Por que a seleção natural não eliminou as doenças hereditárias???

terça-feira, novembro 01, 2016


Why Hasn’t Natural Selection Eliminated Heritable Disease?


Photograph by Tim Ellis / Flickr

John Charles Martin “Johnny” Nash was a teen when he first started hearing a voice in his head. A born-again Christian, he interpreted this voice as God speaking to him. Once, he walked into the middle of a busy highway because the voice said he should. He was an accomplished chess player and math whiz, but playing and calculating became increasingly hard. It wasn’t long until a psychiatrist diagnosed him with schizophrenia. It was the same mental disorder his brilliant father John Forbes Nash, the late Nobel Prize winning mathematician who was famously portrayed by Russell Crowe in A Beautiful Mind, suffered from.

Schizophrenia affects more than 21 million people worldwide, and is highly heritable. So are thousands of other devastating diseases. This poses a conundrum for scientists who study evolution. Why would genes for such diseases persist when they decrease a person’s fitness? One theory that attempts to explain this paradox is called “balancing selection”—the heritable genetic mutations that code for some diseases tend to also be beneficial in some way. Richard Lewontin and John Hubby came up with the idea in 1966, positing that deleterious genes will circulate within a population to help maintain genetic diversity. Too little diversity, and some individuals will suffer from deleterious diseases; just enough and some individuals will benefit from the “heterozygote,” or hybrid, advantage.

The most famous example of balancing selection is sickle cell disease, an incurable form of anemia that inhibits blood and oxygen flow through the body. When a person has two copies of the sickle cell allele—one from each parent—they get full-blown sickle cell disease, which usually results in death before a person reaches reproductive age. Having just one copy isn’t even half as bad, quite the opposite—it actually confers resistance to malaria.

Scientists have speculated that some of the numerous gene variants that can give rise to the schizophrenia, which are also associated with alcoholism and manic depression, may confer creativity, enhanced memory, and a gift for numbers. How this worked at the level of the gene wasn’t clear, but recent research from an international group of scientists sheds new light on this complex process. The group, led by Tobias Lenz, an evolutionary immunogeneticist at the Max-Planck Institute for Evolutionary Biology, in Germany, found evidence that balancing selection may involve not just the individual alleles that make up a gene, but neighboring ones as well.

Experts have established that at least 11,000 genes in the human genome can, in certain variants, cause disease. Lenz and his colleagues were not the first to propose that some of these disease variants may persist due to balancing selection in neighboring genes rather than individual alleles, but they are the first to offer hard evidence.