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Revisiones bibliográficas. Documentación científica en Ortopedia y Traumatología, medicina deportiva, artroscopia, artroplastia y de todas las patologías del sistema Músculo-Esquelético
jueves, 27 de enero de 2011
Agua limpia más barata para las emergencias humanitarias
Más memoria de forma natural
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Casi un tercio de la humanidad navega en Internet
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Una aplicación informática libera a los investigadores del microscopio
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El ruido del tráfico, un nuevo factor de riesgo para el ictus
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Nonfiction: Nabokov Theory on Butterfly Evolution Is Vindicated
Nonfiction: Nabokov Theory on Butterfly Evolution Is Vindicated
Roger Vila
A male Acmon blue butterfly (Icaricia acmon). Vladimir Nabokov described the Icaricia genus in 1944. More Photos »
By CARL ZIMMER
Published: January 25, 2011
Vladimir Nabokov may be known to most people as the author of classic novels like “Lolita” and “Pale Fire.” But even as he was writing those books, Nabokov had a parallel existence as a self-taught expert on butterflies.
Get Science News From The New York Times »He was the curator of lepidoptera at the Museum of Comparative Zoology atHarvard University, and collected the insects across the United States. He published detailed descriptions of hundreds of species. And in a speculative moment in 1945, he came up with a sweeping hypothesis for the evolution of the butterflies he studied, a group known as the Polyommatus blues. He envisioned them coming to the New World from Asia over millions of years in a series of waves.
Few professional lepidopterists took these ideas seriously during Nabokov’s lifetime. But in the years since his death in 1977, his scientific reputation has grown. And over the past 10 years, a team of scientists has been applying gene-sequencing technology to his hypothesis about how Polyommatus blues evolved. On Tuesday in the Proceedings of the Royal Society of London, they reported that Nabokov was absolutely right.
“It’s really quite a marvel,” said Naomi Pierce of Harvard, a co-author of the paper.
Nabokov inherited his passion for butterflies from his parents. When his father was imprisoned by the Russian authorities for his political activities, the 8-year-old Vladimir brought a butterfly to his cell as a gift. As a teenager, Nabokov went on butterfly-hunting expeditions and carefully described the specimens he caught, imitating the scientific journals he read in his spare time. Had it not been for the Russian Revolution, which forced his family into exile in 1919, Nabokov said that he might have become a full-time lepidopterist.
In his European exile, Nabokov visited butterfly collections in museums. He used the proceeds of his second novel, “King, Queen, Knave,” to finance an expedition to the Pyrenees, where he and his wife, Vera, netted over a hundred species. The rise of the Nazis drove Nabokov into exile once more in 1940, this time to the United States. It was there that Nabokov found his greatest fame as a novelist. It was also there that he delved deepest into the science of butterflies.
Nabokov spent much of the 1940s dissecting a confusing group of species called Polyommatus blues. He developed forward-thinking ways to classify the butterflies based on differences in their genitalia. He argued that what were thought to be closely related species were actually only distantly related.
At the end of a 1945 paper on the group, he mused on how they had evolved. He speculated that they originated in Asia, moved over the Bering Strait, and moved south all the way to Chile.
Allowing himself a few literary flourishes, Nabokov invited his readers to imagine “a modern taxonomist straddling a Wellsian time machine.” Going back millions of years, he would end up at a time when only Asian forms of the butterflies existed. Then, moving forward again, the taxonomist would see five waves of butterflies arriving in the New World.
Nabokov conceded that the thought of butterflies making a trip from Siberia to Alaska and then all the way down into South America might sound far-fetched. But it made more sense to him than an unknown land bridge spanning the Pacific. “I find it easier to give a friendly little push to some of the forms and hang my distributional horseshoes on the nail of Nome rather than postulate transoceanic land-bridges in other parts of the world,” he wrote.
When “Lolita” made Nabokov a star in 1958, journalists were delighted to discover his hidden life as a butterfly expert. A famous photograph of Nabokov that appeared in The Saturday Evening Post when he was 66 is from a butterfly’s perspective. The looming Russian author swings a net with rapt concentration. But despite the fact that he was the best-known butterfly expert of his day and a Harvard museum curator, other lepidopterists considered Nabokov a dutiful but undistinguished researcher. He could describe details well, they granted, but did not produce scientifically important ideas.
