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Sex might seem like one of those little gifts from evolution. But it’s pretty inefficient from an evolutionary perspective. It’d be much easier to reproduce if you could do away with finding the right member of the opposite sex to help you create the next generation. So why did evolution come up with sex?
Biologists have hypothesized that one driving force might have been parasites. Now scientists have had a chance to test that theory. Asexual reproduction leads to clones. Being genetically identical, clones are also weak in the same ways, and thus more likely to all succumb to a parasite. But sex keeps shuffling the genetic deck.
Well, there’s a snail common in New Zealand lakes that does both—some populations have sex and some reproduce asexually. So researchers spent 10 years monitoring the two populations, and the number of parasites living off both groups. As expected, cloned snails that were plentiful at the beginning of the study suffered big losses as they became infected with parasites. But the sexual snail populations remained stable, results published in the journal American Naturalist. So, next time you’re feeling sexy, thank a parasite.
—Cynthia Graber
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13 Comments
Add CommentThis has been studied in the New Zealand Mud Snail for a long time. I'm in a lab that works on it.
Reply | Report Abuse | Link to this"Itd be much easier to reproduce if you could do away with finding the right member of the opposite sex to help you create the next generation. So why did evolution come up with sex?"
Reply | Report Abuse | Link to thisI didn't realize we were working with the presupposition that evolution always produced the most ideal solutions.
So if parasites begin to kill too many heterosexual couples, our bodies will magically require three-ways to impregnate? Does anyone else sense just how stupid of an idea this is? Entire body systems just change in an intelligent fashion to become dependent upon others rather than simply boosting the immune system? If this is so, why do the assexual snails even still exist (after all, the parasite threat wasn't too bad for them to survive)? Are there any other examples of assexual humans or apes? And if sharks (for example) can reproduce assexually, why then even bother with sexual contact?
Reply | Report Abuse | Link to thisMaybe, folks, we were created to be physically interdependent for a reason.
But the fact that some species or populations reproduce asexually, implies that there is some advantage to that as well. I'm not sure what that would be though.... Presumably there are times when the asexual populations do better (or at least as well) as the sexual ones. Otherwise they would have become extinct.
Reply | Report Abuse | Link to thisAnd this partly answers JohnJSal's point: these is no single "ideal" solution. There are a number of solutions, some of which are better in some environments. So eveloution should come up with a mixture of "good enough" solutions. After all, you need the diversity in populations to allow selection and hence evolution to occur.
And to jaqcp: "entire body systems just change in an intelligent fashion"
That is not how evolution works (but if it were, I suppose it would be evidence of an Intelligent Designer who stuck around to also be an Intelligent Redesigner :-) ).
"rather than simply boosting the immune system"
I'm sure the snails have developed immune responses to protect against parasites as well. As I say, there is no one perfect solution: a bit this and a bit of that all helps individuals and the population to survive.
"examples of assexual humans or apes"
Nope. It is rare (presumably because of the evolutionary benefits conferred by sexual reproduction) and seems mainly to occur in inverterbrates and simpler vertebrates. (And single celled organisms, obviously.)
"we were created to be physically interdependent for a reason"
And in that case why would some populations of snails be created to reproduce asexually and some sexually? Or does your reason (love?) only apply to humans?
JohnJSal & jaqcp what's with the attitude? Can't you ask a question without sarcasm and arrogance? If you had actually read the article rather than hunting for a reason to disagree with it you would have seen the answers to your questions. First, the author did not state that asexual reproduction was "ideal?" Please lookup "straw man fallacy". She stated it was "easier". Nature does not necessarily find optimal solutions but one has to ask, if design "a" is more efficient, then why did nature settle on design "b"? This goes to jaqcp's rant. Once again we hear a straw man singing, "If I only had a brain" Where does the author say that, "body systems just change in an intelligent fashion to become dependent upon others?" See the problem with straw man is that when you change what someone said to make them sound stupid you only make yourself sound stupid because you are the only one that actually said it.
