Sexual traits condition-dependence

Hill, G. E. 1990. Female house finches prefer colorful males Sexual selection for a condition-dependent trait. Anim. Behav. 40 563-572. 

The genetic contribution to size of the sexual trait (/) has two parts, one that is condition independent f) and the other that is condition dependent [f v), where condition is denoted by v. The genetic variance of s can be estimated as (Rowe Houle 1996),... 

So if condition-dependence is strong (i.e., / >0), genetic variation in the sexual trait will largely depend on genetic variation in condition, as for most traits... 

To some extent all traits show condition-dependent expression. For example, most traits scale with body size. The important question is whether we expect condition-dependence to be greater in sexual traits We can study this by... 

Where costs of sexual trait exaggeration strongly depend on condition (large k), we expect condition-dependence (f) to be large as well. 

In the first experiment (David et al 1998), condition was experimentally manipulated by allowing larvae to develop at different food densities (a 20-fold spread, from low density to very high density). As predicted, male eyespan showed a marked decrease in size as larval density increased (Fig. 2). But so did the non-sexual traits (Fig. 2). This revealed that condition-dependence was a general feature of sexual and non-sexual traits in insects. 

Traits, that are affected primarily by sexual selection can evolve to an extreme not favored by natural selection acting alone. Recent theoretical work (Lande 1980, 1981, 1982 Kirkpatrick, 1982) shows that the expected change in a phenotypic trait over time (i.e., the equilibrium value of this trait) is strikingly different if the trait is affected primarily by natural selection or primarily by sexual selection. Under sexual selection, the values of phenotypic traits at equilibrium will depend largely on the initial conditions that affected those traits. Closely related species... 

---