Optimization and related concepts
I argue in Chapter 6 that organisms are never optimally designed. Designs of organs, developmental programs, etc. are legacies from the past and natural selection can affect them in only two ways. It can adjust the numbers of mutually exclusive designs until they reach frequency- dependent equilibria, often with only one design that excludes alternatives. It can also optimize a design's parameters so as to maximize the fitness attainable with that design under current conditions. This is what is usually meant by optimization in biology. An analogy might be the common wooden-handled, steel-bladed tool design. With different parameter values it could be a knife, a screw driver, or many other kinds of tool -- many, but not all. The fixed-blade constraint would rule out turning it into a drill with meshing gears. The wood-and-steel constraint would rule out use as a hand lens.
Maynard Smith ( 1974, 1982) is both pioneer and current doyen of ESS (Evolutionarily Stable Strategy) theory (see also Parker 1984; Hines 1987), which now provides the standard method of analysis for frequency- dependent equilibria. This theory would have served Darwin well in coping with special difficulties he perceived ( 1859, Chapter 6) in 'diversified habits in the same species.'
Prevalence of maleness and femaleness as mutually exclusive designs is the most easily appreciated example of an ESS resulting from frequency- dependent selection. Any frequenter of a singles bar knows that membership in the minority sex confers an advantage in the games played in such settings. The same principle applies to a natural population. If there were consistently more females than males, the males would, on average, reproduce more successfully. Selection would favor any tendency