I'm not sure I fully understand your question.  By definition, phenotypic plasticity results in non-evolutionary changes.  Since the alleles that allow phenotypic plasticity are differentially expressed with the environmental gradients, there is no introduction of new alleles during this process.  In other words, the selection process that favored the plasticity of gene expression has already occurred.  Phenotypic plasticity improves survival, but it's a different process than the introduction of new alleles.  There is no adaptation.  The adaption has already occurred and is expressed as the phenotypic plasticity itself.

Phenotypic plasticity is more important in sessile organisms, such as plants, becaue they can't move away from unfavorable environments, but it does play an important role in mobile ones as well.

Jay, thank you very much for providing an answer. Apologies for the confusion! (i have amended my question a bit) All I am trying to get help with is that the current consensus in the literature is divided on whether plastic traits are adaptive in nature, they may or may not be! i.e. if  the presence of a plastic trait improves the reproductive/survival potential of the individual and enables population viability! Any thoughts?

Thanks for clarifying.  Phenoptypic plasticity is an adaptation to changing environments.  I don't think there's much debate on whether or not it's an adaptation in itself.

The questions arise around the mechanism of gene expression, as there are several ways to up and down regulate genes.  Then throw in epigenetics and methylation and the the sorts, things get really fun to discuss.

Phenotypic plasticity is proposed to introduce genotypic diversity  through the process of genetic assimilation, by which an ancestrally plastic trait becomes "genetically fixated" at different values on different populations or sets of individuals. The idea is that phenotypic plasticity is a developmentally wired response whose outcome results from parsing out some environmental variable; under certain circumstances (for example, a shift to a more stable/predictable environment), development of the plastic trait switches to depend on an internal/genetic variable rather than an environmental one. At that point, the trait's phenotypic state becomes no longer responsive to the environment, but genetically determined.

Thanks Eduardo. Some interesting thoughts there. Have you/could you recommend any key refs?

I would recommend you to look up the key papers from Mary Jane West-Eberhard, who pioneered the idea that phenotypic plasticity had a key role in evolution.

She even has a wikipedia entry at http://en.wikipedia.org/wiki/Mary_Jane_West-Eberhard

A more recent discussion putting phenotypic plasticity, the more novel concept of niche construction into the framework of evolutionary theory came out recently in Nature, check out http://www.nature.com/news/does-evolutionary-theory-need-a-rethink-1.16080.

http://en.wikipedia.org/wiki/Mary_Jane_West-Eberhard

http://www.nature.com/news/does-evolutionary-theory-need-a-rethink-1.16080

Thank you for this Eduardo. This (nature discussion) looks like an interesting article.   

I second Eduardo's recommendation to look at MJ West-Eberhard's work, and that of her mentor, Richard D. Alexander.  

Niche construction is muddled in semantics, particularly on the topic of cultural evolution. 

Here are some articles on costs of phenotypic plasicity; included some citations:

Auld JR, Agrawal AA & Relyea RA (2010) Re-evaluating the costs and limits of adaptive phenotypic plasticity. Proc. R. Soc. 277B:503-511.

„When the optimal phenotype differs among environments, adaptive phenotypic plasticity can evolve unless constraints impede such evolution. ... Meanwhile, our understanding of the limits of plasticity, and how they may be underlain by the costs of plasticity, is still in its infancy. Based on our re-evaluation of these constraints, we discuss areas for future research“ (p 503).

Snell-Rood EC, Van Dyken DJ, Cruickshank T, Wade MJ & Moczek AP (2010) Toward a population genetic framework of developmental evolution: the costs, limits, and consequences of phenotypic plasticity. BioEssays 32:71-81.

„... it is clear that our understanding of the costs and limits of plasticity remains incomplete“ (p71).

Van Buskirk J & Steiner UK (2009) The fitness costs of developmental canalization and plasticity. J. evol. Biol. 22:852-860.

„We present a metaanalysis of 27 studies (of 16 species of plant and 7 animals) that have measured selection on the degree of plasticity independent of the characters expressed within environments. ... These findings suggest that costs of plasticity are often weak, and may influence phenotypic evolution only under stressful conditions“ (Abstract).

DeWitt TJ (1998) Costs and limits of phenotypic plasticity: Tests with predator-induced morphology and life history in a freshwater snail. J. evol. Biol. 11:465-480.

„The benefit of plasticity can be reduced if facultative development is associated with energetic costs, developmental instability, or an impaired developmental range“ (p 465).

„Although the benefits of phenotypic plasticity are clear, the costs and limits that constrain its evolution are more obscure“ (p 466).

„surprisingly little empirical work has explicitly tested for costs of plasticity. ..., yet no empirical studies have used this approach. As well, little empirical work has been published on limits of plasticity, but models of these limits are common“ (p 466).

DeWitt TJ, Sih A & Wilson AS (1998) Costs and limits of phenotypic plasticity. Trend Ecol. Evol. 13:77-81.

Pigliucci M (2005) Evolution of phenotypic plasticity: where are we going now? Trends Ecol. Evol. 20:481-486.

„Research of costs of plasticity is still in its infancy, but is both theoretically important and empirically challenging, and should become a major area of future inquiry“ (p 483).

