Undeniable (8 page)

Evolution happens as each generation of living things interacts with its environment and reproduces. Lamarck got at least that part of it right. Those natural designs that survive to reproduce pass on their genes. Those that don't successfully reproduce disappear; their genes disappear as well. It's survival of the hang-in-there's, or the made-the-cuts, or the just good-enoughs.

 

8

MY PROM AND SEXUAL SELECTION

Being a nerd, I did not anticipate going to my high school prom. Nevertheless, I did. I was driven to do so, apparently, by the shape of Leith's legs, a (clearly) female classmate. This fascination with sex is, near as anyone can tell, not something we get to choose. Our ancestors bequeathed it to us. It's another one of those deeply shared evolutionary traits. It's a drive we cannot disengage.

Along this line, I cannot help but recall a day at a beach in Delaware, when my mother's first cousin Monique showed up to sun herself. I was about seven at the time. My mother's mother was French, so my first cousin once-removed was French, too. She had a certain European flair. And there was another thing: Monique was in her twenties, and wearing a bikini. (I am not skilled enough to have prepared a sketch.) I remember the grown-ups staring at me, because I was staring at her. The thing is, I distinctly remember that I did not know why I was staring at her. Of course looking back, she was, as it is expressed in modern parlance, a total babe. But I did not have the linguistic skills to express that, nor did I feel anything like what I would later feel as a teenager. I was just stuck staring. I take this as incontrovertible evidence that our brains are set up to support or carry out sexual selection, without our even knowing it.

Sexual selection is the second fundamental idea in Darwin's theory of evolution, ranking next behind natural selection. Sexual selection is the process by which organisms of the same species select genes to be passed to subsequent generations. It is what drives so much of what so many of the species on Earth do all day, and all night.

Natural selection in the general sense is the interplay between organisms and their environment. Nowadays, a century after Darwin, we might describe the process as the interplay between organisms and their ecosystem. Slightly better suited organisms outcompete the not-quite-so-well-suited organisms. This happens when random processes produce genes that happen to fit in well with the environment and ecosystem extant at the time. It was Darwin's greatest insight, and still forms the cornerstone for the modern understanding of what drives evolutionary change.

But along with the interplay between individual and ecosystem is another interplay between individual and other individuals of the species. They are competing with each other for energy and opportunities that enable them to reproduce. For plants, the fundamental resources are sunlight and nutrients in soil. For little fish, it might be zooplankton, tiny animals in the sea. For big fish, it's little fish. For you and me, it's food and water. But from an evolutionary standpoint, no amount of sunlight, fertilizer, nutritious food, or cozy blankets is enough. Organisms have to pass on their genes in order to have successors—in order to keep their genes in the gene pool. To that end, plants and animals go wild.

There are celebrated passages in
The Bible
about the lilies of the field. The authors of the Book of Matthew remarked that these beautiful flowers neither toil, nor do they spin yarn to be made into clothes. It's a passage encouraging the disciples of Jesus not to worry about Earthly things, specifically what exactly they'll be wearing while they're out proselytizing.

Lovely as that passage may be,
The Bible
missed an important point here about nature and evolution, specifically about sexual selection. In fact lilies, like every other sexual organism on the planet, work pretty hard to produce a means to mate. If you haven't already, stop and think about how much energy a plant puts into creating a flower. In general, a green plant such as a lily, rose, hickory tree, Ponderosa pine, or bull kelp, has leaves, needles, or fronds to collect sunlight. And in general, the other structures such as stems, trunks, or stipe serve to support the leaves in an efficient or efficient-enough fashion. What else does a plant do besides look to soak up light? The answer is simply: make more plants, which is not easy.

Plants go to great lengths to reproduce. It takes a lot of a lily's energy to produce flowers. It takes a great deal out of an oak tree to make thousands of acorns. In that case, the tree is, in turn, counting on squirrels to forget where they hid a few acorns, so that a new oak tree might grow nearby. Apple trees and orange trees go to all kinds of trouble to grow fruit, so that some guy like me or my local Los Angeles “citrus mice” (rats) will wander off with a piece and spit out the seeds on suitable moist soil. Palm trees grow coconuts the size and toughness of cannonballs, so that they can float their seed to another island. Just consider how much less energy it would take a lily or a cornstalk to grow and thrive if it didn't have all these seeds to sow.

And there is more. It's not just that these organisms are growing viable seeds. It's also that these organisms are growing structures such as flowers, pistils, stamen, eggs, and pollen to get a mix of genes, before the seeds are sent off. It's all sexual.

A rosebush has woody canes to give it structure. It has thorns to discourage animals from climbing on the canes or perhaps using them for a nest. Producing canes takes energy. But just look at the resources and energy rose plants expend creating elaborate blooms and hips (rose seeds). They produce attractive flowers not to avoid germs or to make it through a cold tough winter, but to get their genes mixed with the genes of other individuals that are selected on account of sex. They do it to attract pollinators, like bees and birds, that stop by for some nectar and carry some pollen when they fly away.

Oranges are so wonderfully sweet and appealing, and sheep wool is so soft and toasty, that it's easy to imagine how our ancestors could believe the whole setup was put there just for us. But this is clearly not the case. Ecosystems come to be over enormous amounts of time. We show up for just a few decades or generations. Our ancestors, leastways the ones who labored over those passages in
The Bible
, missed the idea that the reason it all seems to fit together so well is that it's been developed, from the simple to the complex, over a span of eons. And sex made it happen quite a bit faster, or a bit more efficiently, than it would or could have otherwise.