Reply | Report Abuse | Link to thisClearly there are many pressures on organisms. A large number of organisms on this planet are asexual. There are also a large number that are sexual. The study, which is only very briefly summarized here, is trying to determine what pressures would have given an evolutionary advantage to sexual reproduction. As is pointed out, asexual reproduction benefits from the lower cost ("easier") of not having to find a mate and benefits the organism by passing on their entire genome. The disadvantage is that cloned organisms, or rather populations, are less robust as they present a single immune interface to the world. Once it has been cracked, all individuals in the population are vulnerable. Just like if all computers in the world were protected by the same password. This problem has not only been seen in snails, as in this article, but in cloned crops such as corn. The flip side to this is sexual production which is more expensive and you have to share the genetic legacy with your partner but the resulting population is more robust so disease has a harder time moving through the population. Jaqcp's comment that asexual organism's could have benefited from; "simply boosting the immune system" makes it clear why he disagrees with the study; because he/she doesn't understand genetics. Even if asexual organisms did boost their immune system, they would still all be clones. And again, once the defence is broken, regardless of how good it was on day one, the entire population would be vulnerable.
"So if parasites begin to kill too many heterosexual couples, our bodies will magically require three-ways to impregnate?" Apparently slime moulds have 13 sexes. They can't move that well so if they only had two sexes and one slime mould landed beside another their chance of being compatible would only be 50%. But with 13 sexes the chances are 92%. Note: numbers may be wrong but the implications are the same.
Reply | Report Abuse | Link to thisAnd finally this gem, "Maybe, folks, we were created to be physically interdependent for a reason." That was the point of the article. Trying to determine what those reasons might be.
People, lively discussion is fun and essential to good science. Sarcasm and ignorance, not so much… If you want your words to be respected, show some respect to others, otherwise what's the point?
P.S. and it helps to have some clue of what you are talking about
Robert: I don't feel like I've created a straw man argument. If anything, I think Ms. Graber could be accused of that as well. In the first paragraph, she presents the case as if sexual reproduction were a hindrance of some sort, and in fact even uses the word "inefficient," (which I feel she then seems to conflate with the idea of "ease" in the next sentence) yet as you say the next couple of paragraphs then shows why sexual reproduction *isn't* such a problem after all.
Reply | Report Abuse | Link to thisI suppose my original comment, whether it seemed overly mean or not, was written with the attitude of "why should we even start with the idea that sexual reproduction should be efficient, easy, ideal, etc. at all?
John: You simply don't get it. The point is it is easier for an organism to reproduce asexually. So one has to ask if that is true then why did humans evolve to reproduce only sexually. Meaning there has to be some other reason for humans to reproduce by a means that is LESS efficient than asexual reproduction. Meaning that sexually reproducing higher organisms were superior to asexually reproducing ones and thus won out in the evolutionary race.
Reply | Report Abuse | Link to thisThe article is very clear. Take a biology class.
Hasn't this been common knowlege for a while? The whole parasite-driven sexual-evolution?
Reply | Report Abuse | Link to thisIsn't the major benefit of sexual reproduction because it is difficult? Asexual reproduction would mean we could all go off on our own and procreate, so the only way of removing "inferior" or less favoured characteristics are through environmental factors. Those who can't run as fast get eaten by the tigers and stop reproducing.
Reply | Report Abuse | Link to thisWith sexual reproduction, we need at least a partner and the additional selection process means that we have to pass some physical or behavioural screens in order to reproduce at all. Those who are bigger, smaller, greener, quicker, or just plain nicer to others all can have an advantage and those who aren't picked can't just split down the middle and be done with it. So isn't sexual reproduction harder to achieve, but more selective?
I would think that the selection process would evolve some time after the sexual mode of reproduction. Sex on its own, however, gives enough benefit through extinction resistance to be valuable even without mate selection.
Reply | Report Abuse | Link to thisThis is not a new discovery in evolutionary biology at all. I believe it is closely related to the Red Queen hypothesis?
Reply | Report Abuse | Link to this60 seconds of "just-so"
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