Women After All: Sex, Evolution, and the End of Male Supremacy (7 page)

To begin with, there is more than one answer. One of them can be found by going back beyond Charles to another Darwin, his grandfather Erasmus. That ancestral Darwin wrote not only a scientific treatise but also an epic poem about evolution; contrary to popular belief, the idea was in play for centuries before Charles puzzled out how it works. Erasmus Darwin, a doctor who turned down a chance to become personal physician to King George III (of American Revolution fame), devoted much of his life to studies foreshadowing
those of the grandson who eclipsed him. In his big book on plants—
Phytologia; or, The Philosophy of Agriculture and Gardening—
he wrote that

from the sexual, or amatorial, generation of plants new varieties, or improvements, are frequently obtained; as many of the young plants from seeds are dissimilar to the parent, and some of them superior to the parent in the qualities we wish to possess.

For these reasons, “sexual reproduction is the chef d’oeuvre, the masterpiece of nature.”

Perhaps it would take a man to miss the downside of sex, to call it
the
masterpiece of nature. Erasmus was sexually liberal (and liberally sexual) himself, Charles’s father being only one of his fourteen children by two wives and then the governess, after he was widowed. He may have had a fifteenth with someone else’s wife. But he was a great proponent of women’s rights and an enemy of slavery, and he understood what was evolving across the ocean. He wrote Benjamin Franklin in 1787, calling him “a Philosopher and a Friend” as well as “the greatest Statesman of the present, or perhaps of any century, who spread the happy contagion of Liberty.” Given his love for infant America, maybe Erasmus didn’t trust himself to take care of King George, who was trying to kill it in its cradle.

But Erasmus Darwin’s wider fame came from long poems that played a role like the most successful popular science today, although it’s hard for us to grasp the popularity of book-length poems back then. Erasmus’s most celebrated was
The Loves of the Plants,
which gave a genteelly pornographic account of plant reproduction. Since gardening was a popular women’s art, and botany therefore a popular subject of study for them,
The Loves of the Plants
became a not-so-secret source of guilty pleasure, and it was condemned as such. Dr. Darwin describes, for instance, the sex life of
Lychnis,
an
English field flower known as ragged robin, in which bright purple or pink females compete to be fertilized by males:

Each wanton beauty, Tricked in all her grace

Shakes the bright dewdrops from her blushing face;

In gay undress displays her rival charms

And calls her wondering lovers to her arms.

The Loves of the Plants
had many, some perhaps furtive, women readers, who could not in that era easily or respectably read anything more explicit than this.

But the point is that he got something right about what his grandson would call the question hidden in darkness: by producing young that are different from the parent, sex provides grist for the mill of evolution. Today we say that sex reshuffles the deck of genes whenever sperm and egg form and meet, and the rate of evolution depends on the available genetic variation. But this only pays back part of the twofold cost of sex. And since we know that change is possible in asexual species, and even sexual species may not change for millions of years—
if it ain’t broke, don’t fix it,
says selection—there has to be more to sex than just speeding up evolution.

A second answer is called the “tangled bank” hypothesis, named after this famously eloquent passage by grandson Charles:

It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions
of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms.

This is most of the last paragraph of
The Origin of Species,
one of the greatest science books of all time. It sets forth in one sentence—“These laws . . .”—the long and short of his pivotal theory, and we can see how it was presaged by the grandfather he never knew. And in the two famous sentences that follow, Charles showed something of Erasmus’s poetic gift:

Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Thus ends
The Origin,
including a less risqué but clear account of sexual reproduction, Grandpa Darwin’s “masterpiece of nature.”

But Charles D. makes a separate point about the value of variation, on the “tangled bank” he asks us to envision. Its countless forms, each at least slightly different, depend on one another for the simple gift of life. Being unique, each can exploit the bank’s entangled opportunities in a slightly different way. In other words, if your offspring are identical to you and to each other, you will each have the same needs and will compete head-on with one another
and
your identical mother. Variation creates elbow room; each offspring lives by slightly different means. Another way to make elbow room, even in sexual species, is dispersal. When an oak spreads her seed
on the wind, her offspring compete less with her and one another. But not every species can cast its seed, so variation is needed.

This idea has recently proved true for a species of sea squirt. It’s a translucent yellowish or turquoise vase-shaped thing, about the size of your thumb if you go down to the heel of your hand, although the little vase seems to split into two spouts at the top. When a bunch of sea squirts fix their bases close together, as they often do, they resemble a hand-sized blob of gelatinous wavy tentacles. Often found on rocks, boat hulls, and other submerged surfaces, the sea squirt is a pest, which means it’s a great evolutionary success, and we now know one reason is that when a female squirt mates with multiple males, she increases the genetic variation in her young. This, in turn, decreases competition among them when, as often happens, they hang around together.

But there is a third explanation of sex, now the most widely accepted: the Red Queen hypothesis.

The Red Queen comes from neither of the Darwins but (indirectly) from a younger contemporary of the grandson, another Charles, named Dodgson—the mathematician better known as Lewis Carroll. “It takes all the running you can do,” the Red Queen warns Alice in
Through the Looking Glass,
“to keep in the same place.” So the Red Queen hypothesis simply says, “It takes all the evolving you can do to keep in the same place,” because your environment keeps changing and, to paraphrase Heraclitus, you can’t step onto the same tangled bank twice.