Once it arose, at least 1.2 billion years ago, sex became popular among living things—I mean, it's everywhere. We can speculate about how sex began. Under a microscope, we can readily observe bacteria exchanging genes across what resembles a microscopic string or thin tube. It's called a “pilus,” Latin for “hair.” It's not hard to also imagine bacteria exchanging just portions or tiny bits of genes. Next, imagine one bacterium sending to another small bits of genes (bits of bits) that are lighter in weight, and sending the material very quickly, while another bacterium favored sending larger heavier molecules more slowly. As we observe them now, bacteria only share one to one, and there is always a donor and recipient. Suppose though, that one bacterium sends a few genetic molecules through its pilus, and then a few minutes later the recipient bacterium turns it around and sends some other genetic information to that first donor through its own (different) pilus.

In primordial times, a situation apparently developed in which one bacterium sent many smaller bits in exchange for another bacterium's heavier larger bit. This practice apparently gave the microbes that settled into the two roles an advantage. Each must have performed its job a little better than other microbes that continued the practice of having both organisms on either side of the exchange sending longer or intermediate-length chains of genetic code. Specialization apparently introduced efficiency. One thing led to another and another, and these primitive Earthlings invented sex. Just looking around us, we can see that this exchange of many small pieces of genetic code for one larger piece works well. (Why it works well is less immediately obvious; more on that later.) Sex must give some living things an edge. Otherwise it, and we, wouldn't be here.

Once established, sexual selection is easy to understand. It enables a species to select within its own kind, and individuals within that species to compete in the open world, the open ecosystem, if you will. Another way of looking at it is that sexual selection is a second filter upstream of natural selection: Before offspring are introduced into the world to see if they are good enough to produce their own offspring to advance into the genetic future, the parents have to select each other. If they don't, there are no offspring to be introduced to the world.

The speed of sexual selection also contributes not only to the remarkable diversity we see in nature, but to the complexity of organisms as well. When offspring carry new combinations of genes that can take advantage of resources in the environment faster than other organisms in their ecosystem, they are bound to succeed. If they take in resources such as nutrients and water more quickly or more efficiently than others around them, it is reasonable that their genetic innovations will incrementally become more complex, because they can support more complex genes.

If you're like me, and I know
I
am, you might wonder why we have only two sexes. If two sexes are better than one from an evolutionary diversity-generating, germ-fighting, complexity-adding, outcompeting standpoint, why not have three sexes, or four sexes? Then, you—and all living things—might be able to innovate genetically like crazy. As a first hypothesis, this makes sense—at least it makes sense to me. But, we must keep in mind that evolution by means of both natural selection overall and sexual selection specifically can enable an organism to become more complex only from where an organism is at any moment. Getting multiple microbes of multiple generations to interact simultaneously may have been too rare or provided too little of an advantage over a faster one-to-one scheme, so we have just our two sexes. The next generation of an organism can introduce innovations only from the complexity or level of innovation its ancestors already had. It's not the survival of the absolute fittest multiparent scheme imaginable. It's survival of the good-enough.

Although most of us Earthlings presume to deal with two sexes, in the world of fungi, things are somewhat different. They interact two at a time, all right. But they have what are currently called mating types. They can be sexually compatible with many other individuals, many other mating types of their species that are in a sexual sense quite different. In this sense, an organism like the split gill fungus has 28,000 different sexes, i.e. 28,000 mating types. For us two-sex creatures, it's fascinating. Putting the unusual gene-sharing practices of the domain of fungi aside, the other sexual creatures like us work the problem with just the two—male and female.

No doubt there have been times in your life when you wanted to make multiple copies of something you created. These could be fence pickets, birthday party invitations, or laser gyroscope navigation systems for small jets. As you set about the task, you are probably going to make the second copy from the first. One is going to become two. The same holds true in nature. Let's say molecules used chemical energy from their surroundings to make copies of themselves. When they made a copy, it's just that: one copy. Then that copy could make a copy, and so on. We're talking about the molecular level here. So, the design that emerged over billions of years—not millions, but billions—is a molecule that splits itself in half.

In the case of the primordial gene-sharing microbes, the ones that apparently led to the invention of sex, they were only able to share with one other microbe at any one time. We do not observe multiple gene-exchange partners in microbes. And we observe only two sexes today.

Nature ended up with the deoxyribonucleic acid molecule, DNA. So binary splitting is favored or inherent in the process. With that, binary combination is as well. This is what happens in natural systems without us involved. It's also what happens in human systems. When we play the World Cup soccer matches, we play two teams at a time. Any athletic or bridge tournament involves groups of two teams. It's hard to imagine a system that works any other way. If you have three teams on a field, it might work for a while. But pretty soon, somebody teams up with someone else and the competition becomes one-on-one, or two total at any time.

I'll grant you that we can have more than one individual or more than one team in a horse race or a poker game. But if this whole sex business started with microbes exchanging genes one-to-one, it's a hard system to undo. In other words, once molecules started reproducing by dividing, it would be difficult indeed for a third team or molecule to develop and compete with a binary system. Perhaps there is an ecosystem on a planet orbiting another star—heck, perhaps there's one on Jupiter's moon Europa—with a tripartite sexual system. It's a concept that Isaac Asimov explored elegantly in his novel
The Gods Themselves
. But a binary setup seems to be what happens when we let nature sort it (or her) self out.

Speaking of two teams, books have been written, operas composed, game shows created, and countless news stories produced that deal with the battle of the sexes. That men and women have such difficulty getting along sometimes, or perhaps most of the time, would be a mystery, if it weren't for evolution.

We are all driven to select a mate. Those of us who don't do not pass our genes on to the future. A human female is apparently driven to select a mate that will provide for her and her offspring. If nothing else, such behavior or motivation would be consistent with the tenets of sexual selection. A human male selects a mate that is, by his reckoning, well suited to carrying his genes forward. The female has to make her genes appear valuable by “playing hard to get,” as the old saying was and is so often said. But really … all things in moderation.