What is the evolutionary advantage of having two genders?
Well, in sepcies that only have one parent, the offspring are gentically identical (barring mutations). This works fine if the species has a stable low-level niche in the food chain and can multiply fast enough to fend off predation and whatnot.
The evolutionary pressure is much great on larger species, though. If an offprsing has two parents and gets a genetic mixture from each, positive traits can be reinforced through natural selection while weaker traits die off. Two parents offer the minimum necessary to ensure a good genetic mix-em-up allowing gradual refinements. Some science-fiction stories talk about alien species with three or four or more (!) parents required to produce a child, which always struck me as too pointlessly complex to succeed.
Further, gender dimorphism allows for specialization. Females get the more complicated reproductive system (the male system typically consisting of a relatively simple factory for hapless haploids), while males get the larger muscle mass and whatnot. The genders end up complementing each other and the system has worked pretty well thus far.
Bryan summed it up, but I just wanted to add that gender can be seen even in bacteria - some have an F factor, make a ‘pili’ which could be considered homologous to a penis, and donate their plasmids to bacteria with no F factor. Of course no one wants to give only half of their DNA to their offspring, it would be best to be identical. But nature forces us to adapt, and one quick way to adapt is through the sharing of DNA.
I read an article recently that said the main reason for two genders was to reshuffle the genome to fight off pathogens - you can imagine that a virus would wipe out the human population pretty quickly if we were all identical.
another important aspect to sex is that it allows the DNA to repair, by swapping out the faulty segments with a good copy
But that’s a benefit of sexual reproduction, not different sexes. They’re separate issues; earthworms, for example, reproduce sexually (ie, offspring have DNA from two different earthworms) without having sexes.
[nitpicker mode]
May I first praise you highly for using the word sexes. I was taught “sex”, male or female, is a characteristic of biological reproduction; “gender”, masculine, feminine or neuter, is a grammatical characteristic of words (or an issue set within of sociopolitical discourse). But evidently the language is on the way to ditching the distinction.
[/nitpicker mode]
As mentioned: the great advantage is sexual reproduction, which can just as well be achieved through hermaphroditism (earthworms, or even more glaringly, plants).
The advantage for some life-forms of there being two distinct sexes is, as mentioned above, more along the lines of efficient-division-of-labor considerations (and most of the clades that evolve from a 2-sex common origin will tend to have the trait “grandfathered” into their genetic make-up). Two specialists each doing his thing tend to be able to get more things done, better, than two generalists trying to do everything at once. As to why two and not three or four, as Bryan Ekers said any more would be an unnecessary complication vs. the binary method.
It seems to me that the optimum sexuality level is somewhere between 1 and 2, 1 being cloning and 2 being full scale mammal-style sexuality. Plenty of species get by with just asexual reproduction. But there are many species that reproduce sexually some of the time but also reproduce asexually, plants being the foremost example, but there are also animals like coelentorates that do this. So some sexuality is good, but too much is bad. Once you get to a full-scale shuffling of level 2 sexuality you don’t get any benefit to adding more, and you add additional complications that could potentially cost a lot.
Now somebody should bring up fungal mating types, and explain them, but it’s not going to be me.
I don’t know about the evolutionary advantages, but I do know that sex is more fun if someone else is participating.
And here I thought the OP was about, “Why are there only two sexes? Why not three or more?”
I still don’t see how having two separate sexes evolved from hermaphroditism. Granted [i[once* you have two different sexes, then you can have sexual dimorphism; but that almost certainly occurred after sexual specialization. A lot of species have virtually no difference between sexes.
It’s a bit blinkered to say the reason we have two sexes is because females have a specialized reporductive system and males have big muscles… cart before the horse and all that, yes?
In the beginning, there were hermaphrodites. Everybody had both sets of naughty bits, and it was good. Then the first male was produced… or at least a mutant occurred with non-functional female reproductive organs. Since this mutant could impregnate, but not be impregnated, it got to spread its genes around, while not having to go through that messy business of changing poppy diapers and raising teenagers. So then the population consists of males and hermaphrodites… though there is no sexual dimorphism beyond the nonfunctional female reproductive organs on the male.