Snell-Rood E (2012) Selective Processes in Development: Implications for the Costs and Benefits of Phenotypic Plasticity. Integr. Comp. Biol. 52:31-42.

Thanks to Reinhard for providing a superb list of references.

I don't want to take this thread off on a tangent, but one of the interesting issues to ponder about phenotypic plasticity is how learning -- in particular social learning & "culture" -- fit into concepts of phenotypic plasticity.  This is an area for potentially productive discussions between evolutionary biologists and social/behavioral scientists.  The unusual attributes of human culture provides some difficult challenges for models of phenotypic plasticity.  Apologies for the self-referencing below; easiest way to provide sources in bibliographies.

Flinn, M.V. (2013). The evolutionary biology of culture. In K. Summers & B. Crespi, (Eds.) Foundations of Human Social Evolution, Pp. 94-103. Oxford: Oxford University Press.

Flinn, M.V., Ponzi, D., Nepomnaschy, P. & Noone, R. (2012). Ontogeny of stress reactivity: phenotypic flexibility, trade-offs, and pathology. Current Topics in Neurotoxicity 3, (Mal)adaptive aspects of developmental stress, Laviola, G. & Macrì, S. (Eds.), Pp. 154-172. Berlin: Springer.

Flinn, M.V., Nepomnaschy, P., Muehlenbein, M.P., & Ponzi, D. (2011). Evolutionary functions of early social modulation of hypothalamic-pituitary-adrenal axis development in humans. Neuroscience and Biobehavioral Reviews, 35(7), 1611-1629.

Flinn, M.V. (2006c). Cross-cultural universals and variations: The evolutionary paradox of informational novelty. Psychological Inquiry, 17(2), 118-123.

Flinn, M.V . (2004). Culture and developmental plasticity: Evolution of the social brain. In: Evolutionary perspectives on child development. K. MacDonald & R.L. Burgess (Eds.), chapter 3, pp. 73-98. Thousand Oaks, CA: Sage.

Flinn, M.V. (1997). Culture and the evolution ofsocial learning. Evolution and Human Behavior, 18(1), 23-67.

Brilliant! Thanks Reinhard and Mark for taking time out and sending me this detailed list of articles. 

Your question has an important place in modern evolutionary biology. There are some contrasting points of view and a bundle of terminology that can bring a lot of confusion about this topic. First of all we can consider pp as trait that can be modeled by natural selection like any other trait, this makes things quite clear and confortable. Problems arise if we consider the role of pp (phenotypic plasticity) in driving evolution of novel traits, that’s your question. I think it could be useful to focus on few important general features of organisms to face your question. First point is the correlation among different traits; pp can be related to a single trait but this could be linked to others. This can promote the maintenance of cryptic genetic variation. A second point is the epigenetic marking that can be related to the plastic change in a trait in a specific environment.  I can provide some references maybe not still been mentioned, hope can be useful.

http://onlinelibrary.wiley.com/doi/10.1111/evo.12348/abstract

http://rspb.royalsocietypublishing.org/content/early/2011/06/07/rspb.2011.0971.short

http://www.sciencedirect.com/science/article/pii/S0169534710001059

http://www.biomedcentral.com/1471-2148/11/5

this is new:

http://www.nature.com/hdy/journal/vaop/ncurrent/full/hdy20158a.html

Thanks Andrea, it certainly will be useful. I am going through other refs provided here as well. I had a question for everyone in this discussion and get a handle on the understanding the concepts discussed here: Why is it that there is a greater diversity of species in the tropics relative to the temperate zones? What factors drive them? I appreciate that this is not straightforward and involves multiple disciplines (i.e. physiological, anatomical, developmental etc). 

Plasticity is not always adaptive. Plasticity is the environmentally induced change in the phenotype that is not caused by a change in the DNA sequence. If it is linked to an increase in fitness, then, and only then, it is adaptive. In fact, a recent paper in Nature by Ghalambor et al discusses the consequences of adaptive and maladtive plasticity in terms of genetic selection.

I suggest the following reading: Perplexing effects of phenotypic plasticity, by Juha Merilä which explains the findings and the implications of the Ghalambor paper:

https://www.researchgate.net/publication/281445717_Evolutionary_biology_Perplexing_effects_of_phenotypic_plasticity

Also in the wild, 1/3 of adaptive phenotypic changes do not appear to be caused by genetic changes. The exact role of plasticity, its underlying mechanisms and how it is involved remain to be clarified but that is a great result showing that there is a large potential lying there...

https://www.researchgate.net/publication/5675549_Gienapp_P_Teplitsky_C_Alho_J_Mills_J_Merila_J.._Climate_change_and_evolution_disentangling_environmental_and_genetic_responses._Mol_Ecol_17_167-178

Regards

Article Evolutionary biology: Perplexing effects of phenotypic plasticity

Article Gienapp P, Teplitsky C, Alho J, Mills J, Merila J.. Climate ...

In terms of how environmentally induced variation can be transmitted across generations and play a significant role in evolution, I suggest the reading:

https://www.researchgate.net/publication/51226751_Beyond_DNA_integrating_inclusive_inheritance_into_an_extended_theory_of_evolution._Nat_Rev_Genet

Article Beyond DNA: Integrating inclusive inheritance into an extend...

Thank you Benoit Pujol. It is much appreciated indeed.