Temporal instability of lake charr phenotypes: Synchronicity of growth rates and morphology linked to environmental variables?

Pathways through which phenotypic variation among individuals arise can be
complex. One assumption often made in relation to intraspecific diversity
is that the stability or predictability of the environment will interact
with expression of the underlying phenotypic variation. To address
biological complexity below the species level, we investigated variability
across years in morphology and annual growth increments between and within
two sympatric lake charr Salvelinus namaycush ecotypes in Rush Lake, USA.
A rapid phenotypic shift in body and head shape was found within a decade.
The magnitude and direction of the observed phenotypic change was
consistent in both ecotypes, which suggests similar pathways caused the
variation over time. Over the same time period, annual growth increments
declined for both lake charr ecotypes and corresponded with a consistent
phenotypic shift of each ecotype. Despite ecotype-specific annual growth
changes in response to winter conditions, the observed annual growth shift
for both ecotypes was linked, to some degree, with variation in the
environment. Particularly, a declining trend in regional cloud cover was
associated with an increase of early stage (ages 1-3) annual growth for
lake charr of Rush Lake. Underlying mechanisms causing changes in growth
rates and constrained morphological modulation are not fully understood.
An improved knowledge of the biology hidden within the expression of
phenotypic variation promises to clarify our understanding of temporal
morphological diversity and instability.