Book 1 Reading and Speaking (1108795), страница 16
Текст из файла (страница 16)
Not all populations show the same evolutionaryspeed. For example, Africans show a slightly lower mutation rate. "Africans haven't had to adapt to a fundamentallynew climate," because modern humanity evolved where they live, Cochran says. "Europeans and East Asians,38living in environments very different from those of their African ancestors and early adopters of agriculture, weremore maladapted, less fitted to their environments."And this speedy pace of evolution will not slow until every possible beneficial mutation starts to happen—themaximum rate of adaptation. This has already begun to occur in such areas as skin color in which different sets ofgenes are responsible for the paler shades of Europeans and East Asians, according to the researchers.The finding raises many questions.
Among them: "the medical applications of this kind of knowledge [as wellas] exactly what most of the selected changes do and what drove their selection," Cochran says.But the history of humanity is beginning to be read out from our genes, thanks to a detailed knowledge of thethousands of them that have evolved recently. "We're going to be classifying these by functional categories andlooking for matches between genetic changes and historic and archaeological changes in diet, skeletal form,disease and many other things," Hawks says.
"We think we will be able to find some of the genetic changes thatdrove human population growth and migrations—the broad causes of human history." (From Scientific AmericanOnline, December 10, 2007)*This article wrongly characterized the HapMap genotype dataset used for this analysis as "genes" ratherthan "DNA sequences."Exercise 4.
Are the following statements true or false, according to the text? Explain your answer.1. Homo Sapiens has demonstrated great biological progress over its history.2. The rate of human evolution has slowed down due to vast numbers of people living.3. Transition to agricultural lifestyle and domestication of animals provided a reliable source of food forpeople.4. It also helped to improve human health condition.5. Different populations demonstrate different rates of evolutionary changes.6.
Humans have developed numerous adaptations and thus have achieved the maximum rate ofadaptation.7. Genetic research sheds light on human evolution.Exercise 5. Are humans still evolving? Discuss the following questions:a. What changes have taken place since the emergence of Homo Sapiens? Give as many examples as youcan.b. What adaptations to their environment and lifestyle have people developed?Exercise 6. Read the text which provides some information about the changes of human phenotype.Why are we getting taller as a species?Humans increased in stature dramatically during the last 150 years, but we have now likely reached theupper limit.
The average height of a human man will probably never exceed that of basketball player ShaquilleO'Neal, who stands 7 feet and 1 inch tall.This answer comes from Michael J. Dougherty, assistant director and senior staff biologist at Biologic Image:SportsLine USA, Inc.In fact, over the last 150 years the average height of people in industrialized nations has increasedapproximately 10 centimeters (about four inches). Why this relatively sudden growth? Are we evolving to greaterheights? Before answering these questions, we need to remember that evolution requires two things: variation inphysical and/or behavioral traits among the individuals in a population; and a way of selecting some of those traitsas adaptations, or advantages to reproduction.For example, finches that have large, powerful beaks also have an advantage cracking large, tough seedsduring periods when small, soft seeds are scarce.
As a consequence, large-beaked birds are more likely to eatbetter, survive longer and reproduce than small-beaked birds. Because beak shape is an inherited trait, moresuccessful reproduction by large beaked birds means that the genes predisposing finches to large beaks aretransmitted to the next generation in relatively larger numbers than those genes encoding small beaks.
Thus, thepopulation of finches in the next generation will tend to have larger beaks than finches in their parent's generation.Let's use this basic operating principle of evolution to predict, retrospectively, the direction of change inhuman height if evolution were the cause of the change.
We know from studies conducted in industrial Englandthat children born into lower socioeconomic classes were shorter, on average, than children born into wealthyfamilies. We also know that poorer families had larger numbers of children. Given those initial conditions, whatwould evolution predict? The average population should have become shorter because the shorter individuals inthe population were, from an evolutionary fitness perspective, more successful in passing on their genes. But thisdid not happen. Instead, all segments of the population--rich and poor, from small and large families--increased inheight.
