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Much of what we know about sex, or think we know, stems from the animal kingdom. No surprise there—we're animals and the nuances of the genetic tango are easier to study in organisms larger than infinitesimal blobs.
Trouble is, animal sex is specialized to the point of distraction. Most researchers have learned to avoid seeking universal sexual truths by examining animals' twig on the tree of life, but some still rely heavily on single animal models whereas others hawk dated taxonomic ideas without realizing it, says protistologist Frederick Spiegel of the University of Arkansas in Fayetteville.
"Huge numbers of trees are killed over the origin and function of sex, but some people writing and teaching this material still have animal sex in the back of their minds. It's biased, and it's backwards," says Spiegel, author of a commentary on sex published online May 10 in Proceedings of the Royal Society B.
Enter the amoeba: a collection of blobby, typically asexual microorganisms that taxonomists have historically swept under the rug as evolutionary oddities. If biologists want to understand sex's universal features, especially its benefits and costs, Spiegel argues there's no better critter to start with.
"Sex is one of the most primitive characteristics of all eukaryotic life," he says. "There are only a few eukaryotic lineages where we've never seen sex, and they're all amoebae. With these asexual organisms, we can compare and ask some truly synthetic questions about sex.”
Amoebas are single-celled blobs that house their DNA in nuclei, just like all of their eukaryotic relatives (humans included). Although some amoebas presumably cannot have sex and divide by mitosis, others are among the eukaryotes that can have sex—a process that can most simply be defined as ripping a genome in half and later recombining it. The practice fuels diversity by juggling genes and ultimately helps lineages weather catastrophic change over generations as natural selection acts on them.
The historic generalization of amoebas leans to the less sexy side, which is almost certainly wrong, concludes a study published online March 23 in Proceedings of the Royal Society B (for which Spiegel was a reviewer and on which he based his commentary). Although some amoebas haven't been caught having sex, the authors' taxonomic work suggests amoebic ancestors did do it—just like the common ancestor that led to all modern eukaryotes.
It's not certain what pushed some amoebas into celibacy, but they may have evolved in stable environments that didn't require the energy costs of, or genetic advantages conferred by sex. After dropping the ability to sexually shuffle their genes, perhaps they simply got by reproduction via mitosis. Whatever the case may be, it's fertile ground for more research.
"Sex is an expensive process associated with big changes in an environment," Spiegel says. "I like to tell my students, 'When the going gets tough, the tough get horny.'"
In addition to dealing with asexuality, early taxonomists had to sort amoebas that resembled plants, fungi and animals. When Robert Whittaker debuted his popular five-kingdom classification system 42 years ago, he plucked amoebas out of various kingdoms and deemed them all protists. Genomic research and the field of systematic biology have since refined this sorting, but Spiegel argues outdated ideas continue to distort evolutionary relationships among organisms.
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11 Comments
Add CommentHere's something I don't understand:
Reply | Report Abuse | Link to thisThese are single-celled organisms undergoing "a reduction of two cells into one". I can understand plasmid exchange between bacteria, where both parties are still around to benefit from their new genes, but how can evolution drive a process which, in the short term at least, leaves you with half the population you started with (and each individual amoeba losing half its original genes)? It seems a bit presumptuous of these amoebae to assume that their survival rate will be more than doubled by this random genetic shuffle. And yet, they must be doing something right, because they're still here...
<<<I guess they would claim that complimentary male and female critters developed gender one small step at a time. >>>
Reply | Report Abuse | Link to thisNo thoughtful biologist claims this. To begin with, it's clear that sexual recombination ("sex") preceded the development of maleness and femaleness ("the sexes")." To see this just look at in single-celled organisms such as yeast and many protists, whose genomes undergo sexual recombination as part of an essentially symmetrical reproductive process where it doesn't make sense to talk about "male" and "female." You're demonstrating the same implicit animal-centered bias that Spiegel is identifying as a problem, by assuming that you can think clearly about the origin of sex by considering only highly derived modern forms of it. This suggests you should consider rereading the article to make sure you get its point. ;)
[By the way, "gender" is a linguistic term and not a biological one. The gender of an animal (or at least the word for that animal) depends on the language of the speaker ("rabbit" is neuter in English but masculine in French, for example), while its sex is a matter of biology. Similarly, calling meiotic recombination an example of "gender" is just wrong. Biologists won't be offended if you just call sex sex -- it's not a dirty word to us! :) ]
Great point on the two-cells-into-one point, kinnikinick. I contacted the editor, and a clarification should show up in the text any minute. (In short, that line was a reference to animal-like gametic sex, which is very tightly coupled to reproductive processes.)
Reply | Report Abuse | Link to thisTo answer your other question: What amoebas are doing right touches outstanding questions about sex. What are the universal costs and benefits? Can we even ask if there's anything universal about it? Etc.
My only contribution to this discussion is that gender is NOT a synonym for sex.
