It’s been known for a while that many species, including some vertebrates, can reproduce without sexual reproduction. But of course to observe this, one usually needs to keep animals in the lab and then see them produce offspring without ever having mated. But a new article in Current Biology by Andrew Fields et al. (reference below, abstract only at link) is the first report of vertebrates having done this in the wild.
First, a brief note. Biologists don’t really understand why animals have sex at all, for under most conditions an individual would leave more of its genes by reproducing parthenogenetically (without sex), than by reproducing sexually. If you bud off a diploid organism (one having all of your pairs of chromosomes) rather than producing an egg or sperm (having half your chromosomes) to fuse with another gamete, you leave an offspring having every one of your genes. Ditto if you simply produce an offspring from an unfertilized egg having all of your chromosomes rather than half.
If you have sex, however, you dilute your genetic contribution by half: the offspring has half of its genes from you, and half of its genes from your mate.
Thus, any gene that impelled you to reproduce parthenogentically would leave many more copies of itself than a gene that impelled you to reproduce sexually. In fact, it would leave twice as many copies, and would sweep through a population. The population would comprise only asexually reproducing individuals. This genetic deficit of having sex is called the “twofold cost of sex.”
But since the vast majority of non-microbial species have sex, why do they? They’re losing their genes by so doing. There must be a big advantage (at least twice that of being parthenogenetic) to explain the prevalance of sex.
There are several hypotheses, and some have empirical support; for example, the idea that reproducing sexually, by allowing you to mix your genes with those of other individuals, gives you a genetic flexibility to counter the evolution of other organisms, such as parasites, that are constantly adapting to parasitize you. But the advantage of that must, again, be very large compared to the twofold cost of having sex. Another explanation is simply that an ancestral species did overcome that problem when evolving sexual reproduction, and once you reproduce sexually it’s just physiologically and developmentally very difficult to revert to parthenogenesis. (Humans, for example, can’t conceivably reproduce without sex—except of course for one mythical woman!)
One caveat: if you produce haploid offspring, having only half your genes (the equivalent of an unfertilized egg), then you gain no reproductive offspring (as far as I know). Such an offspring is genetically the same—as far as your evolutionary potential goes—to reproducting sexually, as each offspring carries only half of your genome.
I don’t think the public realizes that the fact of sexual reproduction is pretty much an evolutionary mystery—one of the biggest mysteries we have. But we do know that some organisms that normally reproduce sexually can revert to asexual parthenogenetic reproduction under some conditions. The paper by Fields et al. summarizes the data:
Facultative parthenogenesis — the ability of sexually reproducing species to sometimes produce offspring asexually — is known from a wide range of ordinarily sexually reproducing vertebrates in captivity, including some birds, reptiles and sharks. Despite this, free-living parthenogens have never been observed in any of these taxa in the wild, although two free-living snakes were recently discovered each gestating a single parthenogen — one copperhead (Agkistrodon contortrix) and one cottonmouth (Agkistrodon piscivorus).
It’s easy to see that if you can switch from sexual to asexual reproduction, it would be good to do that when population density is low, and you might not be able to find a mate. At least then you leave some genes. But if you can do that, why bother to ever have sex? Again, there must be some advantage of sexual reproduction about which we’re ignorant.
But on to the paper. The authors studied a very rare fish, the smalltooth sawfish (Pristin pectinata), a subtropical Atlantic and Mediterranean fish that is critically endangered. Here’s one from Wikipedia described as being in the Georgia Aquarium:
They are big, sometimes over 7 meters long. But what the hell is that saw for? Wikipedia notes that it serves as a device to both detect and manipulate prey, as well as to defend itself (by slashing) when it’s attacked:
Recent studies have demonstrated, however, that the sawfish utilize their rostrum to both sense and manipulate prey.
A sawfish’s saw is made up of thousands of sensory organs that allow them to detect and monitor the movements of other organisms by measuring the electric fields they emit. The sensory organs, also called ampullary pores, are packed most densely on the dorsal side of its beak. This allows the fish to create an image of the three-dimensional area above it, even in waters of low-visibility. This provides support for the bottom-dwelling behavior of sawfish. Utilizing their saw as an extended sensing device, sawfish are able to “view” their entire surroundings by maintaining a position low to the sea floor.
