SEXUAL SELECTION, STABILIZING SELECTION AND FLUCTUATING ASYMMETRY IN 2 POPULATIONS OF PUPFISH (CYPRINODON-PECOSENSIS)

Fluctuating asymmetry of morphological traits is thought to reflect th e capacity of a genotype to produce an integrated, functional phenotyp e. I tested three predictions. (1) In a polygynous breeding system, un der intense sexual selection on males, breeding males should show grea ter symmetry in bilaterally symmetrical traits than non-breeding males or females. (2) If these traits are under stabilizing selection, high ly symmetrical individuals also should be modal phenotypes, thus near the mean value for that trait, whereas individuals with increased asym metry should represent marginal phenotypes, near the extremes of the d istribution for that trait. (3) Differences in the intensity of sexual selection should be reflected in differences in the degree of fluctua ting asymmetry between sexes among populations. I examined the relatio nship between male breeding status and the degree of fluctuating asymm etry of four bilaterally symmetrical traits, preorbital and preopercul ar pores and pectoral and pelvic fin rays, in two populations of Pecos pupfish which differed in the intensity of sexual selection. These tr aits do not function in male-male competition or female choice, thus a re not directly affected by sexual selection. In Mirror Lake breeding males, as a group, were most symmetrical for all four traits, while no n-breeding males and females showed higher levels of fluctuating asymm etry. Similarly, symmetrical individuals also represented modal phenot ypes for four traits (breeding males), and for three traits (non-breed ing males and females). These patterns were not seen in the Lake Franc is population, where breeding males were as asymmetrical as non-breedi ng males and females, and the degree of fluctuating symmetry did not d iffer between modal and marginal phenotypes for any of the four traits . When ecological conditions favour intense sexual selection, either t hrough female choice, male-male competition, or both, breeding males r epresent the most fit phenotypes. Thus sexual selection reinforces the effects of stabilizing selection on characters that do not function a s secondary sexual trails. However, when sexual selection is relaxed, differences between sexes disappear. (C) 1997 The Linnean Society of L ondon.