Why sex evolved

The activity is more likely to pop up in heterogeneous environments

By | October 13, 2010

Despite the obvious benefits of sex, it's an activity that's biologically disadvantageous under most conditions. Now, a new study published online today (13 October) in Nature helps explain why sex may have evolved, despite its downside.
Single asexual female
monogonont rotifer.

Image: Kuiper and Becks
Specifically, the paper tracked a eukaryote for nearly 100 generations and found that the species was more likely to switch from asexual to sexual reproduction if it encountered varying physical settings, suggesting sex may help species adapt to diverse environments. "The paper is an outstanding breakthrough," evolutionary biologist linkurl:Sally Otto;http://www.zoology.ubc.ca/%7Eotto/ of the University of British Columbia told The Scientist. "It's the first study to track -- in real time -- the evolution of sex in a multicellular eukaryote, finding that higher rates of sex evolve in a spatially complex environment," said Otto, who was not involved in the research. The evolution of sex has long puzzled biologists, as its disadvantages seem to outweigh its benefits in most situations. When an organism reproduces asexually, for example, it passes on 100 percent of its genetic information to the next generation. A sexually reproducing organism, on the other hand, only passes on half its genes -- a huge evolutionary cost. But sex does have its advantages, such as much more genetically variable offspring, and has evolved in many species. But under which conditions? To answer this question, many researchers have turned to theoretical models of populations, manipulating various environmental parameters to see when sex might be favored, and have generated many promising hypotheses. For example, these theoretical models have predicted that sex is more likely to evolve in heterogeneous environments because some individuals will find themselves in different surroundings that they must quickly adapt to in order to survive and reproduce. Sex allows them to do this by creating new gene combinations that may have an advantage over existing, potentially maladaptive genotypes. In spatially homogeneous environments, on the other hand, there is less need for adaptation to new surroundings, and sex is less likely to evolve. Experimental evidence for this and other theories, however, has been hard to come by. To address this deficiency, evolutionary ecologists linkurl:Lutz Becks;http://www.eeb.utoronto.ca/people/research+and+postdocs/becks and linkurl:Aneil Agrawal;http://labs.eeb.utoronto.ca/agrawal/ of the University of Toronto studied populations of monogonont rotifers, which can reproduce both sexually and asexually. Specifically, they reproduce asexually until populations reach a certain density, at which point females begin signaling to one another to produce males -- the first step in sexual reproduction. By monitoring these rotifers cultured under different conditions, Becks and Agrawal looked for differences in rates of sexual reproduction among the different populations. The team cultured the rotifers in pairs of subpopulations in one of three conditions: both with high-quality food (homogeneous), both with low-quality food availability (homogeneous), or one with high-quality and one with low-quality food (heterogeneous). They then simulated migration by transferring a proportion of each subpopulation to the other environment, which only differed in the heterogeneous treatment. After 6 and 12 weeks, or approximately 45 and 90 generations, the researchers exposed all rotifers to the high-density, sex-initiating signal, and measured the percentage of individuals that switched to sexual reproduction. As predicted, rotifers in the heterogeneous environment were more likely to make the switch than those in either homogeneous treatment. "The experimental data seem pretty clear to me," said evolutionary geneticist linkurl:Bill Birky;http://eebweb.arizona.edu/faculty/birky/BirkyLab.html of the University of Arizona, who was not involved in the research. "They do seem to have demonstrated a short-term advantage for sex in heterogeneous environments." "This study provides the first empirical demonstration that sex may be critically important in bringing together genes from near and far into more fit combinations," Otto said in an email.
Sexual females carrying resting eggs
(the darker eggs) and asexual females
(with the lighter shaded amictic eggs).

