Relationships between traits and fitness are defined within a
context, but the context varies continuously
Selection coefficients and gradients measure the slope of the
relationship between the realized values of a trait and the relative
fitness (that is, individual fitness divided by average population
fitness) of each individual within a population. But these trait-fitness
relationships, or phenotypic selective pressures, make sense only in a
given environmental context.
However, environmental contexts vary endlessly. The environment that
affects a trait-fitness relationship is complex and acts at different
levels. First, at the within-individual environment level (the
phenotypic environment), values of a trait would have different fitness
outputs depending on the values of other characters within the
integrated phenotypes. These different fitness outputs may occur even
when the abiotic and biotic environment is equal. This is due to the
existence of indirect and correlated selection, as shown above.
Second, at the population environment level, a single trait may be
related to different fitness outputs where trait values can be positive,
neutral or negative depending on population parameters such as
population density (see for example
Yang et al 2018).
Third, at the community level, due to mutualistic and antagonistic
interactions between species, the expected effects of traits on
community composition loop back to modify the relationships between
traits and fitness.
Last, at the ecosystem level, the abiotic and biotic environments
continuously vary, spatially and temporally. These variations are
particularly important, regarding the potential unpredictable shifts
driven by Global Change.
Given all the direct and indirect sources of selection previously
described, and because of the complexity laid out here of the selective
pressures over the integrated phenotype and the varying environment, it
is reasonable to argue that any trait would be related to fitness in at
least one of the potential environments. That is, traits would be
related to fitness depending on environmental variation. For example, a
plant population would undergo strong selection on root traits during a
drought year, and the same plant population would undergo strong
selection on flower traits during a year with severe limitation of
pollinators. Nonetheless, if we study the same population during a year
with optimum precipitation levels and plenty of pollinators, we cannot
argue that root traits or flower traits in this species are
non-functional traits.
If in any one of all possible environments a potential link exists
between the distribution of values of a trait and fitness, then all
traits are potentially linked to fitness, and thus are functional by
definition. Evolution happens through ecology, and the object of study
in ecology -diversity at different levels- is the consequence of
evolution. We cannot rend one from the other. Rather, ecology and
evolutionary biology should be integrated in the study of causes and
consequences of trait diversity, a possible start point is to
acknowledge that all traits are functional.
Acknowledgement s Thanks to R. Lande, M. Kulbaba, C.M. Herrera
and L. Sampedro for comments on the manuscript.
Data accessibility statement. NA
Competing Interests. Since this paper explains why the use of the term
“functional trait” is redundant, and even misleading,we can guess that
scientists working in the area of “functional traits” would disagree
with the publication of the arguments here explained.
Author contributions. NA
Funding information. Mar Sobral was benficiary of a grant from Xunta de
Galicia I2C (Spain) when performing this work.