1 . Modern medicine’s ability to keep us alive makes it tempting to think human evolution may have stopped. But if we look at the rate of our DNA’s evolution, we can see that human evolution hasn’t stopped – it may even be happening faster than before.
Evolution is a gradual change to the DNA of a species over many generations. It can occur by natural selection, when certain traits created by genetic changes help an organism survive or reproduce. Such genes are thus more likely to be passed on to the next generation, so they increase in frequency in a population. Gradually, these changes and their associated traits become more common among the whole group.
By looking at global studies of our DNA, we can see evidence that natural selection has recently made changes and continues to do so. Though modern healthcare disrupts a key driving force of evolution by keeping some people alive longer, in countries without access to good healthcare, populations are continuing to evolve. Survivors of infectious disease outbreaks drive natural selection by giving their genetic resistance to offspring. Our DNA shows evidence for recent selection for resistance of killer diseases like Lassa fever and malaria. Selection in response to malaria remains in regions where the disease remains common.
Humans are also adapting to their environment. Gene change allowing humans to live at high altitudes have become more common in populations in Tibet, Ethiopia, and the Andes. The spread of genetic changes in Tibet is possibly the fastest evolutionary change in humans, occurring over the past 3,000 years. This rapid increase in frequency of a mutated gene that increases blood oxygen content gives locals a survival advantage in higher altitudes, resulting in more surviving children.
Diet is another source for adaptations. Studies show that natural selection favoring a change allowing adults to produce lactase – the enzyme (酶) that breaks down milk sugars – is why some groups of people can digest milk. Over 80 per cent of northwest Europeans can, but in parts of East Asia, where milk is much less commonly drunk, an inability to digest lactose is the norm. Like high altitude adaptation, selection to digest milk has evolved more than once in humans and may be the strongest kind of recent selection.
Yet, despite these changes, natural selection only affects about 8 per cent of our genome. But scientists can’t explain why some genes are evolving much faster than others. We measure the speed of gene evolution by comparing human DNA with that of other species. One fast-evolving gene is human accelerated region 1 (HAR1), which is needed during brain development. A random section of human DNA is on average more than 98 per cent identical to the chimp comparator, but HAR1 is so fast evolving that it’s only around 85 per cent similar. Though scientists can see these changes are happening – and how quickly – we still don’t fully understand why fast evolution happens to some genes but not others.
1. Which of the following statements may the author agree with?| A．Evolution occurs among several people overnight. |
| B．Genes may change and some are beneficial to people’s lives. |
| C．Evolution is done when the whole population possesses a certain gene. |
| D．The changed genes leading to higher survival rates are chosen deliberately. |
| A．explains | B．causes | C．upsets | D．heals |
| A．some people can resist infectious diseases like malaria |
| B．children in Tibet tolerate living environments with thin air |
| C．northwestern Europeans digest lactose better than East Asians |
| D．the human gene HAR1 resembles that of a chimp to a lesser extent |
| A．What Is Natural Selection? |
| B．Are Humans Still Evolving? |
| C．Why Will Certain Genes Evolve? |
| D．How Do Mutated Genes Function? |
