During World War II, some British comedian complained that American servicemen were: "over paid, over fed, over sexed, and over here!" So, it comes as no surprise that the following article should come from an American journal:
Parasites May Have Had Role In Evolution Of Sex
ScienceDaily (July 6, 2009) — What's so great about sex? From an evolutionary perspective, the answer is not as obvious as one might think. An article published in the July issue of the American Naturalist suggests that sex may have evolved in part as a defense against parasites.
Despite its central role in biology, sex is a bit of an evolutionary mystery. Reproducing without sex—like microbes, some plants and even a few reptiles—would seem like a better way to go. Every individual in an asexual species has the ability to reproduce on its own. But in sexual species, two individuals have to combine in order to reproduce one offspring. That gives each generation of asexuals twice the reproductive capacity of sexuals. Why then is sex the dominant strategy when the do-it-yourself approach is so much more efficient?
One hypothesis is that parasites keep asexual organisms from getting too plentiful. When an asexual creature reproduces, it makes clones—exact genetic copies of itself. Since each clone has the same genes, each has the same genetic vulnerabilities to parasites. If a parasite emerges that can exploit those vulnerabilities, it can wipe out the whole population. On the other hand, sexual offspring are genetically unique, often with different parasite vulnerabilities. So a parasite that can destroy some can't necessarily destroy all. That, in theory, should help sexual populations maintain stability, while asexual populations face extinction at the hands of parasites.
The scenario works on mathematical models, but there have been few attempts to see if it holds in nature.
Enter Potamopyrgus antipodarum, a snail common in fresh water lakes in New Zealand. What makes these snails interesting is that there are sexual and asexual versions. They provide scientists with an opportunity to compare the two versions side-by-side in nature.
Jukka Jokela of the Swiss Federal Institute of Aquatic Science and Technology, Mark Dybdahl of the University of Washington and Curtis Lively of Indian University, Bloomington began observing several populations of these snails for ten years starting in 1994. They monitored the number of sexuals, the number asexuals, and the rates of parasite infection for both.
The team found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time. As parasite infections increased, the once plentiful clones dwindled dramatically in number. Some clonal types disappeared entirely. Meanwhile, sexual snail populations remained much more stable over time. This, the authors say, is exactly the pattern predicted by the parasite hypothesis.
"The rise and fall of these female-only lineages was surprisingly fast and consistent with the prediction of the parasite hypothesis for sex," Jokela said. "These results suggest that sexual reproduction provides an evolutionary advantage in parasite rich environments."
So we may well have to thank parasites—in spite of their nasty reputation—for the joy of sex.
More from Science Daily (July 6, 2009):
Ancient Fossils Shed Light On Anatomical Changes Accompanying Evolution Of First Land Vertebrates
Cartoon depictions of the first animals to emerge from the ocean and walk on land often show a simple fish with feet, venturing from water to land. But according to Jennifer Clack, a paleontologist at the University of Cambridge who has studied the fossils of these extinct creatures for more than two decades, the earliest land vertebrates — also known as tetrapods — were more diverse than we could possibly imagine.
"Some looked like crocodiles, some looked like little lizards, some like moray eels, and some were snake-like," said Clack. "They occupied all sorts of niches and habitats. And they varied tremendously in size — from about 10 cm long to 5 meters."
Long before mammals, birds, and even dinosaurs roamed the Earth, the first four-legged creatures made their first steps onto land, and quickly inhabited a wide range of terrestrial environments. These early land vertebrates varied considerably in size and shape, said Clack.
To understand the anatomical changes that accompanied this diversity, Clack teamed up with two biologists who work on living fishes — Charles Kimmel of the University of Oregon, and Brian Sidlauskas of the National Evolutionary Synthesis Center in North Carolina.
The researchers focused on 35 early tetrapods that lived between 385 and 275 million years ago. As a proxy for body size and shape, they examined the dimensions of a number of bones in a region of the skull known as the palate. By tracing changes in the length and width of interlocking bones in this part of the skull, the researchers hoped to get a more fine-grained picture of skeleton evolution as a whole.
"I tend to think the genetic instructions for making a skeleton come from how you make individual bones first, and then how you fit those bones together as a refinement of that," said developmental biologist Charles Kimmel, who was the first author on the paper.
