Warning: this post has absolutely nothing to do with WoW, the rewritten version of the previous article will come in early afternoon!When I'm not working or collecting gold or kicking M&S from PuGs, I read every popular scientific articles I can find. The one that started me thinking is one I cannot link since it was an off-line open lecture on the local university about Dawkins's selfish gene theory.
I might write about that, that's popular science nowadays. However after the lecturer a creationist stood up and battled with the lecturer. He wasn't a dumb religious fundamentalist but an educated person who obviously read the literature of evolutionary biology. His point was: the two sexes that are abundant in the animal world (and the humans too) have evolutionary drawbacks and therefore couldn't be evolved. The lecturer reiterated the proper point of the "standard creationist bashing protocol": the meiosis has so many advantages (good genes can escape from bad chromosomes, two good mutations in two different animals can meet in a common child, children differ from each other and their parents so some of them can adapt more to changing environment...) that way overcome the overhead of the two sexes (males don't have babies while eat as much as females, so an all-female species like the amobea could have 2x more children and evolution is about maxing children).
However this creationist was a smart one: he answered that meiosis does not need sexes. 99% of the plants for example have meiotic sperms and eggs in the same organism. They are both males and females. They usually cannot fertilize themselves just each other, so they have all the advantages of meiosis, and sexual reproduction, yet do not have a single "useless" male. The lecturer went dead silent. This question was not in the "standard creationist bashing protocol". No one else could answer it and the creationist used the silence to advertise his un-scientific ideas about Adam and Eve.
Of course such challenge cannot remain unanswered. I've read all available popular and lot of scientific article (luckily my 5 years old university entrance card is still working, although definitely shouldn't) and found several disadvantages of having two sexes over hermaphroditism (like in plants):
- during the individual development from zygote to organism very complicated systems must exists to make sure that the individual differentiates into a fully functional male or female instead of some useless mixture.
- one of the sexes will be in shorter supply than the other (even by random effects), and the abundant sex cannot compensate it. If half of the plants die, they repopulate in a year. If half of mankind die, all females, we die out.
- the males are "useless" outside mating season, strike that, since they live in the same niche as the females, they are "parasites", wasting the resources the females could use for raising children
- "cheater" genes can play against the survival of the species. A gene that makes his carrier animal an attractive mating pair becomes abundant even if it decreases the success of the offspring against the environment.
However I found a nice idea, almost forgotten. It was the handicap principle of Zahavi from 1975. It is pretty nasty but makes sense: the purpose of males is to suck. The females expect them to do stupid things like having huge yellow feathers that are visible to predators or race in fast cars. Only those males are successful in mating who emerge victorious from these stupid matches. The point is that a male that survives the stupid game (handicap) must have lot of useful genes what the female wants for her children. The male bird who manage to avoid predators despite his bright yellow color must be a very fast or smart bird. The grey-colored female don't have to be that fast or smart, since she is harder to spot.
Of course the idea is highly debated since absolutely counter-intuitive. The reason why I wrote about it is simple: I think I found a mathematical proof that the idea is true and want to find a professional biologist who can verify or dismiss my calculations. If you are such biologist and interested, please write a mail.
33 comments:
the argument that becuase were not perfect, we can be evolved is such trash.
i can make a similar argument about god not existing "if god is infallible, he could make a rock so large even he could not lift it, but if he can not lift it, he is no longer infallible and there for dose not exist" its the same logic, and a gross misunderstanding of the subject.
evolution isn't about perfection or being the best solution, its about adaptation and and filling a niche in a way that allows the organism to survive usually by being better than its competition.
a common argument used by anti-evolutionist is that certain systems are to complex to have come from evolution. the eye is a prime candidate. its extremely complex. but through many small steps, creatures went from being blind to the modern eye, each developing it in ways that gave them advantages for their particular situation.
not every evolutionary change is beneficial, no dose it being beneficial mean that it will not be passed on. evolution has occurred over millions of years. it is a slow process, with more misses than hits. remember, there have thousands of times more species that have gone extinct than exist in our world today. there never will be perfection, just what works well enough, and on occasion what works better than enough. when a species becomes less than enough due to changes in its environment, it goes extinct.
