Sex: the pleasure is momentary, the position ridiculous, and the expense damnable.
— Lord Chesterfield, 1694-1773
OK, I admit that I chose this title knowing that it had the dubious quality of being click bait.
But it’s accurate as well. I want to discuss sex, partly from the point of view of personal and social costs, but mostly from the point of view of biological costs. If it’s so costly, natural selection must have also found it very useful. What is that all about?
In fact, this is an important subject for serious science. Nick Lane, PhD, at University College London, is an evolutionary biochemist who uses his deep understanding of our cellular biochemical pathways, and more importantly the genetic coding for these pathways, as clues to our evolutionary path. He uses the DNA nucleotide sequences as fossils. Genes have variations called polymorphisms, that may be adaptive modifications, but also are often of no significance. Yet the polymorphisms can tell us how closely species and phyla are related. They can identify all sorts of “missing links.” So, knowing how similar our genes for cellular housekeeping are, say compared to a chimpanzee, we know we parted ways 6 million years ago.
The flower subcontracts some of its sexual functions to the hummingbird.
We can do the same with humans and a rat or a fruit fly and learn where in evolution we diverged. Lane writes that there are 10 amazing inventions that eukaryotes have made. Eukaryotes are complicated cells with nuclei and one of three types of life on this planet. The other two are archaea and bacteria which are called prokaryotes. We, as eukaryotes, are unlike bacteria, but so are amoeba, fish, fungi, trees, and even algae. And one thing all of us eukaryotes have in common, in addition to a nucleus and organelles, is sex. Several other things we eukaryotes have in common is aging and death. But that is a story for another time. Let’s talk about sex. Sex is expensive. And I don’t just mean dinner, a show and flowers.
For we know, strictly biologically, that sex comes with great costs. Sex is a very efficient way to transmit disease. Yes, we know that some diseases spread by contact (like in handshaking) or by respiratory means, such as with COVID-19. But some awful diseases pretty much only spread through sex. With the temptation so great, sexually transmitted diseases have been the demise of many famous men, such as King George III (some say his madness from syphilis is why we needed to have the American Revolution), philosopher Friedrich Nietzsche and composer Robert Schumann. Then there was gonorrhea that took down many an army and, more recently, AIDS that has led to the tragic deaths of 40 million people at the end of the 20th century.
But biologists would say that the most impressive costs from sex is a not so hidden tax on living. In terms of psychology and sociology, we know that the average young adult spends a substantial portion of his/her resources in dating and an inordinate amount of time in fantasizing and planning. And if it doesn’t go well, there’s not just wasted resources but a great deal of loneliness, anxiety and depression. More biologically, the cost for a woman to have a pregnancy, a long period of lactation and then child caring, is monumental. Furthermore, until recently, childbirth caused maternal mortality levels of up to 10%. This is precisely why women have to be very discerning, though this is perceived by some men as leading to the unfair asymmetry of courting. As the writer H.L. Mencken said, “In the duel of sex, woman fights from a dreadnought and man from an open raft.” She has to, as she has so much more at stake.
Let’s begin with the basics. Sex starts with mitosis and then proceeds to miosis where in forming one gamete you toss out another set of perfectly good chromosomes. How wasteful. You go from two to four sets of chromosomes before you reduce to one and make a gamete. Then one gamete meets another, and you’re back to two but with a shuffling of the deck.
Imagine if humans could make clones. Firstly, everyone could reproduce directly (men as well as women), thus doubling the efficiency of reproduction. Secondly, all this investment in courtship and mating could be omitted. And finally, the costs of divorce attorneys and giving away the house to make things right, could be avoided.
But it is probably even worse for other organisms. Rams must butt heads; stallions bite and kick; peacocks invest in a beautiful huge tail that makes evading predators quite difficult; and baboons, slash and even kill, all for the privilege of female access. Plants pay as much for their version of sex. A flower must be made with form and brilliant color and odor to attract the right insects (Figure). And these same insects must be paid off with sweet nectar, which is mostly glucose, a premium currency of life.
And yet almost every form of life, excluding bacteria and archaea, uses sex. You might ask, if a single cell eukaryote reproduced asexually, and made clones of itself, wouldn’t it be spared all this expense, and wouldn’t it out-compete its sexual neighbors? Apparently not as we’ve had over 2 billion years to try. OK, there are a few exceptions, like the common dandelion that can reproduce without sex, but even these exceptions prove the rule by reverting to sex whenever conditions allow.
