Thursday, May 28, 2015

Sexual Selection and the Reason We Have Men

Seen on my Reuters news feed:

I started out wondering why this is being reported as a revelation. Then having read it, I'm aghast at the reporting, which I find to be simultaneously sensationalist and naive. We begin with this:
Since in many species, sperm is males' only contribution to reproduction, biologists have long puzzled about why evolutionary selection, known for its ruthless efficiency, allows them to exist. 
Now British scientists have an explanation: 
Well, actually everybody else has had an explanation for years, and every else's explanation is pretty much the same as what's being reported here, so this unfortunate phrase just makes "British scientists" look like they're late to the party. And that's sad, because they're not.

First of all, this wasn't a study about why men exist. It was a study to determine whether sexual selection provided fitter offspring than random pairings. In plain language, if all the ladies and gents get their pick of mates, does the population fare better than if they have no choice?  Common wisdom says it does.

The team conducted a 10-year-long experiment that would either support or disprove some of the many explanations that exist. This isn't an ah-ha! moment. This isn't a momentous discovery. They were doing the long and tedious work of real scientists. Of course, this doesn't look revelatory, so the reporting "fixes" that and makes them look more silly than diligent. And diligent they were. This is a tour-de-force of number-crunching as the team performed detailed inspection of generation after generation of beetle. Their aim was to provide empirical observations. It's one thing to say "it makes sense that this is what happens". It's another thing entirely to watch it happen.

If you'd like to read it, here's the paper as published in Nature: [HTML].
And here's the full report, including a description of the experimental methods: [PDF]

It would have been nice if Reuters had written an article that was directly related to the study on which they're reporting. Somehow they jumped from "sexual selection" to "why do men exist?"

The use of the word "scientists" brilliantly illustrates the problem I decried in my last post. It's generic and says absolutely nothing about the researchers. Nowhere are the team's credentials related. We are six paragraphs into the story before we learn that it was conducted by professor Matt Gage, I had to look him up myself, which makes this pretty crap-tastic reporting. Gage is a professor of evolutionary ecology at the School of Biological Sciences at East Anglia University. It shouldn't be that difficult to give the man some credit if you're going to quote him.

I looked around to see if other news outlets used the same generic "scientist" label. They do. Even The Washington Post headline editor played the same tune in a story that is far more detailed than Reuters'. I'm happy to say that the WP reporter had a solid handle on the subject, and provided a link to the paper in Nature. Please read the WP article, as it has a number of quotes from Gage that might enlighten what I'm about to write.

Let's have sex

Now, the EAU study was concerned with the benefits of sexual reproduction in a purely mechanical way: what effect do these (re)combinations have on the genome? That's one aspect of sexual reproduction, but not the only one; and not everything can be adequately described in terms of numbers, and some of the propositions offered seem a little shaky to me. The study is good... empirical studies are "just the facts, ma'am"; it's the assumptions that make me skeptical.

For instance, a great many quotes are expended concerning the "wastefulness" of sexual reproduction. I don't find that to be a terribly profound observation. For instance, the WP reports:
"Almost all multicellular species on earth reproduce using sex, but its existence isn't easy to explain because sex carries big burdens, the most obvious of which is that only half of your offspring -- daughters -- will actually produce offspring," lead author and UEA professor Matt Gage said in a statement. "Why should any species waste all that effort on sons? We wanted to understand how Darwinian selection can allow this widespread and seemingly wasteful reproductive system to persist, when a system where all individuals produce offspring without sex -- as in all-female asexual populations -- would be a far more effective route to reproduce greater numbers of offspring."
That thing Gage said right there... that's not what the study tested. The study's about sexual selection, and that presupposes the existence of sex. It doesn't really say anything about why we have sex; at best, it tells us why we shouldn't have forced marriages.

But why we have sex, and more specifically, why we have sexes... that's admittedly more entertaining. So let's put the study aside and talk about that instead. And as I'm not pulling down any grants, we might as well idly speculate while we're at it.

Sex is more diverse than you think

Hermaphrodites impregnating each
other. Nobody's on top.
I'm not exactly sure why Gage excluded hermaphrodites from his thinking. Hermaphrodites are neither female nor male. Or they're both, depending on your point of view. They do have sex, and therefore genetic diversity, but both partners can have children. They even get the perks of sexual selection. If we really want to question why men exist, hermaphrodites present a stronger argument than asexual populations because it eliminates a variable. The question becomes "why have males when you can have all the benefits of sexual reproduction without them?" 

The weak anthropic principle leads me to challenge the very assumption that sons are wasteful. We observe that most animals have two sexes; therefore having two sexes must be advantageous. The question is how.

All life started out by reproducing asexually. Around 3.6 billion years ago there was no sex, and that was the state of things until about 1.2 billion years ago. But once sexual reproduction appeared, it quickly dominated. In only the last billion years, life on Earth has gone from single-cellular lifeforms to all of the diversity that has ever lived, gone extinct, or given rise to new forms. Asexual reproduction lost a two-and-a-half billion year head start. While it's still common, it's only so among plants and the simplest forms of animal and single-celled life. What appears to be "waste" must in actuality be an investment.

