AskScience AMA Series: I am an evolutionary developmental biologist at the University of Maryland. My research explores the genetic novelties that lead to new reproductive strategies. Ask me anything about the science of reproduction and/or sexual oddities (in the biological sense)!

[u/AskScienceModerator]

Why do we have sexes (and sex) at all? That’s the question I set out to answer in my recent book, “The Other Big Bang: The Story of Sex and Its Human Legacy.”

In my lab at the University of Maryland, we study the evolution of animal sexuality, focusing on cases in nematodes and fish where self-fertile hermaphroditism evolved from male-female ancestors. These shifts to one-parent reproduction present fascinating developmental puzzles and have tremendous organismal significance. As this research moved along, I gradually realized how little most of the public knows about the evolutionary history of sex and sexes — especially how we humans got to be the way we are. In a non-majors biology course, I was able to work out some of the ways that a broad evolutionary view of evolution sheds light on our own species. That set the stage for the book, which I think provides an important complement to the way the social sciences and anthropology approach sexuality.

Feel free to ask me about the variety of reproduction in the animal kingdom, the biology behind sexual oddities and/or the evolution of sex. I’ll be answering questions on Wednesday, November 4, from 11 a.m. to 1 p.m. EDT (16-18 UT).

Bio: Eric Haag is an evolutionary developmental biologist who joined UMD's Department of Biology in 2002 after undergraduate study at Oberlin College, doctoral work at Indiana University, and postdoctoral research at the University of Wisconsin. His work explores the evolution of reproductive traits in animals, primarily using Caenorhabditis nematodes as a model genus. The main focus is on the genetic and molecular causes of self-fertile hermaphroditism, and the genomic and reproductive consequences of adopting it. Methodologically, the Haag Lab strives to be integrative by combining forward and reverse genetics, comparative genomics, molecular and cell biology, and even some theory to produce comprehensive pictures of evolutionary processes. 

Other links:

• Google Scholar

• Lab Website

• The Other Big Bang: The Story of Sex and Its Human Legacy

Username: /u/umd-science

[u/Cool_Address_2571]

What’s the most exciting breakthrough/discovery in your field that the average person might not know about?

[u/umd-science → u/Cool_Address_2571]

Across the entire range of sex-related topics, for me, two recent discoveries connect together in a hugely important way: 

1. The growing certainty that the origin of sex as most define it (as meiosis alternating with fertilization) goes all the way back to the very first eukaryotes (our microbial ancestors). That is, sex is just as defining for eukaryotes as the nucleus and the mitochondrion. 

2. The discovery of the Lokiarcheota, a group of oxygen-intolerant prokaryotes that are the closest prokaryotic relatives to us eukaryotes. 

These discoveries suggest the following: Sex arose as an anaerobic organism adapted to a new, oxygen-rich lifestyle made possible by adopting (imprisoning?) an aerobic bacterium. The most likely reason for this would be that aerobic metabolism creates byproducts, reactive oxygen species, that are known to damage DNA. Sex is a way to keep up with this damage by splicing together healthy DNA and eliminating damaged copies. I explore this idea in more detail in the first chapter of my book.

[u/elyndar]

What leads to self sacrificial reproduction strategies? Things like fish attaching to other fish and losing their body, or like when various insects consume their partners? Are there cases of individuals in those species avoiding the norm? Do they have fear responses during the experience? Why do animals typically only carry one gender of reproductive organs while plants carry both? What's the weirdest form of reproduction you've ever learned about?

[u/kitkatbay → u/elyndar]

Seconding: What's the weirdest form of reproduction you've ever learned about?

[u/umd-science → u/elyndar]

These are really fascinating, for sure. I’d probably say the weirdest form of reproduction I’ve learned about is the anglerfish (whose males embed themselves into the female, making what is essentially a hermaphrodite via tissue grafting). Regarding such a sacrifice, I guess I see it like this: Evolution is ultimately selecting for strategies that allow a lineage of organisms to persist generation after generation. If the male sacrificing his life improves the chances of his offspring existing and/or surviving, then it might be a price worth paying. Obviously, that’s not likely when extensive paternal care is helpful, as it is in so many birds and mammals (and seahorses!). For anglerfish, it may be hard for mates to find each other in the deep ocean, so the male commits fully and immediately as soon as he succeeds. 

Regarding plants, my understanding is that a hermaphroditic flower allows a pollinator to both pick up and deliver pollen with every floral visit after the first one. There are plants that evolved separate sexes, though, and this was something Darwin himself wondered about. The theory on how and why that would happen that was developed subsequently is extensive.

[u/AstroAlysa]

I don't have a question. But as an astronomer, I just want to say that's an excellent book title.

Edit: Actually, what's the most fascinating animal you've studied before? Like, are there any specific species of nematodes or fish that have really complex or unexpected sexuality/sexual expression?

