Sexual Choice: When Evolution Turns into Intimate
Charles Darwin once said: “The sight of a feather in the tail of a peacock makes me sick when I look at it.” And he was right. Have you ever looked at – really looked at – a male Indian peafowl Pavo cristatus and wondered how such bizarre and impractical plumage could even exist? According to Darwin’s theory of evolution, natural selection should have made them extinct by now.
So what’s going on here?
Only later would Darwin find the answer: sexual selection. This is how it works.
Having young ones takes a lot of energy, so in most species the females are picky. When a man develops a trait that women find attractive, he gets more action than any other man and passes on his “sexy” trait to the rest of the population, regardless of whether the trait actually promotes survival.
Women might find a trait attractive for a number of reasons. Redwings Turdus iliacus are programmed to look for red berries on trees, so women may associate a man’s red markings with food.
Redwing, Copyright Glyn Sellors, from the Surfbirds Galleries
Alternatively, an impractical “ornament” like a peacock’s tail can serve as an honest representation of a man’s fitness. If, despite this handicap, a man is still able to feed and escape predators, it shows that he can survive very well. A peacock’s tail actually screams, “Look at me! Only an incredibly sturdy and capable man can survive pulling that ridiculous cock around. You definitely want to mate with me so you can have strapped and capable babies! “
In fact, growing male ornaments like the comb of a Gallus gallus (the ancestor of our domesticated chicken) requires large amounts of testosterone, a hormone that actually suppresses the immune system. Staying disease-free in the face of such difficulties is the sign of a really good constitution. And people are not excluded from it. Studies have shown that muscular men are more prone to minor infections like colds than thin men because high levels of testosterone suppress the immune system.
But sexual selection doesn’t have to have anything to do with a man’s strength. Occasionally, a woman may just develop a random preference for something that may not even exist. The gene for this preference can spread through the population purely by chance, but it can never come into action – unless a lucky man experiences a mutation that makes him the object of their desire.
This may all sound a little crazy, but believe me, it happens. Let me tell you two fish: the swordtail and the platyfish.
Unsurprisingly, the green swordtail, Xiphophorus hellerii, has a sword-shaped tail. The southern platyfish Xiphophorus maculatus (an “older” species that used to branch off the evolutionary tree) does not. Interestingly, the platyfish has a predilection for sword-shaped tails: a predilection for something that doesn’t exist in its own species.
To discover this, the scientists used the most modern and advanced techniques: they put artificial swords on the tails of male platyfish. The effect on women was impressive. When it came to sex appeal, platyfish with artificial swords undoubtedly won. This implies that a fondness for sword-shaped tails lurked long before the sword shape was developed.
There is no reason to believe that humans are any different in this regard – but unlike animals, we can change our surroundings and ourselves to a far greater extent. From the towering powder wigs of the 18th century to modern day bodybuilders who spend so much time pumping iron in the gym that they have joint problems, we too owe it to us to put image before practicality.
But sexual ornaments, like human fashion, are out of date. A trait that works with women will spread across the entire population – but once everyone has it, it’s no longer special. Therefore, new ornaments will evolve to make some men stand out from the crowd. The problem is that all of the old ornaments must be left as they now form the woman’s mental image of what a man should look like. And so we return to the tragic image of the peacock, which is weighed down by innumerable ornate ornaments, of which only a few are perceived by the females. No wonder Darwin felt bad.