Sawfish uncover sand dwelling crustaceans and mollusks, two common prey types, by using their unique anatomical structure as a tool for digging and grubbing about in sand or mud. The sawfish churn up the sea bottom with their exaggerated rostrum to uncover these hidden food sources.
Why are they endangered if nobody eats them? As Wikipedia again notes, “Smalltooth sawfish are extremely vulnerable to overexploitation because of their propensity for entanglement in nets, their restricted habitat, and low rate of population growth.”
While they’ve been observed to reproduce sexually in aquaria, the authors found, by studying wild sawfish, that they can also reproduce parthenogenetically. And they apparently do this in two genetic ways: producing haploid offspring (presumably unfertilized eggs that develop to adulthood) and diploid offspring (which may result from the fusion of two of the four products of meiosis, yielding a diploid egg that also grows to maturity).
How did they find this out? Not by observation, since they were dealing with wild specimens, but through genetics. They sampled 190 individuals captured off of Florida, and released them after taking a sample of either blood or tissue.
They analyzed 40 variable genes in each individual and looked to see if individuals had two copies of their genes (i.e. two different “alleles”, or gene forms, at these genes), or only one. In normal outbreeding, sexually-reproducing populations, individuals will often have different forms of genes at some loci (“heterozygous”), with some having the same form of a gene (“homozygous”). For example, individuals with AB blood type have two different alleles at the Landsteiner blood-group locus, while individuals that are O have two similar alleles.
The similarity of genes within each individual was expressed by an IR (“internal relatedness”) measure, which will be close to zero if an individuals’ parents are genetically unrelated (there will be a lot of heterozygosity in that offspring), but will be close to 1 if individuals are either haploid (having only one copy of each gene, so heterozygosity is zero) or if an individual’s parents are very highly related, as would occur after generations of intensive inbreeding (like brother-sister mating), something that is very unlikely in this fish.
Here’s what they found, with the histogram of IR values shown below the picture of the fish:
Note that the graph is bimodal: the vast majority of IR values are low, but seven individuals (in box at right) had very high IR values. Two of these showed no genetic variation, while five had variation at only one or two of the 40 genes sampled. All of these high-IR individuals are female, as expected under parthenogenesis. (I don’t know how sex is determined in this fish, but if it’s like an XY system, the fish would have to be female because they’d be either XX or XO, since a parthenogenetic female parent doesn’t have Y chromosomes.)
The authors conclude that the two completely invariant fish were haploid, the result of parthenogenesis, which would be the first indication (see above) of a vertebrate reproducing successfully in the wild without sex. (I am taking the authors’ word for this; readers may know of some exceptions. (There are some fish that are parthenogenetic but require fertilization from a male, though the sperm serves a purely developmental function and its chromosomes are not incorporated into offspring.)
What about the five fish that were homozygous at nearly all genes, but variable at one or two? The authors interpret this as a result of parthenogenesis, too, but a form that produces diploid offspring. They hypothesize that this is automictic parthenogenesis: the mother was heterozygous at some of her genes, and when she came to produce eggs (this normally involves producing four haploid egg precursors, most of which—the “polar bodies”—degenerate before fertilization), two of those products fused to form a diploid egg. The two that fused were nearly genetically identical, since they were the fused products of simple cell division of a single haploid cell, but there might have been a rare “recombination” event with a non-identical polar body before this fusion.
So this vertebrate apparently reproduces occasionally without having sex. That would, of course, be adaptive in a species of low density, like this one, for it’s better to leave some of your genes than none.
Are the authors correct in their presumption that they’ve detected asexual reproduction in parents simply by looking at an offspring’s genes? Given the bimodality of the graph above, and no sign of severe inbreeding in this species (in such a case every individual would have a high IR value), I think they’re right. This opens up the possibility, if you can survey many variable genes, of seeing how often asexual reproduction occurs in the wild, and whether, as expected, it occurs more often in low-density populations.
One thing I wished the authors would have done, which is likely difficult and probably impossible in this species, is to do a chromosome analysis of the cells. Those putatively haploid individuals would have only half the normal number of chromosomes, while the diploid ones would have the normal number of chromosomes. This would have rendered their case airtight, but I still think that their conclusion is still pretty solid.