Reply | Report Abuse | Link to thisPer Wikipedia --
Gender is a set of characteristics distinguishing between male and female, particularly in the cases of men and women. Depending on the context, the discriminating characteristics vary from sex to social role to gender identity. Sexologist John Money introduced the terminological distinction between biological sex and gender as a role in 1955. Before his work, it was uncommon to use the word "gender" to refer to anything but grammatical categories.[1][2] However, Money's meaning of the word did not become widespread until the 1970s, when feminist theory embraced the distinction between biological sex and the social construct of gender. Today, the distinction is strictly followed in some contexts, like feminist literature,[3] and in documents written by organizations such as the World Health Organization (WHO),[4] but in most contexts, even in some areas of social sciences, the meaning of gender has expanded to include "sex" or even to replace the latter word.[1][2] Although this gradual change in the meaning of gender can be traced to the 1980s, a small acceleration of the process in the scientific literature was observed when the Food and Drug Administration started to use "gender" instead of "sex" in 1993.[5] "Gender" is now commonly used even to refer to the physiology of nonhuman animals, without any implication of social gender roles.[2]
In the English literature, the trichotomy between biological sex, psychological gender, and social sex role first appeared in a feminist paper on transsexualism in 1978.[2][6]
Some cultures have specific gender-related social roles that can be considered distinct from male and female, such as the hijra of India and Pakistan.
While the social sciences sometimes approach gender as a social construct, and gender studies particularly do, research in the natural sciences investigates whether biological differences in males and females influence the development of gender in humans; both inform debate about how far biological differences influence gender identity formation.
Hey! you can call it what you want to. However, it is clear to me that sex is a more complex way of reproducing than simple cell division. Did I say simple? Sorry, there is nothing simple about that either. The entire issue of the development and maintenance of life is far too complex and specific to have arisen by chance. Only an intelligent Superior Being could conceive of such wonderful constructs. Certainly, elemental chemicals have no desire to build bridges or moon rockets!
Reply | Report Abuse | Link to thisQuizzical, biologists will also agree that modern life is not the result of chance. If you think evolution is the same thing as chance, then you don't understand the theory. (This is an unfortunately common misunderstanding, so you aren't alone here!) There are many excellent recent books out there explaining evolution for the nonspecialist; you might enjoy learning more about the subject. At any rate I'd encourage you not to confine your reading on evolution to the "intelligent design" polemics, which on the whole do a poor job of correctly presenting what evolutionary theory actually is -- the ID authors seem to be mostly interested in knocking down straw men, rather than sincerely engaging in the scientific process.
Reply | Report Abuse | Link to thisWalker,
Reply | Report Abuse | Link to thisI see from the book "The Blind Watchmaker" by Richard Dawkins, that he claims natural selection is not based on chance. I agree. And, he also says that mutations do happen by chance. I also agree with that. But,since mutations are the source of the variety of characteristics that become available to select from, it sounds to me like evolution is based on chance after all.
Maybe you better read that book with open eyes. Dawkins does include a lot of truth in there, but his logic is seriously flawed. That is how the father of lies operates - a lot of truth and just a few of carefully concocted falsehoods that make the whole argument false.
By the way, the father of lies is NOT Dawkins.
Well, I probably can't convince you that I'm not speaking on behalf of the Devil. This is a frustrating part of any effort to bring scientific thinking to certain segments of the religious community -- it's hard for someone to engage sincerely on a scientific question if he or she is distracted by trying to exorcise the demons out of you! ;)
Reply | Report Abuse | Link to thisI probably couldn't have recommended anyone much better than Dawkins as an explicator of evolution, so I'm glad you gave him a shot. (And I imagine he'd appreciate your insistence that he's NOT in fact the father of lies!) Since you obviously appreciate the beauty and complexity of the living world, let me encourage you to continue to read up on mainstream biology. Perhaps I'm biased in favor of biology (I'm a biomedical researcher with a Ph.D. in genetics and development), but I tend to think that the study of life is simply the most interesting thing one can possibly do with one's time on this earth, so I think you can confidently expect to find it rewarding. :) Be open-minded, but skeptical. Demand to see the data. The more you learn about the living world, the more you may be surprised at how your ideas about evolution change.
I am being both open minded and skeptical. That is how I noticed Dawkins argument folds back on it's self.
Reply | Report Abuse | Link to thisQuizzical,
Reply | Report Abuse | Link to thisre: "The entire issue of the development and maintenance of life is far too complex and specific to have arisen by chance. Only an intelligent Superior Being could conceive of such wonderful constructs."
An intelligent superior being such as you describe must surely be even more complex than complexities involving "the development and maintenance of life".
How is that possible?
Rather than explaining the complexities of life, you make them infinitely more complex. And still unexplained.
Actually, I am unable to make the complexities of LIFE any more complex than they already are. The Bible explains the source of all LIFE. But no one claims you have to believe that. I just think that true science and mathematics (not science fiction) proves that abiogenesis is an absurd notion.
Reply | Report Abuse | Link to thisEven Dr. Dawkins, in his book "The Blind Watchmaker" points out that Hemoglobin consists of 4 chains of amino acids twisted together. There are over 140 amino acids in each chain. Since there are 20 different amino acids, That means there are, 20 to the 140th plus power, possible constructions of that one chain.
So, how could such exquisite construction happen one small step at a time? Any partial construction or incompleteness makes the whole thing completely inoperative. That means DEAD.
Remember, "Survival of the Fittest."
Dead tissue is really not very fit! Is it?