Phenotypic plasticity
and the evolution of warning coloration
These are examples of the differential expression of density-dependent colour polyphenism between different host plant-associated forms of Schistocerca lineata grasshoppers. Juveniles from Ptelea trifoliata-associated populations are unpalatable to vertebrate predators by virtue of feeding on their host plant and express density-dependent warning colouration (aposematism). Closely related, but genetically distinct nymphs from Rubus trivialis-associated populations do not derive unpalatability from their host plant. They should not benefit from the expression of density-dependent warning coloration, and accordingly their ability to express density-dependent colour change has been much reduced by natural selection. After Sword (2002).
Understanding patterns of
resource use can have far reaching implications. A good
example is my discovery of the first case of
density-dependent warning coloration found in nature
(Sword, 1999). In a series of laboratory experiments, I
showed that juvenile S. lineata
(emarginata) grasshoppers express
density-dependent phenotypic plasticity in coloration
whereby they are commonly green when reared at low
population density, but become yellow-and-black when reared
at high density. I then demonstrated that the grasshoppers
derive gut-content mediated toxicity to predators simply by
consuming their primary host plant. By linking host plant
use to the expression of density-dependent color change, I
demonstrated that the grasshoppers exhibit a shift in
anti-predator strategy from crypsis at low density to
warning coloration at high density in experiments using
visually-hunting vertebrate predators.
This finding led me to propose a new hypothesis for the
evolution of warning coloration, a debate dating back to
discussions between Darwin and Wallace, whereby plasticity
in coloration can act as an adaptive intermediate stage
(Sword, 1999). My collaborators and I went on to show that
S. lineata populations utilizing different plants
in the field constitute genetically-distinct host
plant-associated mtDNA lineages (Dopman et al., 2002).
These genetically-distinct lineages differ in the toxicity
to their predators that they derive from their respective
host plants. As a result, their reaction norms for the
expression of density-dependent color change have
differentially evolved in accordance with the expectations
of aposematic theory (Sword, 2002; see graphic).
Ongoing work with Dennis Fielding (USDA-ARS) includes
the use of individual-based simulation models to examine
the effects of plasticity on the initial evolution of
warning coloration under a variety of ecological
assumptions (Fielding & Sword, submitted). We also
plan to use simulation models to examine the evolution
of plastic reaction norms for density-dependent color
change to study the ways in which
constitutively-expressed warning coloration can evolve
from an initially plastic precursor via genetic
assimilation.
Relevant publications
Simpson, S.J. & Sword, G.A. (In Press)
Phase polyphenism in locusts: Mechanisms, population
consequences, adaptive significance and evolution. In:
Ananthakrishnan, T. & Whitman, D. (eds.) Phenotypic
Plasticity of Insects: Mechanisms and Consequences.
Sword, G.A. (2002) A role for phenotypic
plasticity in the evolution of aposematism. Proceedings of
the Royal Society of London B 269:1639-1644.
Dopman, E.B., Sword, G.A., & Hillis,
D.M. (2002) The importance of the ontogenetic niche in
resource-associated divergence: evidence from a generalist
grasshopper. Evolution 56(4):731-740. (Cover photo)
Sword, G.A. (2001) Tasty on the outside,
but toxic in the middle: Grasshopper regurgitation and host
plant-mediated toxicity to a vertebrate predator. Oecologia
128:416-421.
Sword, G.A., Simpson, S.J., El Hadi,
O.T.M. & Wilps, H. (2000) Density-dependent aposematism
in the Desert locust. Proceedings of the Royal Society of
London B 267:63-68.
Sword, G.A. & Simpson, S.J. (2000) Is
there an intraspecific function for density-dependent color
change in the Desert locust? Animal Behaviour 59: 861-870.
Sword, G.A. (1999) Density-dependent
warning coloration. Nature 397:217.
Sword, G.A. and Dopman, E.B. (1999)
Developmental specialization and geographic structure of
host plant use in a polyphagous grasshopper,
Schistocerca emarginata (=lineata) (Orthoptera:
Acrididae). Oecologia 120:437-445.
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