James Gorman of the New York Times has a great series of videos, called Science Take, and one of my favorites for showing students is this one, where a pea hen is equipped with a head-mounted video camera, so that scientists can figure out just what she is looking at. The lesson from this experiment was that the pea hen looks at the male doing a variety of things, including turning his back on the pea hen and shaking his tail feathers. He also shivers his gorgeous fan of feathers. The peahen has no such fan. If bodily features are crafted by natural selection, this is a puzzle. “The sight of a peacock’s tail,” Darwin once wrote, “whenever I look at it, makes me sick.” However, he later came up with the idea of sexual selection as an explanation for this sort of extreme sexual dimorphism. The basic idea was that females like certain features of some males, features that are heritable. There is thus a bias, in each generation, favoring these traits, and they can over many generations become quite extreme in males even thought they do not occur in females at all, and even though they are costly and may reduce viability of the males. This proposal explains a great many cases in nature; the predictions it makes are borne out by observation and experiment. But what underlies the peahen’s preference in the first place? Marion Petrie has shown that the elaboration of the male train or fan is correlated with the health of the chicks who will be born later. This is a case where it appears that there is a fitness advantage to the choice by the pea hen to favor the most spectacular males.
Female choice of a mate, however, may be quite arbitrary, as for example when female zebrafish appear to prefer genetically engineered males that glow with a reddish color. Certainly this is fortuitous, an incidental effect of some built-in neural mechanism. Will this lead to evolutionary changes and spreading of the engineered gene? Apparently not. Normal males chase away glowing males and outcompete them for mates, despite what the females like. This reminds us that there are a lot of factors that can enter into the equation for evolutionary fitness, which is all about the number of offspring, not necessarily about who is strongest, most beautiful, or lives longest.
The mechanism for female choice must vary from case to case, but it seems that the genes for the attractive trait and the genes for choosing this trait are associated, either by outright linkage or by a physiological correlation. There are a number of theories on the mechanisms underlying sexual selection, some of which are discussed in a short Wikipedia article.
It is not always the male of the species who develops the most colorful displays. In the straight nose pipefish, where males invest a lot of energy in the raising of offspring, including all the labor of giving birth, it is the females who compete for the males’ affections and who have the most decorative bodily features. This shows that one of the governing factors is the relative work load of one sex vs the other in producing offspring.
In humans the female obviously plays a costly role by giving birth, but the male, through the institution of pair-bonding, also contributes greatly during the pregnancy as well as later, in the provisioning and education of children. In humans there is competition for mates among both males and females. The remark “they make a good pair” thus tells us nothing about the traits of a couple, except that they are, in some important way, well-matched.
Picture from zimbio, where there is a game on how good you are at marriage.
I make no claims about the validity of the game!