Phenotypic Plasticity

Aschania levana, spring morph. © Manuel Valdueza
Araschnia levana, spring morph. © Manuel Valdueza

The nymphalid above, the map butterfly Araschnia levana, produces multiple generations each year. Its pupae come in two forms, dark and light, the difference arising from varying melanin concentrations. These differences persist in the adult. Above you see the adult that emerged from a light pupa; below is the adult from a dark pupa.

Aschania levana, summer morph. © David Short
Araschnia levana, summer morph. © David Short

This butterfly exhibits seasonal polyphenism: the orange morph emerges in spring, while the drab brown morph emerges in summer.

The eulophid wasp Melittoba chalybii is a parasitoid on bees and other wasps, laying eggs in the host for the larvae to feed on. Two forms of larva can be found in a parasitised victim. The first develops during the first 30 days after hatching, while the second takes 60-75 days to develop. This allows the larvae to best exploit their food source, allowing them to extract a very high nutrition:larva ratio, ultimately leading to more pupation and thus more adult parasitoids (see Mathews et al., 2009).

Both of these are examples of phenotypic plasticity. The appearance of the butterfly changes according to the seasons in order to better match the environment. In spring, the orange allows it to blend in well with flowers, giving it more camouflage and thus less predation risk. The environments that species find themselves in fluctuate between states of regularity. Winter is a time of short photoperiod, low temperatures, and less nutrition available; summer is the exact opposite. Spring and fall have their own intermediate properties. A species that has the ability to adapt to each of these special conditions by developing specific phenotypes obviously gains a distinct advantage, allowing it to exploit the conditions for its optimal survival.

More generally, phenotypic plasticity implies that a species has the ability to expand and conquer diverse environments, since its genotype is geared to adapting to different conditions. An insect’s ability to use a variety of host plants is one example of such plasticity, since it means an insect is not dependent on one specific plant that only grows in one environment – hency why phenotypic plasticity is important to consider, as invasive pests tend to be plastic.

Phenotypic plasticity can be expressed at any level. Morphological plasticity includes the Aschania above, or pond snails that develop spiny shells in the presence of predators. Life history plasticity is possible, as with deciduous trees who shed their leaves once specific environmental triggers are encountered – the trees experience a change in temperature or photoperiod, causing a cascade of biochemical changes, causing changing gene expression, which then leads to senescence in the leaves.

Phenotypic plasticity may just be incidental, as with the ability to use various host plants, or it can be selected for and hardwired as a beneficial trait, as with all the other examples I’ve used in this post. This is why phenotypic plasticity is an important factor to study in evolutionary ecology specifically, and evolutionary biology in general. Phenotypic plasticity in that sense merges into the debate on canalisation and the relationship between micro- and macroevolution that I’m interested in: is phenotypic plasticity the result of canalisation, or is it a component needed for increased evolvability? (That’s an excellent essay question for an evolution exam.)

Further Reading:

Gotthard K & Nylin S. 1995. Adaptive Plasticity and Plasticity as an Adaptation: A Selective Review of Plasticity in Animal Morphology and Life History. Oikos 74, 3-17.

Matthews RW, González JM, Matthews JR & Deyrup LD. 2009. Biology of the Parasitoid Melittobia (Hymenoptera: Eulophidae). Annual Review of Entomology 54, 251-266.

Nijhout HF. 2003. Development and evolution of adaptive polyphenisms. Evolution & Development 5, 9-18.

Van Asch M & Visser ME. 2007. Phenology of Forest Caterpillars and Their Host Trees: The Importance of Synchrony. Annual Review of Entomology 52, 37-55.

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