Biology Reference
In-Depth Information
expectation that
L
.
littorea
would be more plastic. Perhaps even more surprising, the two
ecomorphs of
L
.
saxitilis
exhibit less within-treatment disparity (i.e. exhibit greater micro-
environmental canalization) when exposed to a treatment that mimics the environment of
the
other
ecomorph.
Visualizing Norms of Reaction
Norms of reaction for shape can be difficult to depict unless groups differ solely in the
magnitude of plasticity. If that is the sole difference, the norms of reaction can be depicted
just as they are for a one-dimensional trait; groups then differ solely in their position along
a single phenotype axis. The same vector would extend between any pair of means
because all are arrayed along that one axis. But, in more complex cases, when the direc-
tions of the responses also differ, we cannot represent a norm of reaction by a single phe-
notype axis. Several recent studies have contrasted the outcomes of varied treatments on
shape, depicting the norm of reaction by the contrasts between the resultant phenotypes.
One approach is to project the array of phenotypes onto a low-dimensional space; for
example, in the analysis detailed above of the impact of predator cues and wave-action on
shell morphology of
Littorina saxatilis
and
L. littorea
,
Hollander and colleagues (2006)
showed the 12 treatment means projected onto the space of the first two principal compo-
nents of shape variation. Another approach is to present a series of contrasts that depict
the impact of the environmental factor on a single group. For example, a study of the
impact of dietary consistency on marine, benthic and limnetic threespine sticklebacks dis-
played the effects by a series of deformed grids, each showing the impact of a benthic or a
limnetic diet on marine, benthic or limnetic head shapes (
Wund et al., 2008
). Similarly,
another experiment on the impact of diet on shape examined two attributes of diet, its
hardness and calcium content; analyzing pharyngeal jaw shape (and size) of laboratory
stocks of the cichlid
Amphilophus citrinellus
subjected to three diet treatments: (1) intact
snails with shell, (2) peeled snails without shell, and (3) finely ground snails that were fro-
zen, with the fish feeding on the soft thawed outer layer (
Muschick et al., 2011
). Pairs of
treatments were compared by a Discriminant Function Analysis (see Chapter 6) and the
shapes at the extremes of each function were depicted as interpolated outlines. Similarly,
diet-induced plasticity of body shape of arctic char,
Salvelinus alpinus
, was analyzed by
feeding young of each ecomorph a diet that mimics the natural diet of either a benthic or
limnetic population of three ecotypes (
Parsons et al., 2011
). Both ontogenetic trajectories
and age-specific phenotypes were compared between ecomorphs reared on the same diet
in different lakes and within ecomorphs fed benthic versus limnetic diets. Discriminant
function analysis was used to visualize the impact of diet on body shape for each eco-
morph at two ontogenetic stages, depicting the contrasts by regressing shape on the dis-
criminant function scores.
CANALIZATION: QUANTIFYING VARIATION
Canalization refers to the ability to produce the same phenotype despite variation in
genotype and environments of rearing. A long-standing (but still controversial) hypothesis
