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Wednesday, March 8, 2006
New signals abound of our genetic evolution
By ROWAN HOOPER
Good news this week for believers in common sense, opponents of intelligent design, and, incidentally, for writers of columns about natural selection.
First, Japanese volunteers have helped to provide evidence that natural selection has been tinkering with our genetic makeup far more recently than might be supposed. And second, over in Tennessee, feted for country & western music and mocked as a stronghold of the "intelligent design" movement, new evidence has emerged that natural selection is the driving force behind the origin of species.
First the news about recent natural selection. That's "recent," by the way, in the sense that evolutionary biologists use the word, meaning in the last 10,000 years. Researchers at the University of Chicago combed the genomes of 209 people from Japan, China, Nigeria and several European countries, and looked for genetic variations that signal recent evolution. They found more than 700 genes that showed evidence of having evolved at some time in the last 10,000 years.
It's been a busy time for humans, these last 10,000 years. We've invented agriculture and changed our diet dramatically. We've expanded our range extensively, living in new habitats under new climatic conditions. It's no wonder we evolved. But until now the hard evidence for recent evolutionary change has been scant.
Jonathan Pritchard, professor of human genetics at Chicago, looked at genetic data from those unrelated volunteers. His team found roughly the same number of signals of positive selection within each population. They also found that each population shares about one fifth of the signals with one or both of the other continental groups. In other words, those three population "types" (China and Japan were classed as East Asian) have all undergone about the same amount of change.
Strongest signal in the hunt
Among the more than 700 signals that the team found were previously known sites of recent adaptation, such as the salt-sensitive hypertension gene and the lactase gene, which was the strongest signal in the genome hunt. The lactase mutation, which enables the digestion of milk to continue into adulthood, appeared in approximately 90 percent of Europeans. (The mutation is much rarer in Japanese, which is presumably why you occasionally hear Japanese people say that Europeans smell of milk.)
The team used a system called PANTHER (Protein ANalysis THrough Evolutionary Relationships) to classify all the genes in the genome by their biological functions into 222 categories.
Other processes that show signals of selection include genes related to food metabolism, brain development and morphology. All have changed in the last 10,000 years. For example, the researchers found five genes involved in skin pigmentation that had recently evolved in Europeans.
"The idea that skin pigmentation is under strong selection in general is sort of accepted," Pritchard said, "but only one of these five signals was known before." They also found signals in genes involved in hair formation and patterning.
It's no surprise that among East Asians, the researchers found a strong signal of selection in the alcohol dehydrogenase cluster, the enzymes that break down alcohol. It's widely known that many East Asians have a mutation in a related gene that prevents them from metabolizing alcohol.
But the fact that mutation in the alcohol-metabolizing pathway has been positively selected in the last 10,000 years means that it must also have a beneficial effect. Pritchard published his work this week in the online journal Public Library of Science-Biology.
Now back to Tennessee, where battles continue over whether evolution can even be taught in schools. It's the state where the so-called Scopes monkey trial took place in 1925. The Scopes trial was a challenge to the law then in force that prohibited any state-funded school from teaching "any theory that denies the story of the Divine Creation of man as taught in the Bible, and to teach instead that man has descended from a lower order of animals." The defendant, a teacher named John T. Scopes, was found guilty of teaching evolution.
So it is ironic that researchers in Tennessee are now providing evidence that natural selection is a general driving force behind the origin of species.
Despite the name of his most famous book, Charles Darwin most clearly showed the origin of adaptations, not the origin of species. The latter was an obvious logical consequence of his reasoning, but it has been hard to find hard evidence that natural selection explains the origin of the 30 to 100 million different species estimated to inhabit the Earth.
The new study, led by biologist Daniel Funk, of Vanderbilt University, Tennessee, and published last month in the Proceedings of the National Academy of Sciences, does provide that evidence.
Adaptation and interbreeding
Funk and his colleagues looked at whether there was a positive link between the degree of adaptation to different environments by closely related groups and the extent to which they can interbreed. They collected all the data they could on pairs of related species, from plants, insects, fish, frogs and birds, and indeed found a strongly positive association. This means that natural selection really has been the driving force for the origin of species.
"Since our study suggests that natural selection is a general cause of species formation, it seems that Darwin chose an appropriate title after all," said Funk.
John T. Scopes and Darwin himself would be pleased to finally hear it.
A book of Natural Selections columns translated into Japanese, "Nou to sekkusu no seibutsugaku (Evolution, Sex and the Brain)," is published by Shinchosha.
Rowan Hooper is a biologist at Trinity College, Dublin. He welcomes readers' comments at email@example.com