Why Evolution Is True (25 page)

Beginning with a flatwormlike eyespot, then, the model produced something like the complex eye of vertebrates, all through a series of tiny adaptive steps—1,829 of them, to be exact. But Nilsson and Pelger also calculated how long this process would take. To do this, they made some assumptions about how much genetic variation for eye shape existed in the population that began experiencing selection, and about how strongly selection would favor each useful step in eye size. These assumptions were deliberately conservative, assuming that there were reasonable but not large amounts of genetic variation and that natural selection was very weak. Nevertheless, the eye evolved very quickly: the entire process from rudimentary light-patch to camera eye took fewer than 400,000 years. Since the earliest animals with eyes date back 550 million years ago, there was, according to this model, enough time for complex eyes to have evolved more than fifteen hundred times over. In reality, eyes have evolved independently in at least forty groups of animals. As Nilsson and Pelger noted dryly in their paper, “It is obvious that the eye was never a real threat to Darwin’s theory of evolution.”

So where are we? We know that a process very like natural selection-animal and plant breeding—has taken the genetic variation present in wild species and from it created huge “evolutionary” transformations. We know that these transformations can be much larger, and faster, than real evolutionary change that took place in the past. We’ve seen that selection operates in the laboratory, in microorganisms that cause disease, and in the wild. We know of no adaptations that absolutely could not have been molded by natural selection, and in many cases we can plausibly infer how selection did mold them. And mathematical models show that natural selection can produce complex features easily and quickly. The obvious conclusion: we can provisionally assume that natural selection is the cause of all
adaptive
evolution—though not of
every
feature of evolution, since genetic drift can also play a role.

True, breeders haven’t turned a cat into a dog, and laboratory studies haven’t turned a bacterium into an amoeba (although, as we’ve seen, new bacterial species have arisen in the lab). But it is foolish to think that these are serious objections to natural selection. Big transformations take time-huge spans of it. To really see the power of selection, we must extrapolate the small changes that selection creates in our lifetime over the millions of years that it has really had to work in nature. We can’t see the Grand Canyon getting deeper, either, but gazing into that great abyss, with the Colorado River carving away insensibly below, you learn the most important lesson of Darwinism: weak forces operating over long periods of time create large and dramatic change.

Chapter 6

How Sex Drives Evolution

-Charles Darwin

 

 

 

T
here are few animals in nature more resplendent than a male peacock in full display, with his iridescent blue-green tail, studded with eyespots, fanned out in full glory behind a shiny blue body. But the bird seems to violate every aspect of Darwinism, for the traits that make him beautiful are at the same time
maladaptive
for survival. That long tail produces aerodynamic problems in flight, as anyone knows who has ever seen a peacock struggle to become airborne. This surely makes it hard for the birds to get up to their nighttime roosts in the trees and to escape predators, especially during the monsoons when a wet tail is literally a drag. The sparkling colors too attract predators, especially compared to the females, who are short-tailed and camouflaged a drab greenish brown. And a lot of metabolic energy is diverted to the male’s striking tail, which must be completely regrown each year.

Not only does the peacock’s plumage seem pointless, but it’s an impediment. How could it possibly be an adaptation? And if individuals with such plumage left more genes, as one would expect if the raiment evolved by natural selection, how come the females aren’t equally resplendent? In a letter to the American biologist Asa Gray in 1860, Darwin griped about these questions: “I remember well the time when the thought of the eye made me cold all over, but I have got over this stage of complaint and now trifling particulars of structure often make me very uncomfortable. The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”

Enigmas like the peacock’s tail abound. Take the extinct Irish elk (actually a misnomer, for it’s neither exclusively Irish nor an elk; it is in fact the largest deer ever described, and lived throughout Europe and Asia). Males of this species, which disappeared only about ten thousand years ago, were the proud possessors of an enormous pair of antlers, spanning more than twelve feet from tip to tip! Together weighing about ninety pounds, they sat atop a paltry five-pound skull. Think of the stress that would cause. It’s like walking around all day carrying a teenager on your head. And, like the peacock’s tail, these antlers were completely regrown from scratch each year.

In addition to gaudy traits, there are strange behaviors seen in only one sex. Male túngara frogs of Central America use their inflatable vocal sacs to sing a long serenade each night. The songs attract the attention of females, but also of bats and bloodsucking flies, which prey on singing males far more often than on the noncalling females. In Australia, male bower-birds build large and bizarre “bowers” out of sticks that, depending on the species, are shaped like tunnels, mushrooms, or tents. They are festooned with decorations: flowers, snail shells, berries, seed pods, and, where humans are nearby, bottle caps, pieces of glass, and tinfoil. These bowers take hours, sometimes days, to erect (some are nearly ten feet across and five feet tall), and yet they’re not used as nests. Why do males go to all this trouble?

We don’t have to just speculate, as Darwin did, that these traits reduce survival. In recent years scientists have actually shown how costly they can be. The male red-collared widowbird is shiny black, sporting a deep crimson necklace and head patch, and laden with immensely long tail feathers— roughly twice as long as its body. Anybody seeing the male in flight, struggling through the air with its tail flopping behind, has to wonder what that tail is all about. Sarah Pryke and Steffan Andersson of Sweden’s Göteborg University captured a group of males in South Africa and trimmed their tails, removing about an inch in one group and four inches in another. Recapturing the males over the breeding season, they found that longer-tailed males lost significantly more weight than shorter-tailed males. Clearly, those extended tails are a considerable handicap.