The only remaining question for this group is whether Mary could have reproduced this way—and don’t think that some crazy religionists won’t use this as an example of how a virgin can produce an offspring. The problem, of course, is that if Mary had reproduced this way, Jesus would have been either a female or a haploid intersex with an XO chromosome constitution, producing a short female with gonadal dysgenesis (rendering the individual sterile) and a webbed neck. (In this case the “H” in Jesus H. Christ could stand for “haploid”.) Since the New Testament mentions no such abnormalities, I conclude that, on genetic grounds, the Jesus story is a myth.
_______
Fields, A. T. et al. 2015. Facultative parthenogenesis in a critically endangered wild vertebrate. Current Biology 25:R446-447.
Clearly, there is a gap here into which God should be …uhhh… inserted.
We have sex so that we’ll always have something to talk about in confession, obviously. Biologists are so dumb.
I’ll kindly thank you to keep any and all of your gods out of any and all of my gaps.
Or, as the wise man said, Keep your Jesus off my penis.
b&
Nice! Ranks right there with Hayes Caryll’s She Left Me for Jesus, Roy Zimmerman’s Ted Haggard Is Completely Heterosexual, and you-know-who’s Every Sperm is Sacred.
God caused us to have sex so that then, we can have something to feel guilty about!
Checkmate, theistic evolutionists!
You see? Science and religion are compatible!
More like….Religion makes stuff up, until science always explains everything.
Clearly God provided the Y-chromosome, because He’s YHWH.
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* Which I guess also means that God is part hen.
Did anyone consult the Duggars?
I’m sorry. I’m sorry: I am just guffawing ever so silently ( I handle, and it IS, the Front Office, after all !) at this one !
Fascinating science, though.
( Calm down, Blue, and putz on. )
Blue
Titter titter
It is not the first parthenogenetic vertebrate discovered…in fact, approx. 50 reptiles are e.g. http://en.wikipedia.org/wiki/Parthenogenesis_in_reptiles#cite_note-2
Including Whip tail lizards I seem to recall?
This may be of interest –
Clonal Diversity and Clone Formation in the Parthenogenetic Caucasian Rock Lizard Darevskia dahli
http://journals.plos.org/plosone/article?id=info:doi/10.1371/journal.pone.0091674
Also Chinese Quail Coturnix chinensis, & Turkeys….
In the wild?
I believe that’s why the initial claim stresses “facultative”. It’s the first case of parthenogenesis in the wild in a species that usually reproduces sexually. The first bold bit deals with turkeys and such, while the second bit deals with asexual lizards. Drop either of the bold bits and there are a great many prior examples.
As I see this, if a species can reproduce asexually, then it will not be able to produce enough variability for natural selection to work with when drastic, sudden changes in environment occur, conducing eventually and inevitably to extinction. I am not a biologist, so I would appreciate any comment to what I just stated.
Indeed, there is a mechanism called “Muller’s ratchet” that tries to (partially) account for the costs of sex- it describes how asexually reproducing populations will irreversibly accumulate harmful mutations in their genomes, because, in the absence of sexual recombination, there is no mechanism for getting rid of them. This can potentially lead to a so-called ‘mutational meltdown’ and extinction. The rate at which these mutations accumulate is highly related to the size of the population.
However we now know that going through a bottleneck can purge mutations –
Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis
http://www.sciencedirect.com/science/article/pii/S0960982211001242
That is sexual reproduction though…
But would not the purge also apply to asexuals?
I don’t know!
I’m also not a biologist, but that is exactly what occurred to me as the benefit of sexual reproduction.
And yet there’s a whole group of 450 species of bdelloid rotifers that have never been observed to reproduce sexually and have been around for many millions of years. And there are other, similar groups. It’s puzzling.
IIRC there is some evidence supporting this notion; some fungis that can reproduce both asexually and sexually, and which reproduce asexually when their environment is not stressful (lots of food, etc.) but reproduce sexually when it becomes stressful. “Hey, there may be an environmental change in the offing…better switch from cloning to sex!”
That is interesting. It contrasts with asexual reproduction in some vertebrates that seems to occur under just the opposite conditions, when their environment becomes stressed. Like when opportunities for sexual reproduction become improbable.