The naturalist Charles Darwin had some of this wrong. He too noted the universality of sex and its obvious drawbacks, and he too spent a lot of time pondering its advantages. Keep in mind that Darwin was doing this before genes were known. Yet he understood heritability, and he knew some factor was going to pass from parent to child that influenced form and function in the next generation. He thought sex was to provide hybrid vigor. This was a well know phenomenon that basically came down to the problems of inbreeding and the extra strength and rigor of “mutts.” But hybrid vigor can be achieved by avoiding inbreeding, which can be circumvented altogether without sex.
You remember the famous joke when the playwright George Bernard Shaw is propositioned by a beautiful actress who says something like, “Imagine if we had a child. It might have my beauty and your brains.” And Shaw reputedly replies. “God forbid! What if our child had my beauty and your brains!”
Sex roles the dice and the outcomes can vary. If we reproduced as bacteria do, which is to say by cloning, there would still be the role of mistakes in the form of mutations that would give evolution something to work with. But it is sex that shuffles the deck of known cards (genetic variations) which allows each offspring in the next generation to improve or to decrease the value of each hand. This possible net negative outcome would argue against the value of sex.
However, overall, shuffling the deck must have great value. The most obvious thing is that with sex you can evolve more rapidly. But more importantly, you can explore genes, not just one at a time for their fitness, but in various combinations. In fact, genes and their proteins are doing lots of functions and analysis of a single mutation should consider much more than one effect. It’s a systems’ analysis of many component parts. In fact, that’s where the game of trial and error, where errors get eliminated by natural selection, plays out the most.
Novel mutations are rare and less of a driver of evolution. Mutations often need another mutation to do any good. It’s sexual reproduction that brings together both mutations in ways that might be very good or very bad. But sometimes, sex combines two broken parts to produce something cool. Without sex, the degenerating parts accumulate, especially in complex organisms. As a consequence, complexity is unsustainable, so it’s no wonder that only bacteria and archaea, the simplest forms of life, can do without sex.
Finally — and I like this argument the best — sex gives us the immediate opportunity to create variations that can exploit different niches in our environment. One offspring grows best in the shade, another in the sun. This would allow for better exploration of new environments, toleration of fluctuating environments and ecological development where different members of a species actually provide some form of mutual advantage to each other. One man’s trash is another man’s treasure.
For parasites, this type of advantage is very obvious. Malaria fights off the human immune system by covering itself with antigens as an invisibility cloak. By the time the antigens are recognized as foreign and the immune system is primed to attack, the malarial parasite produces new antigens. By combining components from the two parents in various protein permutations, the parasite can go on almost indefinitely, thus always keeping a step ahead of our immune cells. In a similar way, we make antibodies by mixing and matching polypeptides that derive from both parents. Our ability to mount an effective resistance to COVID also derives from all the combinations we get from sex.
For us humans, sex and the variety it produces certainly applies to our children, who will tend to develop different personalities and proclivities. Psychologists have shown that children vary much more than would be expected randomly. If one is bold, another is careful. And that surely applied to my own household with my three kids whose personalities ended up as different as I could imagine (though their characters remain similar) despite what I thought was a pretty homogenous upbringing.
These are just some observations that we, as old-timers (Senior Ophthalmologists) have made with the perspective of seeing sex from many angles. There are others. "You have to know that an older man cannot hang from a chandelier." — Dr. Ruth.
Sexual reproduction is universal for eukaryotes. From single-celled yeast to towering trees to complex animals, the process of reproduction usually depends on two parents. Sex promotes more diversity while reducing the impact of deleterious mutations, thus promoting adaptation including innovation and resilience.
The fact that sex was discovered about 2 billion years ago and has been adopted and maintained by almost 100 million species on our planet means it was not only a good idea but fundamental and necessary to avoid extinction.
The fact that we humans invest, develop and even obsess over sex makes more sense when we appreciate its value to the exquisite gene combos we create with it — our children. But we must also admit that sex spices things up and helps serve a rich cultural environment which makes life much more interesting. Sometimes, you just have to look at this weird thing called sex and laugh. Sex may also be nature’s joke on us.