While maleness offers a benefit over hermaphroditism, a population of all males doesn’t work very well, so an equilibrium condition is met with a certain fraction of the population being male and a certain fraction being hermaphroditic, depending on the biological cost of producing offspring. Anyway, since males can have sex with anything, including other males (in the nonfunctional female reproductive organs, not like that you pervert), at small biological cost, the equilibrium fraction of hermaphrodites is actually pretty low. Since the probability is quite high that a hermaphrodite will have sex with a male, the functionality of the male reproductive organs on a hermaphrodite offer little benefit- since the male will not become impregnanted. Further, not producing the male haploid cells to spew them out into the world for no reason is a minor saving. Thus, any mutation that would tend to render the male reproductive organs of a hermaphrodite less expensive (read, less functional) is encouraged, or at the least not discouraged.
Sexual dimorphism is pretty much a product of increasing specialization of each sex for its given reproductive role.
As others have said, there is more than one question floating around and these need to be distinguished. There is the question of why there is sex. Then there is the question of why there are two roles to be played (as there are even in hermaphrodites). Then there is the question of why, given that there are two roles to be played, organisms specialize in one or the other.
Several explanations for the advantage of sex have been put forth. Some involve the fixation of advantageous mutations, while others involve the effects of the continuous rain of deleterious mutations. There are also clear theoretical disadvantages of sex under certain assumptions, e.g., sex tends to break up co-adapted gene combinations produced by natural selection. Beware vague verbal arguments.
Bryan, I’m not sure what you mean by “the evolutionary pressure is much great on larger species”. Are you talking about population size? Some explanations of sex, such as “Muller’s Ratchet”, are all about small population size. Larger genome size? That can certainly matter if it means a larger target for mutations affecting function and hence a higher rate of selectively important mutation. Larger size of individuals? I’m not sure why that would matter by itself, though it probably correlates with higher per generation mutation rate and lower population size.
In the beginning, sex presumably involved the fusion of bare cells (gametes if you like) of equal size, a condition known as isogamy and quite common among today’s sexual microbes. How this symmetry came to be broken is often discussed under the banner of the evolution of anisogamy, though the issue includes fuller differentiation of gametes into sperm and eggs (oogamy) and also unequal further parental investment (gestation, parental care). There are game-theoretic models that predict this outcome, but they’re not easy to sum up.
Given such asymmetries, why should there be separate sexes rather than hermaphrodites? It’s worth pointing out that hermaphroditism is not so uncommon. Among flowering plants it’s probably more common than separate sexes; it’s certainly not rare. Many animals are also hermaphroditic (various worms and certain coelenterates come to mind), though I don’t think that any vertebrates are (some fish are serial hermaphrodites, meaning that individuals change sex during their lifetime, but I presume we’re talking about simultaneous hermaphrodites). Explanations of the existence of one or the other usually involve ecological considerations: is it more efficient to specialize, or to diversify?
1010011010, I don’t know where you’re getting your arguments, but what you’re saying doesn’t make sense to me. Under the simplest assumptions, maleness will not spread in a hermaphroditic population with equal investment in the male and femal modes of production: it will have no advantage when rare, and will have an increasingly large disadvantage as it becomes common and the population’s sex investment ratio becomes male-biased.
A classic book that deals with these issues is The Evolution of Sex (1978) by the recently deceased John Maynard Smith.
Some new research about sex:
Why sex triumphs over all:
That site had the best quote, but Science Daily has a better overall article on the findings.
If there is an equal investment in male and female modes of production, males are making half the investment of their only competitors, hermaphrodites, since they do not pay the female biological cost.
That doesn’t make any sense. Perhaps males would make only half the investment over some particular time period, but they would have only half the offspring during this time period (they would father the same number of offspring, but would be the mother of none). Note that if they made only half the investment over their whole lifespans, that would be a disaster; half the investment means half the offspring. The point of shutting down female function could only be to put the saved resources into male function, e.g., stop making pistils and gestating seeds and start pumping out more pollen (twice as much on the simplest model).