Image: Kuiper and Becks
But "there are two parts to sex," said evolutionary ecologist linkurl:Terry Snell;http://www.biology.gatech.edu/people/terry-snell/?id=terry-snell of the Georgia Institute of Technology, who did not participate in the study -- producing the signal that triggers some individuals to initiate sexual reproduction, and responding to that signal. Because the researchers provided the signal artificially, it was unclear if the rotifers in the heterogeneous environment would be more likely to secrete that signal themselves, and thus convert to sexual reproduction on their own. "By experimental design, they were simply testing the ability of the females to respond to the signal." To test the other component of sexual reproduction, Becks and Agrawal also tracked how many eggs were produced sexually versus asexually, without providing an artificial signal to initiate sexual reproduction. Again, they found that rotifers in heterogeneous environments produced more eggs sexually than those in homogeneous environments, but the difference was not nearly as great as the differences in propensity for sex. Still, the trend is there, suggesting that "real differences [are] occurring among these populations," Becks said. And as always, there is more work to be done. "I think it will move us further down the road in understanding the evolution of sex," Snell said, "but it's not the last word." Indeed, Becks plans to use the rotifer system to study other predictions regarding the evolution of sex, such as whether the existence of parasites gives sexual organisms a significant advantage, allowing them to evolve mechanisms of resistance. "[These rotifers] allow us to test the evolution of sex within populations," he said. "And we have the means to measure and manipulate it." L. Becks and A.F. Agrawal, "Higher rates of sex evolve in spatially heterogeneous environments," Nature, DOI: 10.1038/nature09449, 2010.
**__Related stories:__***linkurl:Shuffling genes without sex;http://www.the-scientist.com/blog/display/57153/
[22nd February 2010]*linkurl:Butterfly sperm explained;http://www.the-scientist.com/blog/display/55867/
[29th July 2009]*linkurl:Sexual conflict opposes sexual selection;http://www.the-scientist.com/news/display/25282/
[24th October 2006]


Avatar of: Richard Patrock

Richard Patrock

Posts: 52

October 13, 2010

I would like to know what the densities and remaining nutrient levels were within the different treatments. According to the article, this signal is density dependent and therefore population numbers is a covariate. The analysis by Terry Snell is point-on, in that those individuals in the heterogenous environments were more likely to respond to the sex-signal than those in the other environments; I would assume the authors could argue that the populations in the mixed environment had a higher ambient concentration (due to their response to the environment) and the added stimulus was more than enough to trigger a sexual explosion, whereas they did not see this in the other treatments. Given the nutrient component of the design, I wonder how the population trajectories in the treatments went, I would think that food was always limited in the low-nutrient treatment and the population growth was dull; food was not limiting at any point in the high nutrient environment and the pheromone was not produced in any substantial amount. In the heterogenous environment on the other hand, food was probably limiting at the time of the pheromone spike since the food was high initially and then likely consumed, leaving a high density, low nutrient environment and a high concentration of the sex inducing pheromone.


Posts: 37

October 14, 2010

Please note that the advantages of sexuality in changing but not in a stable environment has been previously demonstrated:\nThe advantage of sex in evolving yeast populations\nC. Zeyl & G. Bell, (1997) Nature, 388, 465-7 \n
Avatar of: Dov Henis