onto that ignorant plant argument...plants simply have a different method of reproducing. again, its not about being the best.its a different method that has worked successfully and allowed the various species of plants to repopulate, just because in theory the reproduction method of animals is less efficient or has issues the plants do not, dose not mean that only by the grace of god could it have survived.
im surprised that the plant argument and perfection argument threw the other side of so bad. evolution exist, you cant argue against it, there is proof. any argument i have ever heard against it has been via a simple flawed argument method many ignorant people use. "i make a case against one part of your argument that could possibly be or defiantly is wrong, and there for your entire argument is wrong" and from there they go off on their wild tangents.
as for the evolution theory you speak of, its interesting to say the least.
sorry for the long post. i am a man of science. i hate it when ignorant people make baseless claims that say they have read all the literature but so obviously do not understand the subject, such as any anti-evolutionis/creationisim/intelligent design supporter.
I can see a benefit from having males. A few thousans years ago, having testosteron filled men would be beneficial to the females. Being more muscled, they have an easier time fending of predators or hunting. And so increasing the life expectancy of the females.
Hi there!
What the creationist is saying is that there is no cost to being a hermaphrodite, and that if there is no cost then why do we have separate sexes?
Well there is a cost. An opportunity cost! Being both male and female at the same time requires significant resource investment. You have to make two sets of reproductive organs for example.
These resources could otherwise be used for different things such as gaining greater body size, maintaining a better physical condition, immune defences etc.
In hermaphrodites, body size can determine whether the individual acts like a 'male' or a 'female'. The resource cost of sperm is relatively cheap compared to producing an egg, so a smaller individual may be better off choosing to behave like a male whereas the larger individual can behave more like a female and invest into making more expensive eggs.
Social competition is another factor, as always reproductive success is a driving factor. An individual's surroundings and competition for mates can also drive behaviour. A single hermaphrodite forced to mate with itself or die without any offspring, would theoretically maximise egg production and produce minimal sperm.
A hermaphrodite with a partner may invest equally in eggs and sperm, and a hermaphrodite with many partners may invest heavily in sperm production. Hermaphrodites try to maximise their reproductive success by increasing their sperm production to offset the fact that they do not know if they will be the 'father' of their mates offspring.
Okay so that's hermaphrodites. Somewhere down the evolutionary line, some species may have just been so different, that having two sets of reproductive organs was a waste of resources and separate species came about.
You wrote that if half of mankind dies all females we die out. Well it works both ways. If all the males die out the females die out too!
It is survival of the fittest that is important! The good of the individual not the species.
WoW analogy: This is exactly what we do in making gold. We amass gold for our benefit to the detriment of others 'the dumb people'. If we were for the 'survival of the species', we would just hand out gold to everyone that was poor to 'be fair'. Of course we don't.
In sexual species, there is a tradeoff between males and females. The benefits of reproductive success in being male or female are equal.
But you only need one male to populate the females! Yes if you were that one male you could be greedy and have all the females to yourself but competition will come into play.
Imagine a pride of lions, you usually have a dominant male fighting off lesser males and protecting a group of females.
The drawback is if you are the lesser male, without any mates you wouldn't reproduce and its the end of your line.
A female has the knowledge that any offspring she produces will be hers, and invests huge amounts of resources to feed, protect and nurture her young.
A male does not. So as a trade off, males have the ability to fertilise a much larger number of females, however because of competition, they might not have a chance to produce any offspring.
Hope this helps!
Cheers,
Rex
@mike: sorry but incorrect. The first meiotic species were hermaphrodites. The sexes were mutations. They must had been benefical or they were defeated by their hermaphrodite cousins.
@Carra: sorry but incorrect. The females could fend the attackers off, and in some species like lions or most spiders (where females are much larger) they do.
@Rexkicker: sorry but also incorrect. For evolution reproduction is everything. So the reproduction organs are the benefits and everything *else* is the cost. So a hermaphrodite can have 2 useful organs at the cost of one "useless" liver, while a split-sex being have just 1 useful/liver.
Some sex-changer beings (like certain fish) also behave *like* split-sex being. The males compete for the females by fighting or "cheating". These being could "choose" to be full time hermaphrodites but they prefer to be males and females. On the top of that, those who are anatomically hermaphrodites are still prefer to have only female or only male roles.
