托福阅读背景材料(333)：动物迁徙现象及原因

　　托福阅读背景知识/材料之动物迁徙现象及原因，更多托福阅读背景材料/知识尽在新东方在线托福考试频道！

　　2014年5月11日托福阅读真题再现：

　　版本一：某些动物长大以后离开出生地生存，有些不会。主要讲不可以的。举了两个例子。第一个是松鼠，雄鼠长大后飞走，雌鼠不会。第二个例子是狮子，雄狮子长大了以后会离开，原因是打不过原来的首领，被赶跑。雌性狮子则会留在群落帮忙找吃的。

　　版本二：讲动物离开出生点行为，原因一：某鼠离开出生点，雄150米，雌50米，因为能防止近亲繁殖导致基因病，同时雌性在一起能养小鼠方便。原因二：狮子，群内争斗呀，劳什子排挤呀什么的。

　　版本三： 动物的disperse, 刚开始说为什么动物要离开熟悉的food rich的地方而去其他地方。其中讲了一种动物男女的分布是不一样的，女的离原来的家50米，男的150米， 不同的原因是防止近亲结婚导致孩子多病不易存活，另外女的离家近更有益处，家里可以给她提供保护，然后男的要更远的地方对抗敌人，有可能被竞争者replace而离开，然后有个lion的例子

　　托福阅读词汇：

　　squirrel n松鼠

　　disperse v分散

　　Inbreeding n近亲交配

　　genopathy n遗传病

　　解析：本文围绕动物为何离开出生地这个主题展开论证。做题时需注意记录笔记，对于结构化阅读及最后一题的解答有很大好处。动物行为主题是托福阅读常见考点，结构不难理解。需注意各例证和主题的支撑关系。由于条理清晰，最后一题尽量考虑从正面选出，排除为辅。

　　托福阅读相关背景：

　　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.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.This generally leads to a decreasedbiological fitness of a population,(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.

　　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.(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.

　　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).