I don’t know why you picked males. Your “logic” could just as well say that females should invade because they make only half the investment, or that low-investment hermaphrodites that shut down reproductive investment in both male and female functions by a factor of two should take over.
Josh you need to understand that there is a much larger physiological cost associated with producing large lipid rich eggs, and the production of large, lipid rich eggs is what defines a female. Males just produce sperm. There is a physiological cost involved on producing a spermatozoan but it is miniscule compared to the cost of producing an ovum, and that is why all organisms produce many millions of times more sperm than ova whether they are hermaphroditic or not. The cost of being female is the cost of feeding enough to be able to produce a huge mass of lipids that is capable of sustaining life until the developing organism is able to feed for itself. That is what our binary friend meant when he referred to the female biological cost. Because there is a significant female biological cost being exclusively male is a huge advantage that does not result in the male having half the offspring.
Amongst many slugs for example the animals are hermaphroditic and during mating they undergo the most amazing contortions and literally spar each other with their ‘penises’ in an attempt to impregnate the other individual without being impregnated themselves. That’s because a slug that acts as though it were male rather than hermaphroditic will not have to produce eggs. That individual will be able to father ,say 50, young from that mating with almost zero investment. Instead of producing eggs it can lay down body fat that will enable it to survive the coming harsh season while if it had been impregnated it would waste huge amounts of resources producing eggs and so run the risk of starving to death. That’s not a trivial price to pay for even doubling the number of offspring for one season.
There is no evolutionary advantage to being an exclusively female organism. It’s not cheating anyone but yourself because you are the one investing energy in all your offspring. Males in this scenario are essentially brood parasites, you can think of them as cuckoos in the sense that they place their genes somewhere where another organism is obliged to devote resources to ensuring they survive. Cuckoos lay there eggs in other birds nests because the cost of producing the egg is low compared to the cost of feeding the chick, and in a similar vein males inseminate eggs because the cost of producing sperm is low compared to the cost of producing eggs. Your suggestion that exclusively female organisms should be just as probable is equivalent to suggesting that somewhere there should exist a counter-cuckoo, a bird that builds a nest just to lure other birds into laying there.
Perhaps it might help you understand it to place yourself in the shoes of an hermaphroditic organism. Imagine that humans were hermaphroditic, that whenever a couple had sex both the man and the woman got pregnant. If you wanted to produce the maximum number of offspring in such an environment how would you choose to cheat? Would you choose to be exclusively female, so that every time you had sex only you became pregnant and then had to spend 9 months carrying the child? By cheating in this manner and becoming exclusively female you have only cheated yourself. You will not be able to carry any more children to term in your lifetime but at the same time you will be unable to father any children. You have effectively cut your reproductive rate in half.
Or would it make more sense to cheat by becoming exclusively male? That way every time you have sex you will not become pregnant while your partner will. You can go out and father a new child every night if you wish?
As you can see it makes no evolutionary sense to cheat by becoming exclusively female because females bear 90% or more of the reproductive cost already. Cheating by continuing to bear that cost while getting absolutely no return on the other 10% means that your cheat has lost you 90% of your return. Cheating by being male however results in a 90% increase in returns.
I am not sure I have read you right Blake. The biggest advantage in being exclusively female is that you can guarantee that you will pass your genes on. Males have to hope for pot luck, and many lose out or are cuckold.
Blake, you need to understand that even in a hermaphroditic population one expects a 1:1 ratio of investement in male and female functions (at least under the most straightforward assumptions). This is simple Fisherian (as in R. A. Fisher) sex ratio theory. You’ve alluded to why. If the population were putting 90% of its reproductive efforts into female function, there would be selective pressure to put more into male function. A biologist would expect to find equal investment in male and female functions in, say, outcrossing hermaphroditic plants. Yes, a sperm or pollen grain is cheaper than an egg, not to mention gestation and maternal care where those exist, but as you say, many more sperm and pollen grains than eggs are produced, and things like fighting over mates must be counted as male reproductive effort.
