Developmental Plasticity and Evolution

Mary Jane West-Eberhard

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This is some of the best empirical evidence to date for the theory that phenotypic plasticity enhances the ability of a population to evolve new forms also known as evolvability. Moreover, although polyphenism evolved only once, it appears to have been lost at least 10 times during the subsequent evolution of the lineage.

The accommodation of plasticity in evolutionary biology

Unexpectedly, the loss of polyphenism was followed by an even stronger rate of subsequent evolution of new traits and phenotypes. Thus the loss of phenotypic plasticity was also associated with an increase in evolvability. But how can this be explained? One possible explanation is that a polyphenism requires developmental mechanisms that stabilize two alternative phenotypes, each in a different environment.

If they work well, such stabilizing mechanisms buffer the two phenotypes against moderate changes to the organism's genetic makeup.

Background

This means that many mutations will not effect the phenotype and therefore will not be selected against. Such mutations will gradually accumulate in a population Figure 1. Then, when the polyphenism is lost, the need to stabilize one of the two phenotypes disappears. Thus some of the accumulated genetic variation is no longer buffered and can cause the phenotype to vary more. This new phenotypic variation can now come under selection and lead to diverse adaptations in different lineages.

Understanding Genetics

Changes in the genetic makeup, or genotype, of an organism can lead to changes in its traits and characteristics, also known as its phenotype. The sloped surface represents the hypothetical ideal relationship between genotype and phenotype in different environments, but in the absence of stabilizing mechanisms. A As part of a thought experiment, consider a population where at first all individuals have the same phenotype shown as a red ellipse. This phenotype is stabilized by developmental mechanisms, which allow some genetic variation to accumulate depicted as open ellipses expanding to the right.

The evolution of a polyphenism arrow 1 establishes a new phenotype yellow ellipse in a different environment, but with the same genotype. More genetic variation will accumulate arrow 2 that has no effect on the phenotypes but improves stabilization of the alternative phenotypes in different environmental conditions. B When the polyphenism is lost arrow 3 , the mechanisms that stabilized the second phenotype are also lost.

The accumulated genetic variation can now produce new phenotypes that initially at least fall on the ideal sloped surface, and that establish the foundation for the evolution of new traits and characteristics. It is important to note that this explanation is, of course, a thought experiment that could be supported by statistical analyses.

Stephen C. Stearns and Jacob C. Koella

But the great challenge for the future will be to establish whether it is possible to devise experiments that can prove whether such a mechanism exists in nature. This article is distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use and redistribution provided that the original author and source are credited. Article citation count generated by polling the highest count across the following sources: Scopus , Crossref , PubMed Central.

Developmental plasticity has been proposed to facilitate phenotypic diversification in plants and animals, but the macroevolutionary potential of plastic traits remains to be objectively tested. We studied the evolution of feeding structures in a group of 90 nematodes, including Caenorhabditis elegans , some species of which have evolved a mouthpart polyphenism, moveable teeth, and predatory feeding. Comparative analyses of shape and form, using geometric morphometrics, and of structural complexity revealed a rapid process of diversification associated with developmental plasticity.

First, dimorphism was associated with a sharp increase in complexity and elevated evolutionary rates, represented by a radiation of feeding-forms with structural novelties. Second, the subsequent assimilation of a single phenotype coincided with a decrease in mouthpart complexity but an even stronger increase in evolutionary rates.

Cited 4 Views 1, Annotations Open annotations. Environmental responsiveness and phenotypic plasticity are found everywhere in nature.

All organisms are exposed to an environment and most of these environments are changing constantly, often in an unpredictable manner. Not surprisingly therefore, plasticity is found in all domains of life and at all levels of biological organization. Developmental phenotypic plasticity describes the property of a genotype to respond to environmental variation by producing distinct phenotypes.

The concept of plasticity dates back to the beginning of the 20th century and has continuously been developed to arrive at its current state, where many practitioners consider plasticity to represent a major facilitator of evolutionary diversification. In our lab, we investigate developmental plasticity at an integrative level.

We are trying to answer the following questions: 1. What are the molecular mechanisms underlying developmental plasticity? How do genes end the environment interact to regulate plastic traits? How is environmental information becoming encoded and integrated into the organism? We are using a mouth-form dimorphism in Pristionchus pacificus and its relatives to study the questions mentioned above. Pristionchus waits for the beetle to die before exiting the arrested dauer stage.

At that time, there is enormous competition for food and survival between many animals and microbes all living on the carcass. It is long known that Pristionchus and relatives form teeth-like denticles in their mouth, which allow predatory feeding see figure above. In the case of P. Eu animals form two teeth with a wide buccal cavity, representing predators.

In contrast, St animals have a single tooth with a narrow buccal cavity and are strict microbial feeders. This dimorphism represents an example of phenotypic plasticity Bento et al. It is this aspect of stochastic regulation that allows manipulation of plasticity by genetic and molecular tools. Both genes are part of a developmental switch with loss-of-function and overexpression, resulting in complete, but opposite phenotypes.

Developmental Plasticity and Evolution

Developmental switches had long been predicted to play an important role in plasticity regulation, but due to the absence of genetic models of plasticity little genetic evidence was obtained. More recent studies in our lab began to investigate the epigenetic and potential trans-generational effects in the control of mouth-form plasticity.

Answering this question requires comparative studies that when performed in a phylogenetic context can provide insight into the significance of plasticity for evolutionary processes. Two recent studies have moved this analysis to the macro-evolutionary level, suggesting that phenotypic plasticity indeed facilitates rapid diversification. First, we studied the evolution of feeding structures in more than 90 nematode species using geometric morphometrics Susoy et al.

This study found that feeding dimorphism was indeed associated with a strong increase in complexity of mouth-form structures.

At the same time, the subsequent assimilation of a single mouth-form phenotype coincided with a decrease in morphological complexity, but an increase in evolutionary rates. A second case of mouth-form plasticity increasing morphological diversification came from a striking example of fig-associated Pristionchus nematodes Susoy et al. These nematodes form five distinct mouth-forms that occur in succession in developing fig synconia, thereby increasing the polyphenism from two to five distinct morphs.

Additionally, the morphological diversity of these five morphs exceeds that of several higher taxa, although all five morphs are formed by the same species. These findings strongly support the facilitator hypothesis and they also indicate that ecological diversity can be maintained in the absence of genetic variation as all this diversity is seen within a single species and without associated speciation and radiation events.

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