Evolve as you will, your predators are evolving, too. You get faster, they get faster—you’d better get even faster right away. You get tougher skin, they get sharper teeth. You get toxins, they get fancier guts. You get camouflage, they get color vision. You get bigger, they unhinge their ominous jaws. To respond, you need gobs of variation. You need DNA to spare, genes on hold that you aren’t using yet but may need for a rainy day. And all this running-to-stay-in-place has not even hit critical. It matters most when we switch
from the predators that gobble you up honestly from the outside to the ones that nibble at you insidiously from within.

These are the micropredators—parasites, viruses, germs. And here is what they want from you: no change whatsoever. It takes them long enough to adapt to one individual. Once they’ve done it,
Basta!
They don’t want any more challenges. Suppose Mike (microorganism) X evolves inside one of your daughters and she fights it off with immune cells until she’s 99 percent free of Mike’s progeny. But they have a population doubling time of, what, fifteen minutes? They are back in nothing flat with a new, resistant strain. So she kills off 90 percent of
them.
But then . . . well, you get the idea. It’s why you’re taking a different antibiotic now than you did five years ago.

The thing is, if you’ve reproduced without sex—just cloned yourself—Mike X only has to solve this puzzle once. Ultimately immunity depends on genes. And if you have dozens or thousands of genetically copied sisters, he or she or it has already evolved around what any of them can do. Likewise your mother, daughters, nieces, granddaughters—all are vulnerable. The Red Queen says,
Check.
And then:
Checkmate
.

But suppose you export some copies of your precious genes into a class of offspring that will never themselves reproduce. Keep doing that for enough generations for mutations to make a difference. Now take one of that nonreproducing type—a.k.a. males—and put some of his somewhat different genes together with some of yours. Mix vigorously and simmer. You have found a recipe to make Mike X miserable, a way to resist the Red Queen move after move after move.

The only real drawback is, now you have to put up with Mike Y, who’s not a microorganism at all. He’s a nonreproductive member of your species who carries a pretty pathetic-looking Y chromosome. He’s not exactly a predator, although he does have something in common with predators. He won’t eat you—his destiny is in your hands, or at least your inner organs. Under certain circumstances,
you
may eat
him,
but first you need to collect his wayward genes,
so unless you have other options, you won’t feast on him until after you’re almost finished mating. He’s a member of your own species, sort of. But his chromosomes differ from yours, enough to accord you a new sort of protection.

So, all along you’ve thought that germs come from sex?

Not so. In evolution, sex comes from germs.

The black widow spider and the praying mantis have found a way to have their males and eat them, too. They carry the sperm-donor concept much further than even the most feckless Don Juan. Females accept the gift of sperm, then have the rest of the male for dessert. But it’s not just the melodrama of sexual cannibalism that draws us toward the mantis’s prayerful pose or the widow’s web. It’s that these species, like the virgin whiptails and Grandpa Darwin’s wanton blossoms, are vital scientific anchor points in the web
we
weave to catch the mystery of sex.

The black widow lives in a broad stripe circling the planet, pursuing her venomous trade in all but the coldest climates. She is starkly beautiful, about the size of a raisin sitting on eight long legs, her shiny black shell bearing a distinctive reddish hourglass or studded with two red-orange spots laced with a partial white trim, as if her back were bejeweled with bright, flat mushrooms. The silk she spins is lighter but stronger than steel wire of the same thickness. Her venom, much stronger than a rattlesnake’s, can kill a human child; it routinely kills a broad spectrum of insect prey and, by the way, the black widow male. She is solitary and shy the year round, except for this rather harsh mating ritual. She weighs thirty times what he does, so it’s not exactly a contest.

But it
is
a courtship. Think of a powerful Amazon queen who has won the heart of a Lilliputian. He steps gingerly onto her web and taps the strands, much like the water striders we encountered earlier, but here it’s the males’ tapping that sends out a song of love—or lust, at least—in patterned vibrations. If she is suitably moved, she
allows him closer. She lets him climb her great body, where he continues to woo with fancy stepping. He places his sperm inside her with a special feeler, which breaks off after he makes his deposit; he’ll be a lucky boy if that’s the only body part he leaves behind.

Sometimes the hourglass seems to mean his time is running out, but it’s just part of her siren song. If it’s not his lucky day, she grabs, stings, and wraps him just as she would a mosquito, injects the tidy package with digestive enzymes that turn him into a rich broth, and drinks him down. Aided in part by this delicious, nutritious meal, the widow soon lays a silky, liquid sac bulging with two hundred eggs and weaves a blanket around them, wrapping them neatly and attaching the cocoon to her web. She does this five more times with eggs fertilized by that one brave or crazy male, and in two weeks there are a thousand new baby black widows.

In a 2012 study of a closely related spider, the orb weaver, Klaas Welke and Jutta Schneider found that females prevented from eating their mates had fewer, smaller eggs that survived less well under stress. This is also true of female tarantulas, although if they have already mated once, they may lure a few more males and eat them one by one without even letting them have their sexy moment in the sun. Scientists drily note that tarantula females are “nutrient-limited” and “males are high-quality prey.” Another study of orb weavers, called “Safer Sex with Feeding Females,” showed that males have evolved ways to try to avoid being eaten, but only some succeed.

Female praying mantises have their own charms and slightly different sexual appetites. Some say the males must be praying that they will survive sex (and they do try to survive), but females, if hungry, just seem to pray for their next male meal. Female mantises are world-class hunters that can kill and eat mice, snakes, and hummingbirds bigger than they are. The female can and often does eat the male after sex, but she doesn’t have to wait until he’s finished. She can start by biting off his head while he is at it, and (typical male) his decapitated body can go right on with the show. If it sometimes
seems that men don’t need their brains to have sex, consider the praying mantis male. Talk about a boy losing his head over a girl.