This Evolution 101 post is by MSU graduate student Anselmo Pontes.
What do you think when you see a woman painfully balancing on sky-high heels? How about when you overhear the lame pick up lines guys come up with to get a girl’s attention? We do all kinds of things to impress the opposite sex. Though we can pass the limits of good sense at times, we can’t help but try. What we may not be aware of is that the way we alter our behavior and appearance can change the course of evolution.
Take the guy with the pick up lines. A woman may find him witty, an indirect sign of intelligence, and possibly a resourceful mate. If he manages to partner with the woman and have children, his characteristics will live on. More subtly, the woman’s preference will also be passed on to the next generation. So, this couple will tend to have boys that come up with silly pick up lines and girls that go for them.
The first one to notice this relationship was Charles Darwin. He observed that the choices that animals made when mating affected their descendants. He considered this to be a force of evolution akin to natural selection, and called it sexual selection.
Darwin cited the peacock’s huge, colorful tails, which serve no other purpose than to woo females and may even make them vulnerable to predators. However, peahens, with their discreet plumage, are the ones selecting the mates. Since they prefer males with large and bright tails, these are the kind you would expect to see in the next generation. Over time, the tails tend to become larger and more colorful (and female’s preference for large, colorful tails increases in tandem).
This happens because some preferences, physical characteristics and behaviors are heritable traits, coded in the genes. Therefore, to a choosy female, these big, beautiful tails indicate either good genes and health or a desirable trait that will give her offspring an advantage in future mating efforts.
In most species, the female does the choosing. This is because females are usually the ones that make the most investment in the offspring. Males are a dime a dozen and rarely stay for child rearing. Females are the ones that tend to and nurture the young.
It isn’t only physical beauty that gets attention. Some species attract mates by offering gifts of food, building elaborate nests, singing and even dancing. Bowerbirds, a native of Australia and New Guinea, create masterfully ornate structures akin to theatrical stages called “bowers” with the single function of impressing females. A male bowerbird spends days, sometimes weeks collecting twigs, flowers, rocks, seashells and assorted colorful objects and assembling them into a carefully decorated structure to demonstrate their individual prowess and good taste. Females visit multiple bowers and return to the one they judged the best.
Sexual selection is not only about attracting mates, it’s also about fighting for them. In many species, males fight with each other and only the winners get to mate – sometimes with many females. Such is the case of sea lions and deer, where winners fight to maintain a harem of females, and losers don’t get to mate at all.
Since big antlers intimidate lesser opponents, and long colorful tails attract mates, these traits tend to continue developing in a sort of “arms race” called runaway selection. Eventually though, antlers will grow too heavy to carry and the drag on a long tail will outweigh its advantage. This is when sexual selection tapers off, and traits favoring survival become more important.
Humans have also been shaped by sexual selection. It is suspected to have influenced the size of our brain, the strategic placement of body and facial hair as well as our height and tone of voice. So back in the cave, when the choice was between the rude, scruffy-looking ape and that caring, sleek someone, our ancestors’ pick may have determined how we turned out to be the smooth, quick-witted, fabulous-looking creatures we are.