Interestingly enough I have some articles on the topic. I added an abstract below that may interest you.
__
It has been proposed that resource-poor conditions could promote the establishment of females if hermaphrodites are plastic in their sex allocation and allocate fewer resources to seed production under these conditions.
If this occurs, the seed fertility of females could exceed the doubling required for the evolution of unisexuality under low-, but not high-resource conditions (the sex-differential plasticity hypothesis).
This hypothesis was tested using replicate experimental arrays of the aquatic herb Sagittaria latifolia grown under two fertilizer treatments. The results supported the sex-differential plasticity hypothesis, with females having more than twice the seed fertility of hermaphrodites under low-, but not high-fertilizer conditions. Our findings are consistent with the idea that separate sexes are more likely to evolve under unfavorable conditions.
Reproduction is everything I agree, so being a hybrid you are nearly as good as a pure female or nearly as good as a pure male.
The cost of having two sets of reproductive organs is extremely expensive. The reproductive success of a hybrid male would be lower than that of a pure male due to resources set aside for female function. The reverse also applies that a hybrid female would have a lower success than that of a pure female.
A hermaphrodite population is stable if being a hybrid male and being a hybrid female together allows you to outcompete pure males and females. This is Charnov's model (1979).
Sequential hermaphrodites such as some fish aren't both male and female, they are either male or female.
Size advantage is observed in fish according to different life stages. A trigger occurs when the situation favours switching to become male to gain a higher reproductive success.
So they are born female and then switch to male if its beneficial to do so. A group of females are controlled by a large dominant male. So the large body size provides a greater advantage to being male (through control of a harem) than it would be to female function (increased egg production).
IANAB, I think divergent sexes come about as organisms become more social. This way a group of organisms behave like a larger organism, and each member performs a specialize task, or role. I'm guessing that sex just happens to be an easy way to do that.
Also, traits can become an evolutionary advantage for a time, before it can become a disadvantage and sometimes new evolutionary advantages make it possible to work around them. Such traits like, overactive immune defenses or wisdom teeth, which were once an advantage, but now make life inconvenient for people.
What I don't understand about evolution is that portions of it don't seem to apply to us. Atleast not on a genetic level. Things like poor vision, asthmsa, high blood pressure and countless other inherited deseases should never have survived this long. A caveman with poor vision would have walked off a cliff or into the mouth of a dino. Instead through social grouping these traits were protected by others and therefore passed on from generation to generation.
As our medicine and technology increases we as a civilization seem to be deevolutionizing (pretty sure that isn't a word) Further through improved methods of fertilization people who couldn't have children (and pass on genes) now can. It seems that human's have negated natural selection and only continues too. Our technology increases to compensate for our constantly decreasing health.
I understand evolution exists in the animal world but am confused to why humans are so antievolution. Or do the same rules that apply to animals not apply to us?
You appear to be arguing that the handicap principle is the reason for sexes to have evolved. This cannot be, however, since the handicap principle requires established sexes to begin with. If the males' purpose is to "suck", then surely the hermaphrodites would easily out-compete their 2-sex cousins.
The one sex in shorter supply doesn't make a lot of sense either. Males are prone to higher risk behaviors and will therefore always be in shorter supply. That has no fitness consequence on the population, however.
One problem with hermaphrodites, evolutionarily speaking, is that they will often self-breed rather than outbreed. The evolution of sexes forces them to outbreed every generation. There are lizards in the U.S. of the same species; in some geographic regions, they self-breed and in others they outbreed, and the trigger seems to be climatic stability. In other words, if the environment changes too frequently for hermaphrodites to keep up, 2-sex species will outcompete.
@Artorin: Many of the handicaps you describe would not necessarily have interfered with sexual reproduction (high blood pressure is often only a problem at an age where you are likely to have had children already). Asthma (and other allergies) are currently thought to be the body's response to a "bored" immune-system. Sort of a "what else do I have to do now that there's no mammoth poop around?" And many diseases have been found to be linked to other, seemingly-unrelated advantages. For instance, one of the genes associated with breast cancer is thought also to give a boost to intelligence. Many generations of closed-society breeding has lead Ashkenazi (European) Jews to have a number of genetic disorders that are also strongly associated with an increase in intelligence, so it's easy to see how these disadvantageous disorders might have survived.
