imbalances in clade size observed today are the product of
both past speciation and extinction events. Several authors
have suggested that the selection load, generated by the
evolution of extravagant traits via sexual selection, is
higher in species with more intense sexual selection,
thereby increasing the risk that these species will go extinct
(Dawkins & Krebs 1979; Lande 1980; Kirkpatrick 1982;
McLain 1993; Tanaka 1996; Møller 2000). Darwin was
probably the first to allude to this idea when he framed
his model of trait evolution via sexual selection (Darwin
1871) as directly opposing the evolution of traits via natural
selection (Darwin 1859). In simple terms, ornamental
traits could never evolve by natural selection alone, owing
to their inherent extravagance. Although there is theoretical
evidence that sexual selection may increase the likelihood
that beneficial alleles will become fixed (Whitlock
2000), there is empirical evidence that sexual selection
does not affect the rate of adaptation to a novel environment
(Holland 2002), and that the selection load is real.
For example, in Drosophila melanogaster, Wilkinson (1987)
elegantly showed, experimentally, that there exists an
equilibrium between the opposing forces of natural and
sexual selection. Subsequently, several artificial selection
experiments in the same species have directly demonstrated
that absolute population fitness increases significantly
when sexual selection is relaxed or removed (Rice
1996; Holland & Rice 1999; Pitnick et al. 2001). There
are also several other lines of evidence that suggest sexual
selection may relate positively to extinction risk. For
example, two comparative studies of birds have shown
that male survival rates are lower in sexually dimorphic
species compared with sexually monomorphic species
(Promislow et al. 1992, 1994). Several introductions of