When they mapped the changes in bone length and width onto the tetrapod family tree, the researchers discovered that not all bones changed size at the same rate or in the same direction. This phenomenon can result in an overall reshaping from one lineage to the next, explained Sidlauskas. "Sometimes a change in size can have indirect consequences for the shape of the animal," said Sidlauskas. "When different parts of an animal's body change size at different rates over evolutionary time, that can generate changes in body shape from one species to another."
Moreover, some changes are consistent with an evolutionary quirk known as paedomorphosis, in which species retain in adulthood the youthful dimensions that their ancestors had as juveniles. "Paedomorphosis is definitely there — the descendents of some groups are retaining the proportions that their juveniles had in the past," said Clack.
These results not only help explain why early tetrapods were so diverse in size and shape, but also shed light on an important chapter in the evolution of life on land – the transition from fish to amphibians.
"One of the big questions at the moment is: where did modern amphibians come from?" said Clack. "One of the hypotheses is that they have evolved by paedomorphosis and miniaturization from early tetrapods. This study lends weight to that idea."
The National Evolutionary Synthesis Center (NESCent) is an NSF-funded collaborative research center operated by Duke University, the University of North Carolina at Chapel Hill, and North Carolina State University.
Vaccines & Autism
Well, Jenny McCarthy is up to her old tricks again: initially, she produces a video of herself spouting anti-vaccine propaganda; then, for some reason, the video gets pulled from her website GenerationRescue.org. Go figure!
However, in anticipation of this, Scam Blasting has the video on-line here.
Anyway, the video starts off with the usual nonsense that one expects from her right away, when she says:
I am not anti-vaccine. I am antitoxin. I want the mercury, aluminum, ether, anti-freeze, human aborted tissue, to be removed. I also believe that we are giving too many shots too soon...
WTF is she talking about? Where does she get her 'facts' from?
For some real facts on what's in and what's not in vaccines, read the excellent article by my other brother-in-arms, Todd W., on "The Truth About the 'Evils' of Vaccinations".
Jenny McCarthy then continues to rabbit on about other toxins such as pesticides. Well, funny she should mention that, considering the fact that she, by her own admission in the video, used to smoke marijuana in high-school, as well as regular cigarettes pictured here. Furthermore, nicotine -- the narcotic in tobacco -- is an organic pesticide produced by the plant genus Nicotiana as a natural defence mechanism.
Things then go even further off the rails from there, with Jenny McCarthy making all sorts of ridiculous and unfounded assertions. One of the things that she asserts is that autism isn’t genetic, and then confuses the issue with a statement about epidemics, but she is entirely wrong -- as is the case with everything else she says! Well, what the hell do you expect from someone who is not a trained doctor of medicine. There are strong genetic indicators of autism, but the underlying cause is still unknown. She makes claims about what triggers autism, when in fact no one knows what physically causes autism; however, we do know vaccines do not cause them.
We hear antivaxxers all the time saying that if we don’t know what causes autism, how can we know vaccines don’t? However, that’s just more empty rhetoric. It’s possible to not know the cause of something while still eliminating possibilities; that’s one way how medicine and science work. Start with an event and possible cause — for example, does chocolate cause acne? — then test the idea. If it comes up negative, move on. Furthermore, there is a lot of research on vaccines, and the conclusion is clear. When people like Jenny McCarthy say vaccines cause autism, they are, like ignorant creationists, ignoring the tsunami of evidence showing that they are wrong.
Doctors know a vast amount about children’s health issues; pediatricians devote their lives to them, and the antivaxxers are hell bent on throwing all this knowledge away, and, like damn creationists, replacing it with fairy tales.
Vaccinations have rid this world of smallpox, a scourge that killed millions of people. Measles was on the decline, pertussis was on the decline, rubella was on the decline. We are seeing a surge in outbreaks from these and other easily preventable diseases — including, tragically, deaths of children, deaths of babies — and a lot of it is traceable to the misinformation from the anti-vaccination movement.
Now, I'm not a medical doctor, but neither is Jenny McCarthy; however, I do understand science, so here's my advice: when you need to have your automobile serviced, then talk to the engineer, not to the oily rag! The same reasoning applies with medical issues: consult a real doctor, not some bloody quack with a jar of leeches!