Sexual Inequality - an Evolutionary View
Why should we even have two different sexes? It's perfectly possible to reproduce sexually with just one sex - and a few organisms do just that. Why don't we?
Sexual Inequality – an Evolutionary View
All of our wrangling on child care policy, sexism and morning after pills would turn upside down if we humans had evolved the mating strategy of the nurseryfish. No one knows exactly what sex between nurseryfish entails, but when they’re finished, the male leaves with fertilized eggs stuck on an appendage that grows out of the top of his head.
He protects the eggs while the female swims off for a life of self-actualization, adventure, and further trysts. Nurseryfish are among a few species – including seahorses – where the familiar sexual equation seems reversed.
But why do we humans see the females as the usual caretakers of offspring – shouldering most of the burden of reproduction? The answer goes back to the very definition of male and female. In everything from rhinos to rhododendrons, the sexes are defined by the relative sizes of the sex cells they bring to the reproductive table. The female is defined as the supplier of an egg, which is by definition bigger than sperm and full of all the cellular equipment needed to start a new life. The male brings the sperm, which is small and spare and carries just his genetic material and little else.
It’s not at all obvious why it has to be that way, since sex is all about merging and exchange of genes, and can be done perfectly well with two equal and opposite “mating types” rather than two sexes who share unequally in the burden of reproduction.
A more egalitarian situation probably predated the evolution of male and female sexes, said biologist James Umen of Donald Danforth Plant Science Center in St. Louis. He is looking into the issue by studying two closely related species of algae, one which reproduces using sperm and egg, while the other combines two equal-sized cells – a practice known as isogamy.
Organisms that practice isogamy are neither male nor female but divide into “mating types”, labeled plus and minus,or something similar.
This was probably our ancestral state, said Umen, but things changed when organisms became more complex, going from single cells to many cooperating cells. When multi-celled organisms reproduce, they need to get their offspring off to a proper start, which means one parent or the other has to contribute more than just one cells-worth of material.
It would be nice if both parents could go 50/50, but that situation tends to fall apart thanks to something appropriately called disruptive selection. That begins when some troublemakers start gaming the system by producing increasing numbers of small sex cells (proto-sperm) in order to up their chances of successful reproduction. Others respond to these proto-sperm not by competing with them for numbers, but by taking up the slack, producing larger, more amply-supplied sex cells (proto-eggs).
Anyone who produces middle-sized sex cells loses out, Umen said, because they can’t compete with sperm for abundance and speed, and they’re too small to make a successful progeny if they merge with them. They can’t beat the males and they can’t join them. And so once disruptive selection starts, the sperm get smaller and the eggs get bigger and all the inequality and injustice that goes with having two sexes follows.
If it’s any consolation, the species of algae with the more egalitarian isogamous systems are stuck in a state of single-cell simplicity, while the more complex, multicellular species use sperm and eggs. So it’s possible that the male/female divide is the price we pay for being more complex than pond scum.
That doesn’t mean that males always get to mate and then leave when the fun is over. In some species the males contribute as much parental care as females. In some penguins, it’s the male who incubates the eggs over the frigid Antarctic winter.
Among fish, most species just leave their fertilized eggs to fend for themselves. In others, the females may provide care and in others that job goes to the male. Some male fish carry their fertilized eggs in their mouths. Others build nests in which the females deposit eggs, after which the males stay and tend to them. And then there’s the nurseryfish, in which the male carries eggs on a hook-like appendage that grows from the top of his head.
“Forehead brooding is just one part of this incredibly interesting spectrum of parental care among fishes,” said Ohio State University evolutionary biologist Tim Berra, who has been studying nurseryfish for the last decade. “It’s the only fish we know that has this system.”
Profound questions remain about nurseryfish, he said, such as how the forehead brooding system evolved, and how the eggs get on the hook during nurseryfish sex. Nurseryfish have never been seen mating in captivity.
And in the wild, they live along the North Coast of Australia in waters that are subject to ferocious tides and also happen to be teeming with large crocodiles. Last fall, Berra was stalked by a crocodile that was longer than his 15-foot aluminum boat. The croc eventually got caught in the gill net that Berra was using to capture nurseryfish. He and the crocodile escaped alive (crocodiles can drown if caught in a fish net.) The net, however, was destroyed. The incident made the front page of the local paper with the headline, “5M[5 meter] Croc Shock for Professor”.
Berra said that DNA tests reveal the closest relatives to the nurseryfish are the cardinalfishes – a species in which the male carries the fertilized eggs around in his mouth. It’s not an easy system for the males, who have to forego food while brooding, lest they swallow their genetic legacy. So it would be like being pregnant and on a diet at the same time. But then, evolution doesn’t care what’s fair – it only favors what works.
