Relationships between traits and fitness are complex
The relationships between traits and fitness in a given environment are
defined by multiple selective coefficients that shape the evolutionary
course of populations. Natural Selection explains much of evolutionary
processes that link variation in life expression with success in a given
environmental context.
The total selection pressure on a given trait comprises the effects of
direct, indirect and correlated selection on that trait. Selection of
trait values may be direct, as when the environmental pressures affect
the distribution of the values of a trait in the population. Or
selection may be indirect, as when distribution of trait values is
modified due to correlations with other traits under selection. Indirect
selection occurs because phenotypes are an integration of realized
multiple trait values within a single individual.
Also, selection may be correlated, as when it acts on particular trait
combinations (Lande and Arnold 1983). For example, the combination of
fruit size and seed size may be a cue for dispersers about pulp
quantity. Thus, a negative correlation between these two traits may be
related to higher dispersal success (Sobral et al. 2010). Traits may be
pleiotropically correlated because they are related to the same genes
and biosynthetic pathways. Other reasons for trait correlations include
mechanistic relationships. In these relationships, mechanistic reasons
preclude the existence of infinite combinations. An example of a
mechanistic relationship would occur when the length of a given organ is
correlated with the width. Although these traits could be codified by
independent genes, and modified by independent selective pressures, the
length and width are mechanistically related. The length of a fruit, for
instance, must cohere with the width.
All these kinds of selective pressure can be linear or non-linear—that
is, quadratic. (Phillips and Arnold, 1989). Quadratic selection implies
a trait has different relationships with fitness. These relationships
depend on the trait’s own value, which causes non-linear selective
curves. These curves may be concave or convex, where bimodal or unimodal
distributions of the traits would be selected (Stinchcombe et al 2008).
This analysis implies that the fitness-trait relationship is so complex
that it may vary depending on the values of the trait itself.