Colony organisms would never (or rarely) have a problem with lack of potential mates, so you would not expect to ever really observe mate-unavailable behavior in the wild. Maybe in the lab you can induce it, I don’t know.
Here is some more on fungi who change reproductive strategies. Though in this article they aren’t talking about innocuous fungi that switch strategies under food stress like I was, they’re talking about fungal pathogens that switch strategies as a way to increase/decrease virulence and defeat antibiotics. That’s a bit scary.
Gah, no link! Second try.
There is a popular model called the Red Queen Hypothesis which postulates that asexual populations have a hard time maintaining genetic variability in the face of rapidly evolving parasites and pathogens, while sexual populations have souped up variation and so are better at evolving more rapidly so that some descendants are more resistant. The name is inspired after a line made from the Red Queen from Through the Looking Glass: ‘Now, here, you see, it takes all the running you can do, to keep in the same place.’
Per Matt Ridley’s book?
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Tt came from him, but he popularized it. The form that I described was described by Hartung and Bell.
‘It did not come from him…’
That’s exactly what I thought… It would be great if somebody could comment on that.
There are other examples of that. Many protists – including protozoa but also algae — can be asexual or sexual. As I understand it, they reproduce asexually during the early part of the year when populations and competition and diseases are lower. As the summer progresses and their populations get crowded for competition, they reproduce sexually.
Many aphids also reproduce asexually at low population densities, then they produce males later on and reproduce sexually with ’em.
I will be submitting mat article titled “Mary: A possible case of mammalian (human) parthogenesis.” to a suitable journal very soon.
*my* article.
But of course Jesus would have been named Janessa.
Well, s/he is always pictured with long hair so maybe Jesus really is female. Like the bearded lady in the circus.
Or maybe like Conchita Würst? Jesus when he’s the male persona; Christ when she’s female.
Then it’d be χe, not s/he. That’s why we have Χmas.
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Female pharaohs wore beards – maybe this is just something else that the religion copied from the ancient Egyptians, along with things like the virgin birth.
For a species that washes up in a new environment like an island, it makes sense to be able to have young without mating, but does it stop future mating or does it establish a population that is female only for ever?
Parthenogenisis also occurs in insects – again this makes initial ‘sense’ for colonising a new area I suppose.
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=9250871&fileId=S0007485314000017
Some can swith between sexual & asexual reproduction eg termites – http://www.pnas.org/content/111/48/17212.long
What is interesting to me is why some molluscs are hermaphrodtes (land snails) & more have male/female gonads but remain one sex (as I understand it?), then some change sex as ‘sequential hermaphrodites’ eg Crepidula fornicata.
So isex is complicated!
One more from Nature a couple of weeks back, then I promise to shut up…
Sexual selection protects against extinction
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14419.html
…free-living parthenogens have never been observed in any of these taxa in the wild, although two free-living snakes were recently discovered each gestating a single parthenogen — one copperhead (Agkistrodon contortrix) and one cottonmouth (Agkistrodon piscivorus).
and then
…if Mary had reproduced this way, Jesus would have been either a female or a haploid intersex with an XO chromosome constitution, producing a short female with gonadal dysgenesis (rendering the individual sterile) and a webbed neck…
I’d like to see this question on an exam:
(10pts, extra credit) How might the Virgin Mary have something in common with a poisonous snake, and does this hold up to closer scrutiny? Discuss.
Wouldn’t any environmental novelty put a non-sexually reprocing organism at risk? With only one set of cards to play and change in selective pressure can serious hamper your ability to flourish. Bacteria obviously due just fine but I’m guessing that this is due to their speedy reproduction times.
You don’t have one set of cards to play with; each clone has a different genotype.
Inconceivable!
