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智课网TOEFL备考资料 托福阅读真题文档免费下载【答案+解析+译文】 摘要： 托福阅读真题文档共有30套，应有尽有，全部来源托福阅读题库，80%的学生因为拥有小马版的托福阅读真题文档而成功考得理想成绩，你还等什么？该出手时就出手，赶快点击下载吧！  托福 阅读真题文档里面包含有名师解析和翻译文档，完整版是由 小马 的 托福阅读 专家团队精心撰写，12年来帮助90万的学生成功拿到满意offer，是广大学生必不可少的阅读备考资料，下面我们就来了解下托福阅读真题文档【答案+解析+译文】的部分内容。  托福阅读真题文档免费下载【答案+解析+译文】passage1  Symbiotic Relationships  A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species. There are three main types of symbiotic relationships: parasitism, commensalism, and mutualism. The first and the third can be key factors in the structure of a biological community; that is, all the populations of organisms living together and potentially interacting in a particular area.  Parasitism is a kind of predator-prey relationship in which one organism, the parasite, derives its food at the expense of its symbiotic associate, the host. Parasites are usually smaller than their hosts. An example of a parasite is a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from its host. Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. As an example, plants make chemicals toxic to fungal and bacterial parasites, along with ones toxic to predatory animals (sometimes they are the same chemicals). In vertebrates, the immune system provides a multiple defense against internal parasites.  At times, it is actually possible to watch the effects of natural selection in host-parasite relationships. For example, Australia during the 1940 s was overrun by hundreds of millions of European rabbits. The rabbits destroyed huge expanses of Australia and threatened the sheep and cattle industries. In 1950, myxoma virus, a parasite that affects rabbits, was deliberately introduced into Australia to control the rabbit population. Spread rapidly by mosquitoes, the virus devas