Alternative reproductive tactics (ARTs) have provided valuable insights
into how sexual selection and life history tradeoffs can lead to
variation within a sex. However, the possibility that tactics may
constrain evolution through intralocus tactical conflict (IATC) is
rarely considered. In addition, when IATC has been considered, the focus
has often been on the genetic correlations between the ARTs, while
evidence that the ARTs have different optima for associated traits and
that at least one of the tactics is not at its optima is often missing.
Here we investigate selection on three traits associated with the ARTs
in the swordtail fish Xiphophorus multilineatus; body size, body
shape and the sexually selected trait for which these fishes were named,
sword length (elongation of the caudal fin). All three traits are
tactically dimorphic, with courter males being larger, deeper bodied and
having longer swords, and the sneaker males being smaller, more fusiform
and having shorter swords. Using measures of reproductive success in a
wild population we calculated selection differentials, linear and
quadratic gradients, demonstrate that the tactics have different optima
and at least one of the tactics is not at its optima for body size and
sword length. Our results provide the first evidence of selection in the
wild on the sword, an iconic trait for sexual selection. In addition,
given the high probability that these traits are genetically correlated
to some extent between the two tactics, our study suggests that IATC is
constraining both body size and the sword from reaching their phenotypic
optima. We discuss the importance of considering the role of IATC in the
evolution of tactical dimorphism, how this conflict can be present
despite tactical dimorphism, and how it is important to consider this
conflict when explaining not only variation within a species but
differences across species as well.
Reluctant males: why blue monkey males reject female sexual advances