在2014年5月11日的托福閱讀考試中有這樣一道題：以松鼠和獅子為例講動物遷徙。針對這道托福考題，出國留學(xué)網(wǎng)(www.liuxue86.com)小編來為大家普及一下關(guān)于動物遷徙的背景知識，這樣有助于考生在面對這類題目時方便作答。小編在此提醒大家：動物行為主題是托福閱讀常見考點(diǎn)，結(jié)構(gòu)不難理解。需注意各例證和主題的支撐關(guān)系。
托福閱讀真題再現(xiàn)：
版本一：某些動物長大以后離開出生地生存，有些不會。主要講不可以的。舉了兩個例子。第一個是松鼠，雄鼠長大后飛走，雌鼠不會。第二個例子是獅子，雄獅子長大了以后會離開，原因是打不過原來的首領(lǐng)，被趕跑。雌性獅子則會留在群落幫忙找吃的。
版本二：講動物離開出生點(diǎn)行為，原因一：某鼠離開出生點(diǎn)，雄150米，雌50米，因?yàn)槟芊乐菇H繁殖導(dǎo)致基因病，同時雌性在一起能養(yǎng)小鼠方便。原因二：獅子，群內(nèi)爭斗呀，勞什子排擠呀什么的。
版本三： 動物的disperse, 剛開始說為什么動物要離開熟悉的food rich的地方而去其他地方。其中講了一種動物男女的分布是不一樣的，女的離原來的家50米，男的150米， 不同的原因是防止近親結(jié)婚導(dǎo)致孩子多病不易存活，另外女的離家近更有益處，家里可以給她提供保護(hù)，然后男的要更遠(yuǎn)的地方對抗敵人，有可能被競爭者replace而離開，然后有個lion的例子
托福閱讀詞匯：
squirrel n松鼠
disperse v分散
Inbreeding n近親交配
genopathy n遺傳病
解析：本文圍繞動物為何離開出生地這個主題展開論證。做題時需注意記錄筆記，對于結(jié)構(gòu)化閱讀及最后一題的解答有很大好處。動物行為主題是托福閱讀常見考點(diǎn)，結(jié)構(gòu)不難理解。需注意各例證和主題的支撐關(guān)系。由于條理清晰，最后一題盡量考慮從正面選出，排除為輔。
托福閱讀相關(guān)背景：
Animal Inbreeding
Inbreeding is the production of offspring from the mating or breeding of individuals or organisms which are closely related genetically, in contrast to outcrossing, which refers to mating unrelated individuals.[1] By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from incestuous sexual relationships and consanguinity.
Inbreeding results in homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits.[2] This generally leads to a decreasedbiological fitness of a population,[3][4] (called inbreeding depression) which is its ability to survive and reproduce. An individual who inherits such deleterious traits is referred to as inbred. The avoidance of such deleterious recessive alleles caused by inbreeding is the main selective reason for outcrossing.[5][6]
Inbreeding is a technique used in selective breeding. In livestock breeding, breeders may use inbreeding when, for example, trying to establish a new and desirable traitin the stock, but will need to watch for undesirable characteristics in offspring, which can then be eliminated through further selective breeding or culling. Inbreeding is used to reveal deleterious recessive alleles, which can then be eliminated through assortative breeding or through culling. In plant breeding, inbred lines are used as stocks for the creation of hybrid lines to make use of the effects of heterosis. Inbreeding in plants also occurs naturally in the form of self-pollination.
Offspring of biologically related persons are subject to the possible impact of inbreeding, such as congenital birth defects. The chances of such disorders is increased the closer the relationship of the biological parents. (See coefficient of inbreeding.) This is because such pairings increase the proportion of homozygous zygotes in the offspring, in particular deleterious recessive alleles, that produce such disorders.[7] (See inbreeding depression.) Because most recessive alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be carriers of the alleles. However, because close relatives share a large fraction of their alleles, the probability that any such deleterious allele is inherited from the common ancestor through both parents is increased dramatically. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. However, because the increased proportion of deleterious homozygotes exposes the allele to natural selection, in the long run its frequency decreases more rapidly in inbred population. In the short term, incestuous reproduction is expected to produce increases in spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.[8]
There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar immune systems may be more vulnerable to infectious diseases (seeMajor histocompatibility complex and sexual selection).[9]
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