Evolution: Survival of the Fittest Versus "Decadence"

When we’re first learning about evolution, we always hear about it the same way – survival of the fittest. But there’s also another pressure on the evolution of a species: sexual selection. So what happens when sexual selection and environmental pressure aren’t compatible?

Generally, most classroom examples focus on times when sexual selection and environmental pressure can dovetail into the same explanation. For example, female birds prefer to mate with brightly colored males because healthy feathers mean the male’s diet is good, which means he’s got good genes, which means his genes will help her offspring survive. Simple enough, right?

But, especially when we’re looking at birds, things quickly start to get more complicated. What about the peacock’s tail? Surely all that tail isn’t really necessary to signal environmental fitness, right? While the vibrant colors of a peacock aren’t actually that obvious to its dichromat predators, the tail still seems like an awful lot of energetic effort.

The Club-Winged Manakin and Decadence

For an even murkier example, let’s look at the club-winged manakin. I first heard about this bird thanks to an excellent Radiolab episode called The Beauty Puzzle, which got me thinking about this topic.

The male club-winged manakin courts the female by whacking his wings together at an astonishing 107 times per second. Apparently, the females like the sound and will mate with males who can do it well. The problem? In order to effectively make that sound, the club-winged manakins need solid wingbones.

Let that sink in – a flighted bird with solid wingbones.

The club-winged manakin now has slow, clumsy flight. While the trees in the Amazon Rainforest are dense enough to allow some hopping, these birds are at a serious disadvantage. Since the bones become solid while the birds are tiny embryos (before they’re male or female), both males and females have this disadvantage.

According to scientists, this phenomenon is known as “decadence” – the females are choosing beauty that actually harms the survival prospects of their own young.

But this flies in the face of the basic tenet of evolution: evolution is about survival of the fittest. And yet, these birds are evolving a handicap in the name of sexual selection.

There’s nothing, in theory, to stop this trend from pushing the club-winged manakin to extinction.

Many high school biology teachers just tell you about adaptive evolution – pretty feathers and showy colors exist because they signal that the showy male is extra-healthy.

But in the club-winged manakin, we have at least one clear counter-example. Biologists seem uncomfortable with the concept that some aspects of evolution can be driven by pleasure rather than function.

Survival of the Sexiest?

It is arguably impossible to know if the peacock’s tail really is a great signal of health, or if the bower bird’s bower-building signals his fitness.

We can’t ask the females if they like the looks of a male because of his beauty or because he seems fit. And even if we could, perhaps the females perceive something as pretty because it signals fitness.

We don’t know for sure.

But we can say that there is at least one example where something that the females find “sexy” is a downright disadvantage. There is seemingly no defense to the claim that the solid wingbones of a club-winged manakin resulted from survival of the fittest – it appears to be survival of the sexiest.