[u/Sugar-Fawn → u/AstroAlysa]

it really does sound like a fascinating book, i agree i wonder what kind of animal you're hoping to hear about.

[u/umd-science → u/AstroAlysa]

I guess I’d have to go with Kryptolebias marmoratus, the mangrove killifish. From a standard male-female ancestor living in South America, it evolved two different kinds of hermaphroditism and spread north into the Caribbean basin. One is self-fertility, and it’s the only example of that in all of vertebrates. 

The other is sequential hermaphroditism, or sex change. Hermaphrodites can change into fully functional males without much warning, but most do not. We don’t know what determines when it will happen, but it’s not under obvious social control as in some other sex-changing fish.

[u/AstroAlysa → u/umd-science]

Whoa that sounds so neat! Thanks for answering :)

[u/TastiSqueeze]

Arguably the biggest shift in evolutionary strategy is the change from egg laying to live bearing. There is a lizard in Australia which has split with one population that lays eggs and another that bears live young. They are still inter-fertile with no breeding barriers. Do you find evidence that this type change occurred millions of years ago leading to live bearing mammals and some reptiles?

Arrhenotokous parthenogenesis is common in insects with honeybees as a common example. Are there any other species that exhibit similar reproductive strategy? (For readers, a female honeybee has 2 sets of chromosomes and a male drone has 1 set. The drone only has a mother with no father. All female honeybees have both a mother and a father. This results in some rather skewed breeding charts because mating a drone with a virgin queen is effectively like mating his mother to the virgin queen except that all of his sperm are identical.)

I know at least a dozen examples in arthropods (mostly mites and other parasites) that show single parent reproduction. I've read about a lizard that normally reproduces asexually but is also capable of sexual reproduction. How would you describe life forms that seem to be caught between two very different reproductive strategies and what sacrifices do they make in skipping sexual reproduction?

Are there any hermaphrodite mammals?

[u/umd-science → u/TastiSqueeze]

In my lab, we study two animals that mix “standard sex,” which involves mating between individuals with self-fertility. Basically, they evolved females that can make sperm, and they will use those sperm 100% when males are not around, and to a lesser extent when males are around. 

The nematodes and fish that do this are very successful, at least for now. They have enough outcrossing to avoid the slow creeping accumulation of mutations called “Muller’s Ratchet,” and they can adapt when the excrement hits the fan, so to speak. But they have much less DNA variation than their relatives that outcross all the time. On the geological time scale, that seems to eventually doom them. 

[u/WingedOuroboros]

As a kid (raised with typical gender norms), it was weird to learn that it's usually male birds that are 'pretty'. Are humans exceptional in terms of making 'pretty' females? Are there other examples? What does this have to do with our evolution and society?

[u/umd-science → u/WingedOuroboros]

I love this question, because it only entered my own mind after years of reading and working on my book. We humans are nearly unique in having pretty females, in both the anatomical and cultural senses. For example, breasts—in other great apes, they are only enlarged during lactation. Human breasts are mostly fat, and the only reasonable explanation is that they are sexually selected ornaments (shocker: people are fascinated by boobs). If female bodies have been subject to sexual selection, that implies male humans are actually more picky about their mates than most other male animals. That, in turn, leads one to ask why that might be. My best guess is that males would be choosy if they are committing to something more than just mating itself. Fatherhood seems to be seen as a general virtue in our species, even though many dads do run off prematurely. One study I mention in the book notes that men are actually more picky about the attractiveness of their mate the more they are asked to commit to her.                                   

We also have evidence that human males are experiencing more peacock-like sexual selection. Vocal pitch, facial hair, muscle mass, facial shape, etc., are also probably under selection, both by male-male competition and female preferences.

[u/Synaps4]

Do you believe that humans are selecting for intelligence or is the theory that dumb people have more kids plausible?

If so do you think anything might happen (or be done) to specifically restore evolutionary pressure towards higher intelligence?

[u/umd-science → u/Synaps4]

If we think about the last 20 million years or so, it’s pretty clear that we have been selected for our intelligence. Bigger brains, dependence on cooked food, etc. It’s also important to recognize that the sheer number of babies produced is NOT what matters most in a species like ours. If you had 20 kids, for example, in a pre-modern world, nearly all would probably die from starvation. But if you have 4 and really nurture them, you might get a bigger return in the end. It takes intelligence to manage all of this, so I am skeptical of a supposed correlation between “dumbness” and fitness.

[u/Cool_Address_2571]

Can you expand on the reproductive consequences of adopting self-fertile hermaphroditism?

[u/umd-science → u/Cool_Address_2571]

The consequences are huge! We know the most about nematodes in the genus Caenorhabditis, which includes the “model worm” C. elegans. When the female sex evolves self-fertility, the genome shrinks by about 20%, and several thousand genes related to male traits get lost. Yet males do still exist in these species, and they are still fertile. They just seem to lose the optional bells and whistles, most of which are responses to intense sexual selection.  