This is an interesting report and certainly plausible, but I am a little skeptical (go figure). The paper is based on 16 microsatellite loci each with 4 – 40 alleles (mean = 20). It seems like there is enough variability in the total dataset to make it very unlikely that 2 randomly mating sawfish would contribute exactly (or nearly so) the same set of alleles to their offspring. The authors point to very low probabilities that this was the case. That is all well and good, but I wonder about how they defined their randomly mating population. In other words, was the total dataset the correct one from which to estimate these probabilities? A brief perusal of the paper describing the IR method (STORM Frasier 2008) suggests that how one identifies “associations or groupings within populations” may be important in estimating IR. Fish were sampled from 2004 – 2013. Were they all part of a randomly mating association for all 9 years? It is difficult to judge from the paper. Also, 5 of the 7 outlier individuals with the highest IR scores were captured in the Peace River drainage in 2011 and 2012. IMO, this considerably narrows the temporal and spatial scales at which random mating could have occurred. My guess is that the number of potential parents for the putative parthenogens was something (much) less than the 190 hypothesized in the paper. If this is the case, there may be a pretty good possibility that inbreeding and genetic drift may have driven these results after all. I am with Jerry – Let’s see those karyotypes.
A discussion of the origin of sex on the Sandwalk blog brought up an interesting point I’d never thought of. We diploid organisms find it difficult to figure out how sex could evolve because of the “two-fold cost of sex.” But we’re looking at it the wrong way around.
Sex evolved in haploid, single-celled, eukaryotic organisms. During generations of asexual reproduction, mutations would accumulate, some of them harmful. Sometimes, haploid cells would merge to form a diploid cell, then divide again with their genes somewhat scrambled. Therefore, some of the four haploid products of meiosis would be improved compared to the mutated clone mates of their haploid parents.
The benefits of DNA repair and (possible) improvement are experienced right in the haploid who undergo this fusion and cell division, so it is subject to natural selection directly. And there is no two-fold cost of sex; all the genes that go into the diploid / meiosis process come out again in functional haploids.
Once this process gets going, the diploids can become elaborated, even to the point that we think sex is about producing us, but it’s not. 🙂
Yes, but this doesn’t answer the question of why everybody doesn’t do what the sawfish does once sexual reproduction has evolved. Why not drop it not that we’re sexual? It seems to me that a gene that would make me produce little Jerrys from a sperm would (if I reproduced!) replicate twice as fast as if I had to mate with someone else. I don’t think you’ve explained why disploid organisms don’t back-mutate to parthenogenesis. If the problem were already solved in the way you posit, we wouldn’t have people looking to find out under what environmental conditions we see sexuality vs. parthenogenesis in snails, for instance.
There are two separate issues here. Looking at diploids, it’s hard to see how sexual reproduction could start because its genetic cost is high. Looking at it from the haploid point of view helps get past that hurdle.
The second, different issue is, why keep sexual reproduction now that eukaryotes have evolved it? It seems to me that the genetic variability it introduces, and especially the possibility of combining two different beneficial mutations that arose in different lineages, is the key. If a species reproduces only asexually, it may find itself in a situation that causes it to go extinct, more often than do its sexual relatives.
Of course, many animals and very many plants and fungi do reproduce asexually as well as sexually. Now you’re asking yet another question; why don’t mammals such as humans reproduce asexually? I don’t know and can’t even guess well. Perhaps it has something to do with contingent facts of mammalian reproductive physiology, or the ecology of relatively large and long-lived organisms. Perhaps something else. Now we’re getting far from what I thought the original question was (how can sex evolve given the “two-fold cost of sex”).
“…sexual reproduction is pretty much an evolutionary mystery—one of the biggest mysteries we have.” Let’s start the countdown: how long will it take for a creationist to smugly state that Dr. Coyne “admits” that evolution is “useless” since it can’t explain everything? I predict 48 hours.
Pretty much every self respecting diety in the middle east, and indeed elsewhere, as well as a bunch of known historical figures were apparently born of assorted virgins. Obviously there must have been some other form of genetics working in the past 🙂
From a quick Wiki search:
Mut-em-ua – the virgin queen of Egypt who gave birth to Pharaoh Amenkept
Neith was the virgin mother of the god Ra
Isis the virgin mother of Horos (who has a lot of jebus parallels – although a thousand years or so before)
Nana was the virgin mother of Attis (a Phrygian god)
Dionysos – was the son of Zeus and the virgin goddess Personphone, or alternately the miraculous son of Zeus from Semele (confusion on the maternal line).
Jason, the fatherless son of the virgin Persephone (parthenogenesis?)
