Lamarckian reference apart, it is known that phenotypic plasticity can be an important step for organisms to withstand changing conditions in their environment. But, until what point this capacity to change anatomical, physiological or behaviour phenotypes is crucial to a subsequent adaptive status?!
Using taxa from an important family of parasitic nematodes with developmental plasticity, Susoy and colleagues (2) tried to answer these questions by making use of correlational patterns between different mouth types (morphotypes: narrow or wide) in nematode worms and a) these morphotypes complexity and b) evolutionary rates of the different taxa. The authors selected inbred strains to guarantee general homozygosity (genes with identical alleles – no genetic diversity) and tested for the presence of the two morphotypes by inducing stress (starvation, predation). From these strains they used several genes to preform the phylogenetic analyses.
They found out that when species exhibited the two morphotypes this was associated with bursts in evolutionary rates and mouth complexities. But the evolutionary rates were even higher once one of the morphotypes had been lost. According to the authors this could be due mainly to two reasons: first, during the two morph stage, epistatic effects (gene interaction with other genes) and all the downstream machinery involved in the alternate phenotypes could not be dissociated, leading to slower (or not as fast) evolutionary rates, or second, there was a built-up of genetic variation before the lost of one of the morphotypes, due to relaxed selection, that would be released once the pressure of having two morphotypes was raised.
Although these are fascinating propositions, as the authors also point out, there is no certainty about the mechanisms that drive these correlations to happen. The use of genes with known phenotypes (preferably with no epistatic effects), and their manipulation, would shed some light into this matter and give us causality. Even so, this elegant study reinforces the idea that phenotypic plasticity is crucial for evolutionary events.
1. Bradshaw AD. 1965. Evolutionary Significance of Phenotypic Plasticity in Plants. Advances in Genetics 13: 115-155.
2. Susoy V, Ragsdale EJ, Kanzaki N, and Sommer RJ. 2015. Rapid Diversification Associated with a Macroevolutionary Pulse of Developmental Plasticity. eLife 4: 1–17. doi:10.7554/eLife.05463.