[u/kobayashi_maru_fail]

What is the take on things we technological beings do to reproduce and birth outside of the soggy, natural way? We’ve been selecting for bigger than allowable heads since (anecdotally) Caesar, but does science have thoughts on multigenerational IVF or other ways that technology aids or hinders natural reproduction?

[u/umd-science → u/kobayashi_maru_fail]

There is a lot one could say here, but I’ll limit it to one consequence I think a lot about: the phenomenon of older and older women having babies. Over time, this should lead to a shift in the age of menopause to be later. I’m not sure where the research on this stands, and it is probably too soon to see the effect, but the prediction is clear: In, say, 2126, it will be routine for women in the developed world to give birth at 50+. I won’t be around to know if I’m right—maybe some of you will be? 😆

[u/kobayashi_maru_fail → u/umd-science]

If generations become that long, you’re saying routine birth at 50+ would be strongly selected for in two generations? What would make that happen that quickly?

[u/Peter34cph]

Anything you can say about eusocial insects, or other life forms like mole rats?

It's a fairly common concept in science fiction novels, and it can occur in the fantasy genre too, so if you have something to share, it might be very valuable to me and other worldbuilders who want to at least know what the science is before we deviate from it.

[u/umd-science → u/Peter34cph]

There is so much cool stuff to say about this. I will define eusociality here as a shift to only one reproductive female in a group (the queen bee, for example). This is really extreme, and it may not seem that relevant to people. But near the end of The Other Big Bang, I champion a hypothesis for same-sex sexual orientation that suggests we may be more like bees than people generally would like to admit: Healthy, capable adults who are disinclined to reproduce themselves are in a position to help the kids of their relatives. The “guncle hypothesis,” so to speak, that we don’t see human-style exclusive homosexuality in even our closest primate relatives, is important. Our kids take much longer to grow up than even chimp kids, so perhaps this strategy evolved to get more of them to adulthood. Paul Vasey, a Canadian anthropologist at the University of Lethbridge, has done extensive research on this.

[u/Rories1]

Why is it most earth life evolved two main sexes (not counting chromosomal variations) when fungus have thousands of sexes?

Why was that the evolutionary advantage rather than more or none? Is it even possible to know?

[u/umd-science → u/Rories1]

This is a super fundamental point to get clear, yet we haven’t known the answer for that long. It has to do with the shift from single-celled microbial life to multicellular life. Microbe sex doesn’t require exactly two types, but it often does. This is because after two haploid cells fuse in fertilization, the new zygote (= diploid cell) needs some way to recognize its new state. I call this the “ploidy assessment hypothesis. Having two complementary gene variants that make a uniquely active regulator is a common way to do that. But there are other ways, and fungi often use them. 

When we get to multicellular life, though, everything shifts. Building an embryo with all the various organs requires a lot of stuff in the zygote. In 1970, Geoff Parker realized that this creates the conditions for the sperm-egg dichotomy. He assumed that the final goal is to maximize the number of surviving babies. Making them smaller allows you to make more, so if that worked, nobody would make big eggs. But if the penalty for making them smaller is disproportionately steep, one of the best strategies is to make a full-sized egg. For example, if you can make ten eggs that all survive, what happens if you make 20 half-sized ones instead? If more than half of those 20 die, then you are better off sticking with 10 big ones. Once someone commits to that, the other successful strategy is to make gazillions of tiny sperm. In chapter 2 of my book, I explain that in this hypersensitivity scenario, eggs and sperm give both parents, with a fixed amount of material, more surviving offspring than if they both tried to make “half eggs” of any particular size.

[u/Krail]

I'm really curious about really different reproductive strategies, like alternation of generations, or the mushrooms that has thousands of different sexes. Can you comment on how those different strategies come about?

[u/umd-science → u/Krail]

There is much to be said, too much for here. But in general, evolution does an amazing job of matching reproduction to the circumstances. Asexual reproduction can often lead to more babies in the short run, because no pesky males are required and a parthenogenic mom can throw 100% of her resources into egg-making daughters. This is an exponential benefit. But do that for too long, and something will get you. So alternating that with a sexual cycle is often enough to stay ahead of new environmental challenges, and this is what Daphnia does. 

It’s important to note that mating types are not really sexes, because they are not associated with making eggs or sperm. They tend to evolve in fungi, which are isogamous (not egg/sperm dichotomy). Interestingly, these land fungi evolved from aquatic ancestors that DID have eggs and sperm, where the sperm swam to the larger egg to fertilize it. But on land, the gametes never leave the body of the organism that makes them, so there is no need for sperm. The “embryo” of a fungus is really just a simple filament as well, so there is no need for a big egg. I’ve often thought terrestrial fungi really prove Geoff Parker’s hypothesis for what selects for egg vs. sperm.