Only in the 1990s did a team of scientists systematically review his work and recognize the strength of his classifications. Dr. Pierce, who became a Harvard biology professor and curator of lepidoptera in 1990, began looking closely at Nabokov’s work while preparing an exhibit to celebrate his 100th birthday in 1999. She was captivated by his idea of butterflies coming from Asia. “It was an amazing, bold hypothesis,” she said. “And I thought, ‘Oh, my God, we could test this.’ ”
To do so, she would need to reconstruct the evolutionary tree of blues, and estimate when the branches split. It would have been impossible for Nabokov to do such a study on the anatomy of butterflies alone. Dr. Pierce would need their DNA, which could provide more detail about their evolutionary history.
Working with American and European lepidopterists, Dr. Pierce organized four separate expeditions into the Andes in search of blues. Back at her lab at Harvard, she and her colleagues sequenced the genes of the butterflies and used a computer to calculate the most likely relationships between them. They also compared the number of mutations each species had acquired to determine how long ago they had diverged from one another.
There were several plausible hypotheses for how the butterflies might have evolved. They might have evolved in the Amazon, with the rising Andes fragmenting their populations. If that were true, the species would be closely related to one another.
But that is not what Dr. Pierce found. Instead, she and her colleagues found that the New World species shared a common ancestor that lived about 10 million years ago. But many New World species were more closely related to Old World butterflies than to their neighbors. Dr. Pierce and her colleagues concluded that five waves of butterflies came from Asia to the New World — just as Nabokov had speculated.
“By God, he got every one right,” Dr. Pierce said. “I couldn’t get over it — I was blown away.”
Dr. Pierce and her colleagues also investigated Nabokov’s idea that the butterflies had come over the Bering Strait. The land surrounding the strait was relatively warm 10 million years ago, and has been chilling steadily ever since. Dr. Pierce and her colleagues found that the first lineage of Polyommatus blues that made the journey could survive a temperature range that matched the Bering climate of 10 million years ago. The lineages that came later are more cold-hardy, each with a temperature range matching the falling temperatures.
Nabokov’s taxonomic horseshoes turn out to belong in Nome after all.
"What a great paper," said James Mallet, an expert on butterfly evolution at University College London. "It's a fitting tribute to the great man to see that the most modern methods that technology can deliver now largely support his systematic arrangement."
Dr. Pierce says she believes Nabokov would have been greatly pleased to be so vindicated, and points to one of his most famous poems, “On Discovering a Butterfly.” The 1943 poem begins:
I found it and I named it, being versed
in taxonomic Latin; thus became
godfather to an insect and its first
describer — and I want no other fame.
“He felt that his scientific work was standing for all time, and that he was just a player in a much bigger enterprise,” said Dr. Pierce. “He was not known as a scientist, but this certainly indicates to me that he knew what it’s all about.”
Long and Short of Calcium and Vitamin D
Long and Short of Calcium and Vitamin D
By JANE E. BRODY
Published: January 24, 2011
The new daily recommendations for calcium and vitamin D, issued in November by the Institute of Medicine, have left many people wondering whether they are getting enough, or perhaps too much, in their diets and supplements.
Yvetta Fedorova
Related
The institute’s expert committee, which included bone specialists, concluded that most people don’t need supplements of these critical nutrients and warned of serious health risks from the high doses some now take — including kidney stones and heart disease linked to calcium supplements, and the very falls and fractures that vitamin D is meant to protect against.
For bone health, vitamin D and calcium go hand in hand, because the vitamin must be present for calcium to be absorbed from the digestive tract. But who, if anyone, needs supplements — and how much? Can you get enough from foods naturally rich in these nutrients or fortified with them?
These are important questions, given the steady increase in life expectancy and the already epidemic levels ofosteoporosis and fractures among older Americans, men and women alike. (Women are especially vulnerable, because estrogen loss at menopause can cause a precipitous decline in bone density.)
The answers depend on three things, not to mention which experts you happen to ask: the foods and drinks you regularly consume, your personal and family history of broken bones, and habits that influence bone health.
Dr. Robert P. Heaney, a bone specialist at Creighton University in Omaha, maintains that “at least one-third of all osteoporotic fractures have a nutritional basis.”