This article
Is There Anything Good About Men? And Other Tricky Questions might be of interest to you as it discusses this topic. Have fun!
Here's another way to think about it. Any time a new mutation shows up in a single individual that provides a fitness advantage, the individuals in the population that are genetically predispositioned to outbreed more often have an long-term reproductive advantage because they are more likely to pick up the new mutation through outbreeding. If there are enough advantageous genetic variants floating around in the population, then it will be advantageous to outbreed every generation.
We are just beginning to get a handle on the frequency of new advantageous mutations, but in humans there are hundreds of genes that have been influenced by strong positive selection in the last 40,000 years or so. Even with just a handful of moderately advantageous variants floating around in the population, hermaphrodites would not have a chance.
See but thats my point Phae. I understand how they got passed on but it seems to conflict with natural selection and evolution. While some of the things I mentioned don't appear until later in life other things such as poor vision can develop very early in life yet a significantly high portion of the population suffers from this.
I know technology has a big involvement looking at how the life span of people has doubled in the last 100 years do to this. We aren't living longer because our bodies evolved in a way that makes our lives longer, we live longer because our techonology allows us to.
Poor vision was corrected with the use of glasses and even to the point now where laser surgery can all but eliminate poor vision. Yet the genes that cause it will still be there and only continue to spread. I can see a few hundred years down the road most people being born blind but due to surgery performed at birth flawed genetics will continue to survive.
@Artorin: Humans today are no more "naturally selected" than the plants or animals we have domesticated. We are becoming an artificially selected species, as we are creating our own environments with their own selective landscapes. If the technology we develop eliminates the fitness disadvantage for poor eyesight, you can be sure that poor eyesight will spread, but that is no cause for concern since the only reason it can spread is that it is causing no major harm.
So does that mean that evolution has an end? If humans are powerfull enough to essentially create our own environment does that mean that we are no longer bound by natural selection and evolution?
Is that the natural progression of worlds? Evolution occurs until a species can overcome and create its own environment. Natural selection becomes artifical selection?
@Captain The First: that's very interesting, could you give the reference of that article so I can seek it in the library?
@Artorin: Phaelia is right about asthma and such. Several bad traits shown after reproduction, or not shown at all due to recessive gene.
@Phaelia: at first warm welcome here, yours were the first blog I read. And sorry, but wrong: the separate sexes were not needed for handicap principle. Several existing hermaphrodites, like the Serraninae (fish) are taking male or female roles exclusively. The big fish are "males", fighting with each other for the harem of small "female" fish. But they are NOT males and females but fully functional hermaphrodites who "choose" to act like males and females. This could be the start point of separation of sexes. The roles already existed when the only-male or only-female mutant was born.
And females are ALWAYS in shorter supply. If there is one male and 100 female, who is in short supply? The females since the male still have unused sperms while all the females are pregnant.
@Joel: already read it, but thanks.
@Artorin: Just because we are powerful enough to create our own environment does not mean we are no longer bound by selection. Though I would still consider it artificial selection, it's very different than breeding for a cow that produces more milk. To extend the eyesight example, we have not intentionally created environments that allow poor eyesight to spread. We are still very much bounded by evolution, but many of the selection pressures we face now are man-made. However, if genetic engineering in humans ever becomes a reality, then we may truly start "evolving ourselves."
We're already seeing the beginnings of this with pre-natal screening for genetic disorders. Eventually, this will extend to engineering for positive mutations like those that give boosts to intelligence, though this type of service is likely to restricted initially to those who can afford it (at least within capitalist societies).
At some point, however, the first societies will make the leap of providing widespread genetic planning for members of their society that could not otherwise afford it on their own. Those societies are likely to quickly out-compete those that resist or restrict this type of family planning (for ethical reasons or otherwise), increasing the pressure on other 1st-world countries to follow suit.
I think the world depicted in the movie Gattaca is extremely close to becoming a reality.