Plato – said to be the son of the virgin Perictione and the god Apollo, with Apollo ‘fessing up to her betrothed.
Julius Caesar, Krishna, Odysseus (disputed – but that’s a bit like questioning the “historical accuracy” of the Noah movie!)
The list is goes on and on…..not exactly a novel idea in the Roman Empire or in the middle east.
The chromosomal arrangement of birds would allow for male offspring a as result of parthenogenesis, and turkey is widely eaten at Christmas…..
I noticed they’re all men. Is there any female who was born of a virgin? That would actually be possible! (in theory)
Good question, google didn’t come up with any, at least on a superficial search. Few female heroes in the bronze age and less need to tie them to miraculous birth?
Indeed. It shows that the bronze age was dominated by men. If only those people knew that a virgin birth was theoretically possible if the baby was a girl. Imagine what a boost that would give to the particular religion that preached such a thing!
Beat me to it. My list is from the Hitch:
“Now the birth of Jesus Christ was in this wise. When his mother, Mary, was espoused to Joseph, before they came together she was found with child of the Holy Ghost.” Yes, and the Greek demigod Perseus was born when the god Jupiter visited the virgin Danaë as a shower of gold and got her with child. The god Buddha was born through an opening in his mother’s flank. Catlicus the serpent-skirted caught a little ball of feathers from the sky and hid it in her bosom, and the Aztec god Huitzilopochtli was thus conceived. The virgin Nana took a pomegranate from the tree watered by the blood of the slain Agdestris, and laid it in her bosom, and gave birth to the god Attis. The virgin daughter of a Mongol king awoke one night and found herself bathed in a great light, which caused her to give birth to Genghis Khan. Krishna was born of the virgin Devaka. Horus was born of the virgin Isis. Mercury was born of the virgin Maia. Romulus was born of the virgin Rhea Sylvia. For some reason, many religions force themselves to think of the birth canal as a one-way street, and even the Koran treats the Virgin Mary with reverence. God is Not Great, pages 22-23
He forgot one. The virgin Shmi fulfilled prophesy by giving birth to Anakin through the power of The Force.
In my ancient edition of Genetics by Strickberger, there is a question at the end of one of the chapters that goes something like this:
‘There once was a woman who had given birth to a child, but she claimed to be a virgin. Assuming that her claim was correct, what should be the expected gender of the child?’
I actually used this question for a couple years when teaching genetics until I suddenly realized that it seems sort of … loaded. So I dropped it.
A miracle? So what! One must focus on the big picture : the whole universe came about in six days. Isn’t that a more noteworthy miracle?
Very interesting, Jerry, particularly the discussion of sexual reproduction. I remember from Common Core Bio that there is an open question about why humans enjoy sex, but didn’t realize that the whole question was a mystery.
“why humans enjoy sex”
Really–there’s a question about that?
Offspring do get all Mom’s genes from parthegenesis but that does not mean there is a 50% dilution with sexual reproduction.
Like mates with like so many genes are going to be quite similar.
Here’s a question for christians and muslims (who also believe in the virgin birth). Either Jesus wasn’t born of a virgin or he was a woman. What’s it going to be?
I wonder how theologians will deal with this.
Well, we know, don’t we? When the Holy Spirit came to Mary at the Annunciation, a second set of chromosomes complete with a Y was conjured up in her ovum she had ovulated earlier that day.
Come on now: They believe in magic.If God can speak the cosmos into existence, surely the Holy Spirit aspect of Himself can make some chromosomes.
You’re probably right. I guess I expected too much from theology. I’ve been trying for days now to find an example of reconciliation between evolution and catholic teaching. There is lots of stuff about the pope saying catholicism and evolution are compatible. Yet I haven’t seen a single example of such compatibility.
Well, I think the official Catholic doctrine is that evolution might explain most things, but there had to be a bit of magic to make us the ensouled magic beings (with magic free will) that we are who were capable of acquiring the Original Sin that we have such that we need Jesus for salvation, whatever that means. What’s a little more magic to make a diploid male Jesus?
This is from the Daily Mash, a UK spoof news website:
http://www.thedailymash.co.uk/news/celebrity/jesus-back-as-a-fish-2-2015060298848
That made me laugh out loud.