What you eat and drink, from childhood on, is critical to the amount of calcium in your bones. Dairy foods, especially milk, yogurt and cheese, are the primary sources of calcium in the American diet, and consumption of milk has been falling steadily for decades, especially in adolescence, when most bone development occurs. A British study concluded that frequent milk consumption before age 25 was an important determinant of bone strength among middle-aged and elderly women.
Other foods are not nearly as rich in absorbable calcium, or the amounts normally eaten do not come close to the calcium content of dairy products: 300 milligrams in a glass of milk, 400 milligrams in eight ounces of yogurt.
Sardines and canned salmon eaten with the bones are good sources, and almonds are a fair source if you eat enough of them. And calcium-fortified foods like orange juice, soy milk, breakfast cereals and tofu are now widely available.
Too Much of a Good Thing
But some other desirable foods are problematic, at least when it comes to calcium: you’d have to eat so much broccoli to approach the level in milk that it could be toxic to your thyroid gland. Other vegetables with calcium, like spinach, collards, kale and beans, contain oxalates that block calcium absorption.
For daily calcium intake, the institute now recommends 1,000 milligrams for children 4 to 8, women and men 19 to 50, and men 51 to 70; 1,300 milligrams for children 9 to 18; and 1,200 milligrams for women 51 and older and men 71 and older. The upper limit of safety, the institute said, is 2,000 milligrams a day for men and women over 51.
Thus, if you are a postmenopausal woman who typically consumes only one or two servings a day of dairy, you may be hard put to get 1,200 milligrams of calcium from the rest of your diet unless you take a supplement. Dr. Ethel Siris, director of the osteoporosis clinic at Columbia University Medical Center in New York, said such women could benefit from a supplement of calcium carbonate (600 milligrams a day) or calcium citrate (500 milligrams a day).
Be sure to read the product label carefully — a usual “serving” is two tablets. Calcium carbonate should be taken with meals to assure absorption, but calcium citrate can be taken at any time and may cause fewer digestive problems.
Most calcium supplements now also contain vitamin D (usually as cholecalciferol, or D3), supplying about 250 to 300 international units in two tablets. The Institute of Medicine recommends 600 units a day for everyone from age 1 to 70 and 800 units for men and women 71 and older, with a safe upper limit for everyone over the age of 9 of 4,000 units.
Vitamin D has one advantage over calcium: It is fat-soluble and can be stored in the body for later use. But getting enough of it can be tricky.
The body gets most of its vitamin D not from diet but from skin exposed to the ultraviolet B radiation in sunlight. Unprotected skin on the arms and legs may need about 15 minutes of sun exposure a day in spring, summer and fall to make enough of the vitamin.
Alas, this production is effectively blocked if you follow current advice to prevent skin cancer and wrinkles by always covering up or using ample amounts of sunscreen. Used properly, sunscreens with an SPF of 8 or higher completely block UVB radiation and prevent synthesis of vitamin D.
Also, people who are dark-skinned or housebound or who live in far northern latitudes may fail to make enough vitamin D. And as people age, their bodies are less able to convert the vitamin into the hormone that is its biologically active form.
Milk is fortified with vitamin D at a level of 400 units per quart, and some yogurts have it as well (check the label). Many breakfast cereals are also now fortified. The only naturally rich dietary sources are oily fish from the sea like salmon and mackerel, egg yolks, liver and fish liver oil.
Testing and Maintaining
An increasing number of physicians now routinely test vitamin D levels in the blood of their female patients, and if it is below 30 nanograms per milliliter, will suggest they take a supplement. The Institute of Medicine maintains that a level of 20 nanograms is adequate, but other experts say it should be higher to assure maximum calcium absorption and bone health.
In any event, unless you are a year-round sun worshiper, a daily supplement of calcium with D, or even a separate supplement of 1,000 units of D, is likely to keep you well below the institute’s upper safe limit. Based on current evidence, unless you have a severe deficiency requiring temporary megadoses to correct, there is no reason to go any higher.
At the same time, you’d be wise to get sufficient weight-bearing exercise and avoid several bone-robbing habits: smoking; eating a lot of salty foods; drinking more than two alcoholic drinks a day; consuming more than the caffeine equivalent of two cups of coffee a day (about 300 milligrams); and eating too little protein. As for soft drinks, Dr. Siris advises a daily limit of two 12-ounce cans, and she’d prefer that soda be only an occasional treat.
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