolving environment. Because no super adapted species exist relative to the heterogeneity of environmental systems, the premise behind phenotypic plasticity is that, certain “incompleteness” in the genotype of organisms is compensated by variations in phenotypic expressions arising from the same genotype. In other words, phenotypic plasticity enables many organisms at different stages of growth and development to adaptively match trait expression to particular environmental settings.

How then does phenotypic plasticity affect life history patterns? It must be recognized that phenotypic plasticity is but a factor in a myriad of other factors that are central to the determination of life history patterns. Therefore, where phenotypic plasticity figures in life history patterns, its function should be taken as correlative to other evolutionary functions such as speciation, acclimatization, heterochrony, and allometry. Thus said, if phenotypic plasticity comes as a consequence of an organism’s juggling of its options to match existing environmental settings through variable expression of phenotypes, life history patterns should logically reflect these variations in the distribution of organisms across all possible geologic ecospaces. Following is an illustration of a trade off in the larvae of echinoids. (At left is an echinoid larva that was starved. Compared to the well-fed larva on the right that developed new adult structures, the starved larva developed feeding structures instead like the elongated ciliated arms.

Kathleen K. Smith of Duke University asserted in her article titled, “Heterochrony Revisited: The Evolution of Developmental Sequences”, the idea that evolutionary innovations can occur at any stage in the life history of organisms. She anchored this premise on the observation that a certain amount of change in developmental timing can produce evolutionary change. Having thus correlated developmental change – which in some cases may constitute phenotypic plasticity, to evolutionary change – which for the most part is about a survey of life history patterns, it bears noting where and how phenotypic plasticity can in fact, effect change in organisms belonging to the same taxonomic category, if not the same phyletic origin.

Stephen Jay Gould of Harvard University recognizes the idea “that morphological diversity among taxa arises from diversity underlying developmental processes responsible for ‘building’ morphological features.” Paul Magwene of Yale University in his article titled, “Comparing Ontogenetic Trajectories Using Growth Process Data”, substantiated this assertion on the correlation between developmental change and diversity among taxa by analyzing variation in phenotypic plasticity, its effect on morphology and behavior, and its influence in ontogenetic trajectories.

The aforesaid studies have shown that life history patterns may have in fact been driven by phenotypic plasticity through the mechanisms of heterochronic developmental changes. But as to whether phenotypic plasticity taken as a process independent of developmental mechanisms have direct influence on life history patterns remains uncertain. This is because phenotypic plasticity is largely on account of an organism’s response to a certain environmental stress or constraint but does not imply change in the genetic makeup of the organism. As such, it may be implied that no amount of genetic modification is introduced into the gene-pool of the population. Therefore, this hardly qualifies as a veritable evolutionary mechanism for organismic change.

Phenotypic Plasticity’s significance figures prominently in the discourse on life history patterns. It may even be made a discourse unto itself that is independent of the discourse in evolution. Is phenotypic plasticity then, to a certain level of validity, an alternative to evolution in mapping out life history patterns? Dr. Lee Spetner has this to say: “A change in phenotype in the fossil record is recognized as evolution. There is no way to tell from the fossils whether the observed changes in continuous records were caused by variation appearing in the genotype or only in the phenotype”

In other words, it cannot be deduced from the premises of evolution whether or not “apparent transitional features of a fossil are truly the results of changes in the genotype (i.e. random mutations)” or are simply the consequence of certain species adapting to varied environmental settings as a result of phenotypic plasticity.