Contact Faye Flam at 215-854-4977, fflam@phillynews.com, or @fayeflam on Twitter. Read her blog at philly.com/evolution.
Interesting that humans are the only species that "have a problem" with the roles that evolution has generally guided them to. Everyone else just does what they've evolved to do, with rare exceptions. (Even bonobos and dolphins.) I'm not sure how someone can believe in equality and believe in evolution at the same time. As Faye said above, "evolution doesn't care what's fair".— nerdyseahorse- Evolution gave us Hitler and our sense of fairness. Acceptance of the theory of evolution doesn't require us to endorse Hitler or reject equality.— jxxphilly
- But if you assert that evolution is the basis of our sense of fairness, then it is not grounded -- it is merely an emergent property of evolved brains. (Not too different than thinking spiders are creepy.) One person's sense of fairness is as "valid" as any other person's. There are no universal physical laws to compare against; the universe couldn't care less about chattering monkeys on a galactic backwater. So who can claim that they are fairer than someone else? Especially if evolution is inherently unfair -- the "stronger" (better adapted) outlive/outbreed the "weaker" (worse adapted).
Note: I definitely believe in fairness. But I have a spiritual basis that is the origin/backstop/ratification for such a belief. What do materialists have? Their feelings? Anything they could scientifically prove?— nerdyseahorse - Nerdyseahorse: regarding, "I'm not sure how someone can believe in equality and believe in evolution at the same time", what has one to do with the other? Did evolution give us science, art or mathematics? No. But does that mean they're not important to us?
-david3340— david3340 - David: But we treat a belief in equality as some sort of universal truth. On what basis?— nerdyseahorse
- Nerdyseahorse: I don't think we do. Can you think of any society that has achieved this ideal? Rather, most people would agree with the concept of equality, as it appeals to our sense of fairness, and would presumably lead to happier people and societies. It is something we choose to aspire to.
-david3340— david3340 - David, what I mean is that we have a sense of fairness and the thought that anything that falls short of it is not entirelyfair. This sense is stronger than our sense of what is beautiful or funny or tragic. It seems to have some hefty behind it. Why is that? It seems like it should be just as illusory as how many here consider religious feelings. Yet many atheists congratulate themselves on their non-spiritually-based moralities. This seems a bit hypocritical to me.— nerdyseahorse
- Sigh. That should be "heft". Dumbphone. — nerdyseahorse
- Sigh. That should be "heft". Dumbphone. — nerdyseahorse
- I think that is exactly correct - evolution does what works - crazy as some results may seem. The nurseryfish is an amazing creature. Perhaps this unique set up works on the principle of illusion, in the same way large spots on some species of butterfly wings mimic the eyes, confusing predators. The male may be "safer" than the female in their native habitat, therefore evolution has elected him to be the one to carry the progeny. Also interesting is the mention of the cardinalfish - that is a very delicate situation indeed! I'm not sure of the details in reproduction, how much they have in common with the penguin behavior mentioned, but I have noticed that with the cardinals that gather around my bird feeders, the males take a very active role in rearing their young. Males can be seen feeding the fledglings by mouth, perhaps pre-chewing the seed? I have never seen the females do this - only the males. Thank you, Faye, for another great piece!— fineprintJK1
- It seems that the anti-Darwin people will use ANY argument against both evolution and fairness among and between sexes. Natural selection resulted in our ability to reason. Our chief adaptation among primates is the human brain and its capacity for rationality, although some of your correspondents' responses raise that question!!— cdarwin
- nerdyseahorse: Clearly, it's easy to imagine how behavioral traits such as cooperation (which would naturally encompass ideas like fairness) are adaptive. There's nothing illusory about it. I think of myself as having high moral standards (like most people, I imagine); but what has religion or spirituality to do with it? I don't see an contridiction here.— david3340
- Don't tell nanny Mayor Bloomburg about this - he would have sex outlawed. And here in Philly - Nutter would find a way to tax it.— Beethoven987
- Equality is a human concept. You don't see it very much in any other species.— verve
- Human sex is much more fun.— Keith S.
In pursuit of her stories, writer Faye Flam has weathered storms in Greenland, gotten frost nip at the South Pole, and floated weightless aboard NASA’s zero-g plane. She has a degree in geophysics from the California Institute of Technology and started her writing career with the Economist. She later took on the particle physics and cosmology beat at Science Magazine before coming to the Inquirer in 1995. Her previous science column, “Carnal Knowledge,” ran from 2005 to 2008. Her new column and blog, Planet of the Apes, explores the topic of evolution and runs here and in the Inquirer’s health section each Monday. Email Faye at fflam@phillynews.com.
Reach Planet of the at fflam@phillynews.com.