@Gevlon: Your description of Serraninae does not describe the handicap principle but rather describes a starting stage in the evolution of sex. That is a very transient stage, and it's not clear that any sort of costly signaling could evolve when the sexes are so "fluid."
"Short supply" is never expressed in terms of sperm count vs. egg count. Men produce trillions of sperm over the course of their lives, but we don't use that number to determine population or who is in short supply. Population is always measured as % male vs. % female, and that ratio is generally close to 50/50 with slightly more men born than women but more women living to reproductive age than men.
In your example, if there is one male born for every 100 females, then a gene that makes it slightly more likely to produce a male would be amazingly advantageous and spread very rapidly through the population, because a male has 100x the fitness of a female. This rapid spread would very quickly bring the population close to 50/50.
@Phae: Very interesting but I would argue that our technology has made up for our shortcomings and even in genetic engineering it wouldn't be as much as human evolution as it would be techonological. In another bout of Sci Fi 2001 space oddessy series talks about how eventually people evolve sort of into technology essentially ultimately becoming beings of energy with limitless knowledge. Our bodies become more frail as our minds become stronger eventually exceeding mortal bounds.
Be it mechanical limbs, brail computers, artificial heart valves or even anti depression medications we are compensating for our mutations rather then eliminating them. Glasses allowed people with poor vision to live longer more productive lives when before they would have been undesireable. So we don't create environemnts that promote poor eye sight but we do compensate for it therefore allowing those genes to become more widespread.
@Phaelia: The handicap principle need sexual *roles* and not biological split-sex. So Serraninae can have the handicap principle. Of course its not proof of having it. It just says that it is possible. The exact calculations, (that were not published here as being more complicated than a DPS chart) are the proof. I gladly send you tomorrow if you are interested (I need some time to translate the text)
The 1 male 100 female means 1 alive and reproductive male and not male Zygote. Several species, like seals have worse ratio. 1 male haremlord can have 2-300 females (non-haremlord males do not count as not able to reproduce).
I don't argue with 50-50 (or slightly more) male zygote is evolutionary stable strategy
Maybe you should look to the Daphnia to gain an understanding of sexual/asexual reproduction. Hermaphrodites aside, this model species may provide insight into the benefits/motivations? of either form of reproduction.
@Artorin: It's true that right now we are compensating for poor eyesight with glasses, but why would we continue to do so when glasses are considered unattractive? If we can eliminate it genetically, we will probably do so since people would rather not need to wear glasses or for their children to have to wear them (children are typically not candidates for Lasik corrective surgery). The same is true with artificial heart valves. At some point it will be easier and less expensive to eliminate or reduce the risk factors of needing them than performing the costly and dangerous surgery to install them.
Changes like the addition or substitution of mechanical limbs will be much more difficult to "sell" to the population because of the perception that such things are "weird" and therefore less sexually attractive. A math chip in your brain, however, would probably do very well and is likely something we can't achieve genetically so the actual world we end up with will likely be a mixture of both genetic and technological enhancement, with technological changes limited primarily by what we as a society perceive as sexually attractive.
@Gevlon: We have had a lot of interesting conversation that is easy to get lost in, so I am hopefully going to come back to the central idea: that the handicap principle is behind the evolution of sex. There are plenty of examples of "runaway sexual selection" such as the classic Peacock's tail, but to my knowledge they are in species where sex has long been established, and for every solid example of the evolution of costly signaling, there are hundreds of thousands if not millions of examples of species with very similar males and females (limited sexual dimorphism).
Further, while costly signaling raises the fitness of the individual, the spread of the behavior lowers the fitness of the population relative to other species, so it does not seem likely that it would be a major factor in the evolution of sex, or even come into play until long after the sex roles have been established.
Sex clearly has several disadvantages. As you have pointed out, the biggest disadvantage is in the rate of reproduction. In a species where rapid reproduction matters much more than anything else, the disadvantages of sex may well outweigh the advantages (and is part of the reason that bacteria are so successful). However, sex allows disadvantageous mutations to quickly and efficiently be swept from the population while allowing advantageous mutations to quickly spread through the population. Sexual reproduction will likely evolve wherever it is important to be able rapildy adapting to new and/or changing environments. Though not immediately obvious, by the same logic sexual reproduction should be favored for populations with a small number of individuals since they have a smaller pool of advantageous mutations to pull from.