That's not dust; those are spores.
You don't get more prolific than that.
via Wikimedia Commons
Investments that look wasteful are commonplace in nature. No matter how wasteful it appears, it has paid off if it produces one more generation. Billions of spores can be released from one (asexual) fungus. Likewise, some animals can produce many more offspring than could ever be expected to survive, most of which are simply eaten. Plants do the same, casting seeds upon the wind, many of which will be eaten or fall on infertile ground [1]. These organisms don't look at it as an investment, of course. That's anthropomorphic thinking. But the organisms that weren't prolific... that weren't wasteful... are now extinct, as they didn't produce enough offspring that survived until reproductive age for their species to continue.

But being prolific is only one strategy for survival. We find many more. You'd be hard-pressed to think of one that hasn't been attempted by some organism somewhere on Earth [2].

Rather than being prolific, some organisms have few offspring, but zealously protect them to adulthood. This is the norm among the most complex species. I think it's worth pointing out that the 50% "sex tax" only applies to those organisms that don't engage in parenting; and that these are the same organisms that tend to be highly prolific. Numbers ain't everything.

When parenting is added to the mix, the "sex tax" disappears, as the parents can not only protect the young, but through behavioral roles they can do so while still providing food and security for themselves. The lower birth rate is offset by a higher survival rate. This is independent of the genetic advantages found by Gage and his team. The concepts of family and community are important factors, be they applied to a den of foxes or a human family.

But you don't really need men for any of that.

It's raining men

The first sexual organisms were hermaphroditic, but as with sex itself, once the male/female paradigm was reached, it quickly dominated the animal kingdom. The more complex an organism, the more likely it is to have differentiated sexes.

Gage describes males as pretty much useless after they've donated sperm, but keep in mind that this applies only to those cases where the males disappear after impregnating the female. In those cases, genetic diversity alone is the payoff. So why isn't hermaphroditic sexual reproduction the norm? Like everything else, sexual roles and physical dimorphism aren't completely accidental. They're shaped by environmental pressure and the need to produce and raise offspring. So there must be some advantage to having some segment of the population that doesn't have children.

We can guess. Setting aside the advantages of "sex", we're looking for the advantages of having "sexes". And it doesn't really have to make sense; it just has to work well enough to survive natural selection.

Fuck bacon.
In males you have a class of organism that can not only breed with multiple partners at the same time, but which never itself becomes pregnant, so that those not engaged in breeding are always free to forage and defend the breeders. While the females have the children and most often are the ones who nurture and care for them; males, in general, tend to be more aggressive and adventurous [3]. They tend to engage in dangerous activities that have high potential payoff.  They "bring home the bacon". Again, I challenge the assumption that producing males is "wasteful". Rather, they enable a population to engage in survival strategies that have high individual risk in such a way that they present a very low communal risk. But the individual risk yields high communal benefit.

Does that sound stereotypical and sexist? Well duh... we're talking about biology, not politics. Sexual roles aren't something to run from: they're natural.

via Wikimedia Commons
Honeybees, for instance, have found such utility in a non-breeding population of risk-takers that they have evolved a third sex (or caste) to do specifically that. Worker bees are female in name only. They are infertile, neither providing eggs nor sperm. Within the caste they specialize by age. Young ones build and maintain a hive, but as they age, they begin to forage. Beyond 21 days of age that's all they do. Meanwhile the queens have sex and babies and the drones do nothing but look for sex.

Wired magazine describes sex as the "driving force of evolution". Meh. Sex is a product of evolution. Sex is evolution eating its own tail. Sex provides the raw material of evolution faster by recombining DNA, but we'd get raw material from mutations anyway.

Evolution isn't driven by mere reproduction; it's driven by survival.

If Gage is accurately phrasing the question he's researching (the why and how of it), I think the answer is obvious. "Why should any species waste all that effort on sons?" Because sons are useful in helping the members of a species survive to reproductive age. The utility of men doesn't stop at ejaculation. Higher organisms prefer diverse sexes because sons become fathers, not sperm donors.

This diversification of the sexes led to families and communities: prides of lions, pods of whales, packs of dogs, herds of cattle, flocks of geese, villages of humans.

Like every evolutionary advance, the fact of males is an accident. But organisms that have discovered the utility of males preferentially select for it. Presumably, if that utility didn't exist, then it would not be selected.

more hermaphrodites
So how do we test this? I'd say to start by looking at species that have little to no real opportunity to adopt behavioral roles. And when we look at flowering plants, which are sedentary, we do indeed see that though they are sexual, they are overwhelmingly hermaphroditic. Most flowers contain both male and female parts; some, like the holly, have male and female sexes, but they are in a distinct minority. When behavior is removed from the mix, hermaphrodites win.

So far, it looks plausible. What do you think?

[1] Trees do the same with their sperm, casting billions of grains of pollen into the wind in a valiant attempt at impregnating as many cars as possible.

[2] Other sexual strategies:
  • In some species males are literally parasitic. They don't have a real existence beyond being a sperm delivery system. Anglerfish males attach to the females so completely that they share a circulatory system. For all practical purposes, they become one organism.
  • In addition to true hermaphrodites like earthworms, there are animals that can change their sex. A number of fish, such as wrasses, can do this.
  • Actually, any living thing that swaps DNA and recombines it to make children, no matter the method, is engaging in sex.
[3] Yes, I know all about seahorses. They're an exception.

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