Dov Henis

Posts: 97

October 14, 2010

Whence And Whither\n\n2010 Update Concepts Of Life/Evolution\n\nOn Life's Twist \n\n\nA. Di Mauro's RNA formation concept is great.\nhttp://www.f1000scientist.com/2010/7/1/23/1/\nhttp://blog.the-scientist.com/2010/09/16/what-is-life/comment-page-1/#comment-4755\n\nBut, a step further is required. How had the RNA become alive, i.e. how did it (1) uptake the sun's radiation and how did it (2) catalytically use it to perform work, to keep augmenting its constrained energy by keeping augmenting its self-propagation, WHICH IS THE ESSENCE OF LIFE ? \n\nTHIS IS THE STILL MISSING TWIST IN THE BRANCHING OF LIFE FROM OTHER SURVIVING, ENERGY DIGESTING, MATTER SUCH AS BLACK HOLES. THIS TWIST IS THE BIOLOGICAL ENTITY "CULTURE", LEARNING-ADAPTING, THE ESSENCE OF DARWINIAN EVOLUTION SINCE LIFE'S DAY ONE. THIS MUST HAVE BEEN THE CRUCIAL PROCESS TWIST, EVEN WITHOUT YET KNOWING ITS MECHANISM.\n\nOr, is the mechanism of this twist known now?\n\nIt is now known how the RNAs, Earth's primal organisms, adopt an enhanced energy event's DNA conformation. But what is the mechanism of its recognizing the enhanced energy event? Does it derive from augmented RNA propagation and alternative splicing feedback loop, or is the augmented propagation a result of an energetic feature recognized by the RNA?\n \n\nB. Ponder with plain commonsense, and stick to obvious data. \n\nSleep and chirality are data, evidence that RNAs ARE ORGANISMS, alive, that came into being and were active prior to evolution of biometabolism. RNAs are not just "genes in the organisms". They are Earth's base primal organisms. Plain and simple. Why complicate the obvious simple evident data? \n\nAdd to this the embarrassingly obvious evident data-based concepts of the dual cycle universe, with cosmic expansion as the driver of universal evolution, the continuous melee of mass formats for energy. \n\nOur essential science interests should be the BASIC ESSENTIAL existential matters. Seek them carefully within the infinite number of mechanisms. Focus on the mechanisms that are rational evidence for the basic matters. \n\nThis is the opinion of an old-timer, a self-considered scientist, tired of the preposterous 20th-21st centuries technology culture and of the century-old corrupt science tradeunion-guild-establishment.\n\n\nDov Henis \n(Comments From The 22nd Century) \nhttp://www.the-scientist.com/community/user/profile/1655.page\n\nSeed of Human-Chimp Genomes Diversity\nhttp://pulse.yahoo.com/_2SF3CJJM5OU6T27OC4MFQSDYEU/blog/articles/53079\n03.2010 Updated Life Manifest \nhttp://www.the-scientist.com/community/posts/list/54.page#5065 \nEvolution, Natural Selection, Derive From Cosmic Expansion\nhttp://darwiniana.com/2010/09/05/the-question-reductionists-fear/
Avatar of: Chris Cox

Chris Cox

Posts: 2

October 14, 2010

Mr. Collins is correct. As a lay person with great interest in all things science, I read about the asexual/sexual comparison for DNA propagation advantage over thirty years ago, perhaps in one of Gould's or Dawkins' books, or Scientific American. They realized then that the more mutations that occurred, within reason, the better the chances for a mutation or two that would improve survivability. And most assuredly required for a sometimes violently variable environment in the presence of an unpredicatably variable star. A basic odds game.\n\n\n \n\n
Avatar of: Jeff Anderson

Jeff Anderson

Posts: 1

October 14, 2010

Can anyone explain how sex evolution happened? I must assume that every evolutionary step was necessary and then it stopped, so how come when evolution "decided" that it was needed to create a female, it evolved not only with the basic capacity for sex but also embellished it and made it perfect in every way? Why did curves in the body evolve? It's only icing on the cake, not really needed for it. Did male and female develop together? Wasn't it enough for evolution that man needed it and the woman was able of it? Why did evolution have to give the urge to both? What I'm asking is, if there is any "intelligence" guiding this, or if nature by happenchance gave us more then we really needed to survive?


Posts: 37

October 18, 2010

Chris Cox said I was correct in pointing out that there already was evidence that sex really is advantageous and went on to quote old hypotheses and calculations that showed why it should be [ e.g. Muller's ratchet(which actually only proposed sex being advantage by preventing degeneration in small populations)]. The hypotheses although plausible don't replace factual evidence. It isn't trivial to find a system in which on can do a direct comparison of how beneficial (or not) sexuality is to a population. I quoted another set of experiments with yeast that came to the same conclusion that the authors of this Rotifera paper came to.
Avatar of: anonymous poster

anonymous poster

Posts: 2

October 18, 2010

While it was great to know about what has been done in evolutionary biology, it was not great to realize that the findings were not so great. I'm surprized that this articles was published in nature. However I'm not evolutionary biologist and I'm not confortable with judging the article without reading the original. I just guess that I might have missed great point of the experiment due to poor knowledge in this field or Jef Akst did bad job by not bringing it for non evolutionary biologist who didn't read the original article.
Avatar of: Rivkah Rubinstein

Rivkah Rubinstein

Posts: 11

November 18, 2010

"The evolution of sex has long puzzled biologists.."\n\nThis only verifies how most of my esteemed colleagues actually start with their conclusions in hand prior to selectively ignoring and manipulating the data. They would do far less damage to the puruit of scientific advancement, if they would only focus on their true calling - journalism.

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