Although modern healthcare frees us from death in many circumstances, in countries without access to good healthcare, populations continue to develop. Survivors of infectious disease outbreaks drive natural selection by passing on their genetic resistance to offspring
Author: Lawrence D. Hurst, Professor of Evolutionary Genetics at the Milner Center for Evolution, University of Bath, UK
The ability of modern medicine to preserve our lives tempts us to think that human evolution has stopped. Better health disrupts the central driving force of evolution. Thanks to the extension of life, there is a higher probability that people suffering from diseases that they would have died from in the past, will be able to pass their genes on to the next generation. But if we look at the rate of evolution of our DNA, we can see that human evolution has not stopped - it may even occur faster than before.
The most accurate definition of evolution is a gradual change in the DNA of a species over many generations. This process can occur by natural selection, where certain traits created by genetic mutations help an organism survive or reproduce. Therefore, such mutations are passed on to the next generation, so that their frequency increases in the population. Gradually, these mutations and their associated characteristics become more common among all members of the group.
By looking at many studies of human DNA, we can find evidence that natural selection has recently changed some genes and continues to do so. Although modern healthcare frees us from death in many circumstances, in countries without access to good healthcare, populations continue to develop. Survivors of infectious disease outbreaks drive natural selection by passing on their genetic resistance to offspring. Our DNA shows evidence of recent selection for resistance to deadly diseases like Lassa fever and malaria. Natural selection in response to malaria still continues in areas where the disease remains common.
Humans also adapt to their environment. Mutations that allow humans to live in high altitudes are more common in populations in Tibet, Ethiopia and the Andes. The spread of genetic mutations in Tibet is perhaps the most rapid evolutionary change in humans that has occurred over the past 3,000 years. This rapid burst in the frequency of the mutant gene that increases the oxygen content of the blood gives the inhabitants a survival advantage at higher altitudes, resulting in more surviving children born to them.
Nutrition is another source of adaptation. Evidence from Inuit (what we until recently called Eskimo) DNA shows that the adaptation that allows them to thrive on a high-fat diet of arctic mammals evolved only recently. Studies also show that the natural selection in favor of a mutation that allows adults to produce lactase - the enzyme that breaks down milk sugars - developed not long ago. Over 80% of the Northwestern European population can digest milk after weaning, but in parts of East Asia, where much less milk is drunk, lactose intolerance is the norm. Like adaptation to high altitude, selection (in milk for digestion) has evolved more than once in humans, and it may be the strongest selection.
We can also adapt to quite unhealthy diets. Research on the family genetic changes in the USA in the 20th century found a choice to lower blood pressure and cholesterol levels, both of which are high in the modern diet.
However, despite these changes, natural selection affects only 8% of our genome. According to the theory of neutral evolution, mutations in the rest of the genome may change frequently in the population by chance. If natural selection weakens, these mutations are not efficiently purged, which may increase their frequency and thus increase the rate of evolution.
But neutral evolution cannot explain why some genes evolve faster than others. We measure the speed of gene evolution by comparing human DNA to that of other species, which also allows us to determine which genes evolve rapidly in humans only. One of the fastest evolving genes in humans is HAR1, which is required during brain development. A random segment of human DNA is more than 98% identical to the chimpanzee sequence on average, but HAR1 evolved so quickly that in the region where it is found the similarity is only 85%.
Although scientists can see that these changes are occurring—and how quickly—we still don't understand why rapid evolution occurs in some genes but not others. Originally I thought this was a result of the lack of exclusivity of natural selection, now we know this is not always true.
Recently attention has been focused on the process of biased conversion of the genes, which occurs when our DNA is transmitted through our sperm and eggs. Turning these gametes into an embryo involves separating DNA molecules, reprocessing them, and then fusing them. However, molecular repairs tend to happen in a biased manner.
DNA molecules consist of four different chemical bases known as C, G, A and T. The repair process favors repairs using C and G bases instead of A or T. While it is not clear why this bias exists, it makes G and C bases more common.
A high concentration of G and C at the sites of normal DNA repair causes the rapid development of parts of our genome, a process that misleads natural selection, because both cause rapid changes in the DNA at very targeted sites. About a fifth of our fastest-evolving genes, including HAR1, were affected by this process. If GC changes are harmful, natural selection would have gotten rid of them. But with selection weakening, this process can continue and can even help speed up the development of our DNA.
The human mutation rate itself may also vary. The main source of mutations in human DNA is the process of cell division that creates sperm cells. The older the males, the more mutations occur in their sperm. The longer you delay having children, the higher the rate of mutations will be. This determines the rate of neutral evolution.
The acceleration of evolution that does not occur only by natural selection means that the process will never stop. Freeing our genomes from the pressures of natural selection only opens them up to other evolutionary processes - making it even more difficult to predict what will happen in humans in the future. However, it is quite possible that with the protections provided by modern medicine, there will be more severe genetic problems for future generations.
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