Varying the level of investment in a quantitative trait
is rarely simply advantageous or simply disadvantageous.
There are many components to overall fitness, and increasing
investment in one is usually done at the expense of
others. For example, growing large, though possibly beneficial
in intraspecific competition, raises metabolic costs
and also lengthens the time spent growing and thus delays
the onset of reproduction. In the pygmy swordtail, Xiphophorous
nigrensis, there are large males who perform
elaborate courtship displays and small males who do not
display but seek “sneak” copulations. The difference is
underlain by a genetic polymorphism