Andy Bodle rides the ebb and flow of attraction, reproduction, and the spread of behavior and genes in part two of his three-part series.
“And the LORD God said, It is not good that the man should be alone; I will make him an help meet for him … And the rib, which the LORD God had taken from man, made he a woman, and brought her unto the man.”
What a lovely gesture, Mr God. Except that’s not quite how it went down, is it? (And while I’m here, you might want to think about dropping some of those capitals. No one likes a bighead.)
You see, for about the first 800,000 years of life on this earth, there were no Adams. Until that point, every organism on the planet was an Eve (or, if you like, a Mary; they all reproduced by parthenogenesis, or “virgin birth”). It is males that are the latecomers.
No one knows exactly how maleness first came about—biologist Geoff Parker and colleagues proposed a brilliant hypothesis in 1972, consummately summarized in Richard Dawkins’ The Selfish Gene—but come about, and stick about, they did. (Well, most of them. In a number of species, such as Bynoe’s gecko, whip-tailed lizards and bdelloid rotifers, it seems the male experiment was an abortive one; they seem to have reproduced sexually in the past, but have since reverted to the solo method.)
Forget my last post. It was a straw man/woman. Most sex differences in humans are cosmetic and trivial. The one, huge, ancient, and utterly fundamental difference between men and women—and between the sexes of almost every sexual species on earth—is in our sex cells.
Sperm and eggs differ in three crucial ways. First, the female gamete, the egg, is immobile, while sperm are mobile. Second, the egg is much larger: in humans, around 1,000 times the size of a single sperm. And third, the little guys are far more numerous. A woman can expect to release around 450 of her sex cells in a lifetime; the average man will produce over 500 billion, 250 million per ejaculation.
These disparities may be visible only through a microscope, but their consequences are far-reaching. The immediate ones are these:
1) Males can sire more offspring.
2) Eggs often (but not always; cf fish, insects and amphibians) spend a long time developing inside the female after fertilization.
Now, the single aim in life of any one organism is to reproduce—to produce offspring that can themselves reproduce—and to do so as often as possible. It’s what we’re designed to do. (I don’t mean the words “aim” and “design” in the sense that there is any conscious planning behind it; it’s just a convenient and widely accepted metaphor for the way evolution works.) In a vacuum, then, we should constantly, with the odd break for snacks, be at it like rabbits.
But we’re not in a vacuum. Resources are limited. There’s food and territory and predators and competitors to factor in. And as we’ve seen, since the two sexes function in different ways, the costs and benefits of reproduction are very different.
Critically, in many species, offspring need nurturing before they can survive to reproduce themselves, and there’s no point in producing offspring if you just run (or swim, or fly) off and leave them to die. However, parenting is an extra cost: the more time you spend feeding, protecting and (in humans) teaching the young, the less time you have to make more babies.
So if any one individual can somehow renege on its parental duties and abandon its young, leaving its partner to do the nursing, it will produce more offspring (with other partners), and such selfish behavior will become more widespread in subsequent generations.
The male’s minimum investment in each batch of offspring is tiny—the time it takes to do the deed. But because the egg develops inside the female, rendering her infertile for a time after conception, she has invested a lot in her offspring before they’re even born. She also stands to gain less from extra matings, since she produces fewer eggs. Nine times out of 10, then, it is going to be the female who ends up holding the baby—simply because she started carrying the baby. (Google “Robert Trivers cruel bind” for a more detailed breakdown.)
What’s more, a female can be 100% certain that any offspring she bears are hers—but a male cannot. Imagine that there’s one male with a paternal instinct, and one without. The one who takes time out of his busy breeding schedule to help feed “his” babies may in fact be wasting his time on another male’s young. So the male who deserts is gaining a huge genetic benefit—having someone else ensure that his offspring survive, and yet having the free time to produce more offspring—and the “paternal” male is consigning himself, and his cuddly instincts, to extinction. Deserting, then, is a more effective strategy than being a daddy. (This phenomenon, called paternity uncertainty, also explains why “mate guarding” strategies—tactics to ensure that other males do not impregnate your mate—are so common in nature: from simply keeping a wary eye on her, as macaques do, to biting off your penis and ramming it inside her vagina, as banana slugs do. Psychologist David Buss argues, very plausibly, that human jealousy is our own solution to the problem.)
This, of course, creates a situation that is wildly unfair for females. If male desertion became universal, females’ lives would be permanent drudgery: mating, gestating, lactating, educating, then starting all over again.
However, any female who evolved a means of getting the male to help out, even a little bit, would be at a tremendous advantage. The offspring she did have would be more likely to survive, and she would have more time and energy to produce more young.
And the female has one weapon to help achieve this: she can co-operate, or refuse to co-operate, with any given male. At the very least, she can make mating easy, or more trouble than it’s worth. She can exercise choice. (Again, I’m not talking about conscious choice, but about a random mutation that gives rise to the illusion of choice.)
Since the female has an endless supply of potential suitors practically queueing at her front bottom, and since she can only produce a brood every so often, being a little choosy costs her practically nothing. But the potential genetic gains are enormous.
If, say, a female happened to emerge who had a preference for males that were larger, or faster, or more intelligent, or had blue fins (and blue fins were associated with greater fertility), then her own sons would be larger, faster, more intelligent and more fertile … and just as importantly, her daughters would also prefer larger, faster, etc males.
Similarly, if a female emerged who had a preference for males who somehow made her life easier, or the survival of her offspring more likely—by, for example, bringing her a large parcel of food, or building her a nest—she would have a clear advantage over other females. And again, because her sons would inherit the behavior and her daughters would inherit the preference for that behavior, these traits would spread through the population just as surely as (if more slowly than) the fuck-and-chuck gene did in males. (“Runaway sexual selection” is the Google term for anyone wanting to explore this further.)
Now think about things from the male perspective. Prior to this development, they only had to worry about each other. Natural selection would have favored qualities such as size, strength, speed, aggression and cunning—anything that gave them an edge over their rivals. Suddenly, there’s an extra hoop to jump through: pleasing the female. Now there is some evolutionary pressure on them to develop “paternal” behavior.
Those who take this path will have fewer offspring, because they’re spending time feeding, building nests, and so on. But the offspring they do have will be far more likely to survive. This, very, very roughly speaking, is probably something like how pair-bonding evolved.
But evolution never stands still. Genes are always (blindly) seeking to gain an advantage. In one population, males might practice a particular strategy, females another, and they might complement each other perfectly. But if a mutation arose that created a new behavior, one that was able to exploit this status quo, it could become the dominant strategy in the matter of a few generations. A “cheating” gene, for example, would do exceptionally well in a population of trusting, monogamous pairs … until everyone was cheating on everyone else. Then evolution would favor a gene that *detected* cheating … (Intrigued? Google “evolutionarily stable strategies”.)
… and so preferences, and behaviors, and even appearances, wax and wane and shift, in an endless dance down the ages.
So what stage are we at in 2013? How does all this apply to modern human males and females? I am vaguely considering building towards some sort of point. Maybe next time.
This post originally appeared at Womanology.