Fisherian sex ratio theory predicts a 1:1 allocation of resources, but allows that to be achieved by either a population consisting of separate males and females or a population consisting entirely of hermaphrodites making equal male and female investments (or by various other configurations, including populations containing both hermaphrodites and non-hermaphrodites). So the question is “why separate sexes rather than 1:1-investing hermaphrodites?” One question relevant to this larger question is “will a gene causing maleness rise in frequency in a population consisting of equal-investing hermaphrodites?” The answer, under simple assumptions that include linearity of the payoff for investment (no diminishing or increasing returns for a particular type of investment), is unequivocally “no”.
More generally, under such assumptions there is no reason to expect either hermaphroditism or separate sexes to “win”. If linearity is relaxed, things change. With diminishing returns, one expects hermaphroditism to win, whereas with increasing returns, separate sexes would win. You might have a look at chapter 13 of John Maynard Smith’s Evolutionary Genetics if that’s not clear.
Josh you still aren’t getting it even though you seem to have read all the basic material. You almost got with “Fisherian sex ratio theory predicts a 1:1 allocation of resources, but allows that to be achieved by either a population consisting of separate males and females or a population consisting entirely of hermaphrodites making equal male and female investments”. The answer is contained theirein.
So long as nobody is able to cheats it is possible to have “a population consisting entirely of hermaphrodites making equal male and female investments”. But as the examples I gave above demonstrate as soon as one organism cheats by becoming exclusively male we no longer have “a population consisting entirely of hermaphrodites making equal male and female investments”. What we then have is a population consisting eprimarily of hermaphrodites making equal male and female investments but with a small subset of male cheats who are at a significant evolutionary advanatge.
What happens as a result of that new state is that maleness genes become more widespread. Once maleness reaches a critical value only then does it become advantageous to cheat by becoming exclusively female because it is the female respources that have become limiting.
You’ve apparently rad Fisher so you know precisley why sex ratios sit around the 50% figure so I won’t explain it. In “a population consisting entirely of hermaphrodites making equal male and female investments” there exists an opportunity to cheat because the cheating of one individual will not result in a significant alteration of the sex ratios. Because cheating by becoming exclusively male is possible the population immediately ceases to be ““a population consisting entirely of hermaphrodites making equal male and female investments””.
Nonsense. Use a hypothetical population of hermaphroditic humans where every mating result sin both partners being impregnated. Assume that all individuals mate every day until impregnated. Introduce one a gene causing maleness. That gene will be replicated at every single mating because that individuals partners will all be impregnated, but the individuals carrying that gene will be able to continue to mate every day for 270 dyas (9 months) following mating. IOW a gene causing maleness will be able to reproduce 269 times for every one reproductive act of a hemraphroditic individual.
How can yopu possble say that such a gene will not rise in frequency in such a population when it results in 268 more offspring in any given reproductive period?
Males only get pot luck in species with gender differentiation because females, making the investment, get to pick and choose their partners and as a result one male can father many offspring but many ,males will father none. The male ratio is maintained because although many males never reproduce the ,ales tha do reproduce have many more oyung than any female. In hermaphroditic societies every single cheating male will reproduce just as much as every hermaphrodite because there are no females to chhoose. Every source of ova is working under an assumption that its investment in offspring will also be rewarded by the opportunity to fertilise its partner. In the case of hermaphroditic humans for example why would a male be at any less risk of fertilising a receptive partner than a hermaphrodite? Every mating either results in both partners being impergnated or neither, however by not being able to be impregnated a cheating male also gets to impregnat another partner the following night and so forth.
Blake seems to be explaining things better than I could, so I don’t really have anything crucial to add.
I don’t fully comprehend how Josh dePlume is trying to use Fisher’s sex ratio to apply to hermaphrodites.