This doesn’t seem right to me. If you have just one offspring, then sure, you’ve wasted half your genes. But the vast majority of sexual species have more than one offspring, and the fraction of genes that get passed on to the next generation approaches unity very quickly as the number of offspring increases. (With two offspring, you pass on 75% of your genes; with five, it’s 97%; with ten, 99.9%.) So in fact very few genes are actually lost as a result of sexual reproduction.
Granted, the number of copies of each gene is halved by sex. But as others have pointed out, what the gene gets in return is the synergy of novel gene combinations. Some of these combinations may be less fit than the parent genotype; some will be more fit. Sex spreads a gene’s fitness out over a bell curve, allowing natural selection scope to prune away the less fit tail of the curve while amplifying the more fit tail. It’s another way of ratcheting a gene’s fitness upward without mutation.
That Raëlian lady?
“I don’t think the public realizes that the fact of sexual reproduction is pretty much an evolutionary mystery—one of the biggest mysteries we have.”
I am afraid that there is nothing like a mystery to spur the ridiculously over-optimistic amateur into trying their luck at finding a solution. So here’s mine.
When a theory is deemed valid but uncomfortable facts seem to contradict it, perhaps we should consider whether the original theory is not actually incorrect but merely a special case of a more general one. For want of a snappier title, let’s just call this general theory “Whatever Works”. Suppose then that what actually passes down the generations is not just the genes but the more abstract one called “Whatever Works” which is a plan utilising the genes but not actually being concerned with preserving any one of them. If this plan is expressed as if a computer instruction “shuffle the genes” and perhaps to make other subtle sub instructions such as how the genes should be shuffled etc etc it could be simply this plan “Whatever Works” which is preserved through the generations, not any actual gene.
It would even be possible that different plans could compete with one another under evolutionary forces. No end to this evolution thing!
I have to admit that the above seems too simplistic an explanation to possibly be true but I just couldn’t resist putting it out there.
Just as a post script I relate the following:-
Stephen Fry, in his TV series QI (standing for Quite Interesting) once produced a pack of cards and confidently stated to the panel that he would show them something that had never been seen before. All he did was to shuffle the cards and spread them out on the table in front of him. He then explained that the chances of that particular sequence of cards being repeated was one in a number so large that it exceeded the total number of all the atoms in the known universe, mathematically this is factorial 52 I believe. This illustration merely reflects upon the number of shufflings of genes there would be available to create a complex life form. No likely end to this sex thing I’m pleased to see!
It is interesting that sawfish use their toothy snout to detect prey and as a weapon. There are fossils of ancient sharks / shark relatives with long but curled up toothy snouts, jaws, or head ornaments.
The advantage of sexual reproduction to men is pretty obvious: it takes a village to hold the potential offspring a healthy guy with motile sperm might put into production over the course of a long holiday weekend. (Hell, a man produces enough gametes to repopulate the entire US in a single tablespoon of splooge — which, I think, calls for link to the Monty Python tune “Every Sperm is Sacred.”) Plus, it feels good — but you already knew that.
The advantage to women of sexual reproduction — or why they ever deign to mate with us at all — has long been a mystery to me. But I certainly appreciate it, and endeavor to make manifest my gratitude every time the opportunity presents itself.
For sexual reproduction my bets are one the “Red Queen hypothesis”. In a nutshell: the main benefit is accelerated evolution.
Some info:
http://en.wikipedia.org/wiki/Red_Queen_hypothesis
And some nice research where they compare crossing over in modern humans and an extinct human species, the Denisovans:
“The Red Queen Model of Recombination Hotspots Evolution in the Light of Archaic and Modern Human Genomes”
http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1004790
Where they conclude:
Our refined estimates of the age and life expectancy of human hotspots provide empirical evidence in support of the Red Queen hypothesis of recombination hotspots evolution.
Hotspots are the places where DNA splits up and this is determined by a protein PRDM9.
Sexual reproduction comes at a genetic cost overall, but doesn’t it also massively reduce the reproductive cost of males?
A few highly active males can spread their genes through an entire population without destroying its genetic diversity.
Doesn’t having 2 strands of DNA also allow for much better repair of damaged sections?
Law school was a breeze compared to this stuff.