2016年8月20日托福机经阅读（新东方版）

　　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.