Thus, natural selection, the process whereby differences in reproductive success account for changes in thetraits of a population, does not explain why we are taller.If evolution doesn't explain height increases, what does? Most geneticists believe that the improvement inchildhood nutrition has been the most important factor in allowing humans to increase so dramatically in stature.The evidence for this argument is threefold:39First, the observed increase in height has not been continuous since the dawn of man; it began sometimearound the middle of the nineteenth century. In fact, examinations of skeletons show no significant differences inheight from the stone age through the early 1800s.
Also, during World Wars I and II, when hunger was a frequentcompanion of the German civilian population, the heights of the children actually declined. They only recoveredduring the post-war years. Such data are consistent with recent research indicating that slow growth induced bytemporary malnourishment can usually be reversed. Chronic underfeeding during childhood, however, permanentlyaffects stature and other traits, including intelligence.Second, the trend toward increasing height has largely leveled off, suggesting that there is an upper limit toheight beyond which our genes are not equipped to take us, regardless of environmental improvements.Interestingly, the age of menarche, which is also influenced by nutrition, has shown a corresponding decrease overthis same time period.
Some scientists believe that the increase in teenage and out-of-wedlock pregnancies in thedeveloped world may be an unanticipated consequence of improved nutrition.Third, conditions of poor nutrition are well correlated to smaller stature. For example, the heights of allclasses of people, from factory workers to the rich, increased as food quality, production and distribution becamemore reliable, although class differences still remain. Even more dramatic, the heights of vagrant London boysdeclined from 1780 to1800 and then rose three inches in just 30 years--an increase that paralleled improvingconditions for the poor.
Even today, height is used in some countries as an indicator of socioeconomic division, anddifferences can reveal discrimination within social, ethnic, economic, occupational and geographic groups.For those hoping that humans might someday shoot basketballs through 15-foot high hoops, the fact that theincrease in human height is leveling off no doubt will be disappointing. For those who understand, however, thatour genes are merely a blueprint that specifies what is possible given an optimal environment, a limit on height isjust one of many limitations in life, and certainly not the most constraining.With environmental variables perhaps near their optimum, what are the prospects for evolutionary increasesin height as a consequence of changes to our genetic blueprints? Apply the methods of the thought experimentabove and see.Exercise 7.
Using the information from the text prove that:1. Evolution doesn’t explain the increased height of people.2. Improvement in childhood nutrition has been the most important factor in increasing the stature.3. People will hardly increase their height any more.Exercise 8. In the following text the paragraphs are mixed. Put them in the correct logical order. The firstand the last paragraphs are in their right places.African Adaptation to Digesting Milk Is "Strongest Signal of Selection Ever"East African cattle herding communities rapidly and independently evolved ability to digest lactoseBy Nikhil Swaminathan(A) For many adults in the world, the phrase "got milk?" is quickly followed by "got a nearby toilet?" Lactose,the primary sugar in milk, is a universal favorite in infancy but into adulthood the level of lactase-phlorizinhydrolase, the enzyme that metabolizes lactose in the small intestine, decreases and digestion of dairy productsbecomes difficult.
In some populations, however, such as those located in northern Europe, the ability to digest milkremains most likely as a result of lifestyles based around cattle domestication. In 2002 Finnish scientists localizedthe genetic mutation that conferred this trait in northern Europeans to two regions on chromosome 2.(B) Tishkoff and her students tested 470 people representing over 43 ethnic fractions in the Sudan, Kenyaand northern Tanzania for lactose intolerance using glucose-monitoring kits, familiar to most diabetics.
The teamthen selected the 40 most lactose intolerant participants and the 69 most tolerant and sequenced parts of theirgenomes around the two markers identified in the Finnish lactase persistence study. The researchers determinedthere were 123 single nucleotide polymorphisms--SNPs, or changes to one base in the genetic code--associatedwith digestibility. Of these, three SNPs were more promising than the others and one of them was very commonamong Tanzanians and Kenyans, showing up in 40 to 50 percent of the sequences.(C) Tishkoff believes that because she found so many markers associated with lactose tolerance in thesequencing of her 109 subjects, evolution clearly develops multiple solutions when there is a strong selective force."There are some populations that can digest milk, and they don't have any of these mutations," she says.
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