2016年8月20日托福阅读真题答案解析
2016年8月20日托福阅读词汇题：
likewise=similarly
lucrative=profitable
surge=sudden increase
might=strength
compile=put together
prevail=accepted
coexist=live together
exquisitely=perfectly
predominate=mainly
adopting=start to use
2016年8月20日托福阅读第一篇 人口增长
原文回顾：因为农业发展，公元前8000年人口就忽然增长。然后，因为农业让妈妈可以生更多的孩子。而且婴儿可以喂谷物，而不是像以前那样，只喝奶;大孩子也可以照顾小孩，还可以分担一些农活。所以，人类出生率就变高了。农业人口再语言上和其他一些方面上都优于渔猎人口。所以，渔猎人口逐渐开始向农业人口学习。
相关背景学习： Population genetics
In population genetics a sexual population is a set of organisms in which any pair of members can breed together. This means that they can regularly exchange gametes to produce normally-fertile offspring, and such a breeding group is also known therefore as a gamodeme. This also implies that all members belong to the same of species, such as humans. If the gamodeme is very large (theoretically, approaching infinity), and all gene alleles are uniformly distributed by the gametes within it, the gamodeme is said to be panmictic. Under this state, allele (gamete) frequencies can be converted to genotype (zygote) frequencies by expanding an appropriate quadratic equation, as shown by Sir Ronald Fisher in his establishment of quantitative genetics.
This seldom occurs in nature: localisation of gamete exchange – through dispersal limitations, or preferential mating, or cataclysm, or other cause – may lead to small actual gamodemes which exchange gametes reasonably uniformly within themselves, but are virtually separated from their neighbouring gamodemes. However, there may be low frequencies of exchange with these neighbours. This may be viewed as the breaking up of a large sexual population (panmictic) into smaller overlapping sexual populations. This failure of panmixia leads to two important changes in overall population structure: (1) the component gamodemes vary (through gamete sampling) in their allele frequencies when compared with each other and with the theoretical panmictic original (this is known as dispersion, and its details can be estimated using expansion of an appropriate binomial equation); and (2) the level of homozygosity rises in the entire collection of gamodemes. The overall rise in homozygosity is quantified by the inbreeding coefficient (f or φ). Note that all homozygotes are increased in frequency – both the deleterious and the desirable. The mean phenotype of the gamodemes collection is lower than that of the panmictic "original" – which is known as inbreeding depression. It is most important to note, however, that some dispersion lines will be superior to the panmictic original, while some will be about the same, and some will be inferior. The probabilities of each can be estimated from those binomial equations. In plant and animal breeding, procedures have been developed which deliberately utilise the effects of dispersion (such as line breeding, pure-line breeding, back-crossing). It can be shown that dispersion-assisted selection leads to the greatest genetic advance (ΔG = change in the phenotypic mean), and is much more powerful than selection acting without attendant dispersion. This is so for both allogamous (random fertilization) and autogamous (self-fertilization) gamodemes.
2016年8月20日托福阅读第二篇 鸟类的起源
原文回顾：鸟类是不是从恐龙进化的，之前普遍认为是从树上飞下来而不是在陆地上。之后发现鸟类和恐龙有很多共同点，不过和现在脊椎动物不同，这些都还在研究中，参考的是中国发掘保存好的化石，说很多不飞的恐龙也有羽毛，所以羽毛只有鸟类有不对，而且羽毛是地面恐龙和飞行鸟类的一个过渡。
相关背景学习： Feathers, plumage, and scales
Feathers are a feature characteristic of birds (though also present in some dinosaurs not currently considered to be true birds). They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signaling. There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex.
Plumage is regularly moulted; the standard plumage of a bird that has moulted after breeding is known as the "non-breeding" plumage, or—in the Humphrey-Parkes terminology—"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey-Parkes system as "alternate" plumages. Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, flight feathers are replaced one at a time with the innermost primary being the first. When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop. After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary coverts are moulted in synchrony with the primary that they overlap. A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. As a general rule, the tail feathers are moulted and replaced starting with the innermost pair. Centripetal moults of tail feathers are however seen in the Phasianidae. The centrifugal moult is modified in the tail feathers of woodpeckers and treecreepers, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail. The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation.
Red parrot with yellow bill and wing feathers in bill
Red lory preening
Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland; these secretions protect the feathers' flexibility and act as an antimicrobial agent, inhibiting the growth of feather-degrading bacteria. This may be supplemented with the secretions of formic acid from ants, which birds receive through a behaviour known as anting, to remove feather parasites.
The scales of birds are composed of the same keratin as beaks, claws, and spurs. They are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers. The scales of birds are thought to be homologous to those of reptiles and mammals.
2016年8月20日托福阅读第三篇 美国铁路的发展和影响
原文回顾： 美国的铁路线数量几乎超过了世界其余国家的铁路线总量，铁路的发展使美国成为仅次于英国的第二大工业国家。除了促进交通业的发展，铁路的发展还促进了钢铁业，采煤业的发展。铁路的发展更促进了电报业的发展。
相关背景学习：Rail transport
Rail transport is a means of conveyance of passengers and goods on wheeled vehicles running on rails, also known as tracks.
It is also commonly referred to as train transport. In contrast to road transport, where vehicles run on a prepared flat surface, rail vehicles ("rolling stock") are directionally guided by the tracks on which they run. Tracks usually consist of steel rails, installed on ties (sleepers) and ballast, on which the rolling stock, usually fitted with metal wheels, moves. Other variations are also possible, such as slab track, where the rails are fastened to a concrete foundation resting on a prepared subsurface.
Rolling stock in railway transport systems generally suffers lower frictional resistance than road vehicles, and the passenger and freight cars (carriages and wagons) can be coupled into longer trains. The operation is carried out by a railway company, providing transport between train stations or freight customer facilities. Power is provided by locomotives which either draw electric power from a railway electrification system or produce their own power, usually by diesel engines. Most tracks are accompanied by a signalling system. Railways are a safe land transport system when compared to other forms of transport.[Nb 1] Railway transport is capable of high levels of passenger and cargo utilization and energy efficiency, but is often less flexible and more capital-intensive than road transport, when lower traffic levels are considered.
The oldest, man-hauled railways date back to the 6th century BC, with Periander, one of the Seven Sages of Greece, credited with its invention. Rail transport blossomed after the British development of the steam locomotive as a viable source of power in the 18th and 19th centuries. With steam engines, one could construct mainline railways, which were a key component of the Industrial Revolution. Also, railways reduced the costs of shipping, and allowed for fewer lost goods, compared with water transport, which faced occasional sinking of ships. The change from canals to railways allowed for "national markets" in which prices varied very little from city to city. The invention and development of the railway in Europe was one of the most important technological inventions of the 19th century; in the United States, it is estimated that without rail, GDP would have been lower by 7% in 1890.
In the 1880s, electrified trains were introduced, and also the first tramways and rapid transit systems came into being. Starting during the 1940s, the non-electrified railways in most countries had their steam locomotives replaced by diesel-electric locomotives, with the process being almost complete by 2000. During the 1960s, electrified high-speed railway systems were introduced in Japan and later in some other countries. Other forms of guided ground transport outside the traditional railway definitions, such as monorail or maglev, have been tried but have seen limited use. Following decline after World War II due to competition from cars, rail transport has had a revival in recent decades due to road congestion and rising fuel prices, as well as governments investing in rail as a means of reducing CO2 emissions in the context of concerns about global warming.