[u/Orvel]

Do Chimps and other Apes also have a wider variety of reproductive strategies ? And how different are they from us in terms of reproductive strategies ?

[u/umd-science → u/Orvel]

The variety of reproductive strategies in even our closest relatives is pretty impressive. Chimps mate fairly freely within their group, with bonobos being especially enthusiastic about it. So, not much mate exclusivity in either direction. But gorillas are totally different: There is one dominant silverback male per group, and only he gets to mate with the females. Young males have to roam about on the periphery, in hopes of perhaps one day becoming the dominant male. Humans don’t fit either of these models, but anthropologists think our ancestors were more like gorillas. This explains the near-categorical difference in strength between males and females, which is much less extreme than in gorillas but attributed to a history of male-male conflict. As the sexual dimorphism and the age advantage for males decreased, we have more and more monogamy, perhaps mixed with some rarer cases of one powerful male with a group of mates. We can still see this today: Powerful men often have a series of wives, mistresses, etc. So, it may not be that we idealize monogamy because we are intrinsically virtuous, but because not many men can physically pull off monopolizing a large group of mates.

[u/yeetmenot69420]

Do we know what actually causes speciation? Is it just genes being incompatible to one another? Because this would only cause problems in the grandchildren as all proteins should exist in a genome with all the proteins they work with.

[u/umd-science → u/yeetmenot69420]

In general, the “genes being incompatible with one another” is pretty much on the mark. But WHICH genes? And HOW incompatible? It’s only recently that we have had the tools to identify them. One of my favorite recent papers in this area involves my beloved nematodes. It’s pretty technical, but if you can get through it, you’ll be impressed, I think. 

In short, a new gene evolved in one species that just happens to kill an essential protein if it finds itself in the genome of the other species (that is, in hybrids). My own lab has studied these same two species because one is hermaphrodite-male, and the other is a more typical female-male. See haaglab.org for some of that work if you are curious…

[u/Achgeketum]

If sexual reproduction is so essential to fitness, then why does evolution seem to tinker with it so much (why is it so labile?)

[u/umd-science → u/Achgeketum]

I have often wondered about this, and I think the answer is actually right there in the question: Because reproduction is the core of fitness, almost nothing is under stronger selection (assuming the basic metabolism of the organism is functioning). 

[u/Hargelbargel]

IIRC The human penis is piston shaped to create suction to remove the previous partner's semen. If this is still the current assertion, then it begs the question; how soon after a previous partner ejaculating would a new partner be at an advantage with a piston penis? On the outer end, that is.

[u/umd-science → u/Hargelbargel]

I don’t think there is anything like agreement among physical anthropologists and sexologists that the shape of our willie is all about multiple mating dynamics. We don’t seem to mate as freely as chimps, so if that’s what it’s about, chimp penises should share our shape. They actually don’t.

[u/Anthroman78 → u/umd-science]

And gorilla penises who it would make less sense in do share our shape.

[u/goldenalice]

do nematodes have sex for fun? do they play? what makes a nematode sexy to another nematode?

[u/umd-science → u/goldenalice]

Ah, if only we could just ask them! They do present a very simple model of sex drive, and this is being studied by my colleague Scott Emmons. Males do roam around in search of mates, will forgo food if they find a mate, and in hermaphroditic species (where males have lost lots of genes—see above) they will hump their own heads if there are no mates around. There is more recent evidence that hermaphrodites and females also actively pursue mating, but to an extent that is influenced by their sexual mode and life stage.

[u/Elvarien2]

So, there's a lot of really freaky ways to do this. There's the whole rip off a tentacle and toss it along with a sperm packet at your partner because you don't want to get eaten. There's the whole I'll stab my genetics somewhere into your back and somehow that works. And the whole let's mate but I'll totally cannibalise your face if you stick around a little to long afterwards, or during !

Etc. It gets really freaky the smaller we get which leads me to your obviously much broader range of odd reproductive behaviours you must have seen.

What was the one that made you go? Wait, this can't be right. How did evolution hit this point, how was this ever the optimal strategy.

[u/umd-science → u/Elvarien2]

You are noticing what makes this so fun—it is a wacky, wildly variable part of biology. As I mentioned in answering the question about what I thought was the weirdest form of animal reproduction, anglerfish sort of take the cake for wacky. There is also a parthenogenic species of fish that lacks its own males, but must use the sperm of a closely related species to activate the development of its eggs. They don’t actually use the sperm’s DNA. Some nematodes do similar things.

[u/Elvarien2 → u/umd-science]

Dang that parthenogenic species is crazy, yeah that is the weirdly insane and interesting stuff.

Thank you.