@phaelia: most text mentioning "sexual" reproduction actually talking about "meiotic" or "via gametes" reproduction. However this does not need "sexes". The sweeping of disadvantageous mutations and the quick spread of advantageous ones does *not* need sexes as meiotic (not self-fertilizing) hermaphrodites can enjoy it. The question is about the two sexes. It basically have three explanations in the literature:
* fixation of an error (ridicolous)
* runaway sexual selection (actually not, since hermaphrodites could also "run away" but they never did)
* handicap principle
If you know of others I would appreciate them.
However a very brief version of my calculation (I have normal extensive ones):
Imagine that there are 6 hermaphroditic giraffes, 10,10,11,11 and 12,12 feet tall. Due to selection, only 4 reaches maturity, the 11,11,12,12 feet tall ones. The average height of the children will be 11.5 feet, so in a generation they win 0.5 feet.
If we have 3 female (10,11,12) and 3 male giraffes (10,11,12), 4 reaches maturity, we get the same result, except the females have to carry 2x more offspring to match the hermaphrodites.
However if we place a handicap to the males, decreasing their survival rate, we get the two mature female giraffes (11 and 12) and ONE surviving male (12). Since both females have offspring from this one male, their children will be 11.5 and 12 feet tall, making the average height of the next generation 11.75 (instead of 11.5)
Of course the real case with gaussian survival probabilities is much more complicated, but the result is the same.
The handicap is enforced by the females. If a mutant male is not handicapped, his survival rate increases but the females reject him so he cannot reproduce. The females have selfish interest is choosing a handicapped male as it has proven (by survival against the handicap) that it has several benefical qualities.
maby i wote in a manner that made you think that i think sexs came first, but i did not mean to say that. the point i was attempting to make is that its 2 different methods, each a working method that leads to the same end, reproduction. both are successful, and there for survive.
@Gevlon: I think you are hypothesizing about hermaphrodites that are incapable of self-breeding, or at least that only do so very rarely. It's true that such a species would have the advantages of 2-sexes without the disadvantages. However, this system has a major evolutionary stability problem: if it's advantageous to produce more males when there are fewer males than females, then it's certainly advantageous to produce males when there are no males in the population at all!
Once you have a population of naïve females doing all the work, a cheater can show up that refuses to play nice and instead just focuses on spreading its seed. Once this first "male" is born, its genes will spread rapidly through the population until they reach 50/50 or so. After that, the gene frequencies might bounce around for awhile until the "male" gene gets accidentally copied onto its own chromosome, or until some developmental mechanism is established to "set" the sexual role of the hermaphrodite early in life, etc.
Meanwhile, the hermaphrodites that are playing by the rules are forced into the female role, because they cannot compete with the gamete production/dissemination males, so they are better off specializing as well.
@Phaelia: Several hermaphrodite species have "egg trading" mechanisms, exactly to prevent such cheating. The first (usually the weaker or otherwise less attractive) hermaphrodite release a *few* eggs (female role) and the other one release sperm (male role). Than the second hermaphrodite takes the female role. If it refuses to, the first walks away. So the *existing* hermaphrodites have defense mechanism against being forced to take the more costly female role. They often has long dating mechanism to make sure that the other is not a cheater (long dating is too costly to do it for only the first turn). So they are not so naive as you think.
On the top of that: if the males were just cheaters, than the fitness of whole species would fall below the species of (yet) non-cheating hermaphrodites and die out. While male-cheating mutations would spread quickly in a population, soon the population itself would die out.
@Gevlon: Egg trading as an anti-cheating mechanism can only be effective when most of the female effort is spent in the production of eggs and not gestation, so it would only work for small organisms. As far as the species with the males dying out is concerned, well I personally don't believe in group selection but since its a matter of faith you are of course free to have your own beliefs. :-)
@Phaelia: Group selection is a more or less outdated idea and I don't believe in it either. I just say that the individual animal is competing not only with other members of his species but with members of other specieses.
If males are just "parasites", who forced the hermaphrodites into costly female roles, than the average member of this species has half of the fitness of a member of a hermaphroditic species (of similar niche). So they would die out.
