Selection data: Reproductive success and morphology

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 is rarely considered. In
addition, when intralocus conflict 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 alternative
reproductive tactics 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 intralocus
tactical conflict is constraining both body size and the sword from
reaching their phenotypic optima. We discuss the importance of considering
the role of intralocus conflict 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.