And so are bright colors, as demonstrated in a clever experiment on the collared lizard. In this footlong lizard that lives in the western United States, the sexes look very different: males sport a turquoise body, yellow head, black neck collars, and black-and-white spots, while the less gaudy females are grayish brown and only lightly spotted. To test the hypothesis that the male’s bright color attracts more predators, Jerry Husak and his colleagues at Oklahoma State University put out in the desert clay models painted to look like male and female lizards. The soft clay would preserve the bite marks of any predators mistaking the models for real animals. After only a week, thirty-five of the forty garish male models showed bite marks, mostly by snakes and birds, while none of the forty drab female models were attacked.

Traits that differ between males and females of a species—such as tails, color, and songs—are called
sexual dimorphisms
, from the Greek for “two forms.” (Figure 23 shows a few examples.) Over and over, biologists have found that sexually dimorphic traits in males seem to violate evolutionary theory, for they waste time and energy and reduce survival. Colorful male guppies are eaten more often than are the plainer females. The male black wheatear, a Mediterranean bird, laboriously erects large cairns of stones in various locations, piling up fifty times his own weight in pebbles over a period of two weeks. Male sage grouse perform elaborate displays, strutting up and down the prairie, flapping their wings, and making loud sounds from two large vocal sacs.
³²
These shenanigans can use up a tremendous amount of energy for a bird: one day’s display burns up the caloric equivalent of a banana split. If selection is responsible for these traits—and it should be, given their complexity—we need to explain how.

FIGURE 23
. Examples of sexual dimorphisms, showing marked differences in the appearance of males and females. Top: the swordtail (
Xiphophorus helleri
); middle: King of Saxony Bird of Paradise (
Pteridophora alberti
), whose males have elaborate head ornaments that are sky blue on one side and brown on the other; bottom: the stag beetle (
Aegus formosae
).

BEFORE DARWIN, sexual dimorphism was a mystery. Creationists then—as now—could not explain why a supernatural designer should produce features in one sex, and only one sex, that harm its survival. As the great explainer of nature’s diversity, Darwin was naturally anxious to understand how these seemingly pointless traits evolved. He finally noticed the key to their explanation: if traits differ between males and females of a species, the elaborate behaviors, structures, and ornaments are nearly always restricted to males.

By now you might have guessed how these costly traits evolved. Remember that the currency of selection is not really survival, but successful reproduction. Having a fancy tail or a seductive song doesn’t help you survive, but may increase your chances of having offspring—and that’s how these flamboyant traits and behaviors arose. Darwin was the first to recognize this trade-off, and coined the name for the type of selection responsible for sexually dimorphic features:
sexual selection
. Sexual selection is simply selection that increases an individual’s chance of getting a mate. It’s really just a subset of natural selection, but one that deserves its own chapter because of the unique way it operates and the seemingly nonadaptive adaptations it produces.

Sexually selected traits evolve if they more than offset the male’s diminished survival with an increase in his reproduction. Maybe widowbirds with longer tails don’t evade predators very well, but females might prefer the longer-tailed males as mates. Deer with bigger antlers might struggle to survive under a metabolic burden, but perhaps they win jousting contests more frequently, thereby siring more offspring.

Sexual selection comes in two forms. One, exemplified by the Irish elk’s huge antlers, is
direct competition between males
for access to females. The other, the one that produces the widowbird’s long tail, is
female choosiness
among possible mates. Male-male competition (or, in Darwin’s oft-pugnacious terminology, “the Law of Battle”) is the easiest to understand. As Darwin noted, “It is certain that with almost all animals there is a struggle between the males for the possession of the female.” When males of a species battle it out directly, be it through the clashing antlers of deer, the stabbing horns of the stag beetle, the head butting of stalk-eyed flies, or the bloody battles of massive elephant seals, they win access to females by driving off competitors. Selection will favor any trait that promotes such victories so long as the increased chance of getting mates more than offsets any reduced survival. This kind of selection produces armaments: stronger weapons, larger body size, or anything that helps a male win physical contests.

In contrast, features such as bright colors, ornaments, bowers, and mating displays are molded by the second type of sexual selection, mate choice. To female eyes, it seems, not all males are the same. They find some male traits and behaviors more attractive than others, so genes that produce those features accumulate in populations. There is also an element of competition between males in this scenario, but it is indirect: winning males have the loudest voices, the brightest colors, the most alluring pheromones, the sexiest displays, and so on. But in contrast to male-male competition, here the winner is decided by the females.

In both types of sexual selection, males compete for females. Why isn’t it the other way around? We’ll learn shortly that it all rests on the difference in size between two tiny cells: the sperm and the egg.

Is it really true, though, that males who win contests, or are more highly ornamented, or perform the best displays, actually get more mates? If they don’t, the whole theory of sexual selection collapses.

In fact, the evidence strongly and consistently supports the theory. Let’s start with contests. The northern elephant seal of North America’s Pacific coast shows extreme sexual dimorphism for size. Females are roughly ten feet long and weigh about fifteen hundred pounds, while males are nearly twice as long and can weigh up to six thousand pounds—bigger than a Volkswagen and more than twice as heavy. They are also
polygynous:
that is, males mate with more than one female during the breeding season. About a third of the males guard harems of females with whom they couple (up to one hundred mates for an alpha male!), while the rest of the males are doomed to bachelorhood. Who wins and who loses the mating lottery is determined by fierce contests between males before the females even haul out on the beach. These contests get bloody, with the big bulls bashing their massive bodies together, inflicting deep neck wounds with their teeth, and setting up a dominance hierarchy that has the largest males at the top. When the females do arrive, the dominant males herd them into their harems and drive off approaching rivals. In a given year, most pups are sired by just a few of the largest males.