BTW hermaphrodites could defend themselves against the cheating of males by other mechanisms like "marriage" (several birds choose mate for life), the cheater can cheat once but never again.
They could also try to actively punish cheating by attacking the cheater (female locusts attack their mate if he makes the "LFG female" noise).
So I think the "fixation of error" or "male = cheater hermaphrodite" is not convincing.
@Gevlon: I believe the infidelity birth rates among birds that "mate for life" is something like 20%, which demonstrates just how difficult it is to maintain "hermaphroditic sex roles" in larger animals. I also imagine it is very easy to mutate into a psuedo-male; practically any mutation that would break or even slow down the female reproductive system should do the trick. So any population of hermaphrodites should have plenty of "males" to pick from, and the behavior should spread rapidly if it's favored within the group. I don't think a male/female group would have to compete with a hermaphroditic group of the same niche because the hermaphroditic group is inherently unstable.
Also, there is more to fitness than maximizing reproductive rate, so I do not think the evolution of 2-sexes would cause a 50% fitness reduction to the group. Certainly, reproductive rate matters for invasive species or colonization events, but most of the time the size of the group is relatively stable. Looking at brood size adjustment in birds, females in many species clearly have the capacity to ramp up their reproductive efforts in response to additional resources, so the fitness cost of males to the group could be quite minimal.
@hermaphrodites do exist, like 99% of the plants. So I wouldn't call them unstable. You could claim that hermaphroditism is *so* unstable that it cannot survive, but it's obviously not true as they do exist.
Even if hermaphroditism would be unstable, the existing hermaphrodites should rule their niche until they also split into sexes.
The other problem is the existence of females. Males van be cheater hermaphrodites, but why would any hermaphrodite choose to be female? By doing so she gives up the chance to be father of *some* offspring, even if most of her offspring come from female role. But a few is bigger than zero.
And finally: there are split-sex and hermaphroditic races. Why there is no intermediates who are just in the middle of transition?
They should have males and hermaphrodites who are doing some kind of desperate defense (egg-trading, long mating) against the invading males. You can find some 100K of hermaphroditic and millions of split-sex races but *no one* in the middle of this important evolutionary step? Why?
I found a paper very relevant to this discussion from Eric Charnov and John Maynard Smith, two of the biggest names in the business when it comes to evolutionary ecology modelling. I did not review their math, but they concluded that hermaphroditisim is only stable with low mobility (plants or low mobility animals), when the resources needed for female reproduction are different than male reproduction (seasonally pollenating plants), and when males and females have to expend roughly the same same amount of resources for reproduction (insect-pollinating plants). Otherwise, the male/female system is stable and hermaphrodites cannot invade.
http://www.nature.com/nature/journal/v263/n5573/pdf/263125a0.pdf
@phaelia: I'm at home and could access the whole article only from the university, now I just see the abstract. Could you mail the pdf to me?
The more intellectual creature is, the more time it needs to grow up and be independent. You can see a lot of examples looking at animals of all species. So it takes more and more time just to brood a child, and eventually comes to a situation where pregnant creature or creature with children can't leave without assistance of others. So we need some to brood, and some to gather food etc. And while hermaphroditic creature has no reason to feed and guard children of other hermaphroditic creature, male has very strong reasons to feed and guard females and his children, as it is his children too, and otherwise he can't have any. So by having 2 sexes we have a possibility to extend brood and growth time.
Actually we see hermaphroditic forms only on stages of evolution before livebearings.
Had read this before, but after thinking about it there is an argument which is similar to what Phaelia posted. Keep in mind:
1) even tiny animals living in the ocean feeding off of whale bones usually have sexes
2) behavior does not always have an effect on choice, since (again for ocean animals, etc.) the reproductive cells are released directly into the water sometimes
3) sometimes the large female spider eats the smaller male spiders of a species, etc.
So similar to what Phaelia said, it might be the result of specialization [slight penalty when not specialized adds up over evolutionary time], and also speed of propagation or loss of specific traits, due to the differences of behavior and effect of genes. (Such as the slightly higher variance of performance of males described in another article here)
Post a Comment