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题目材料：
Lichens consist of a fungus and an alga living in symbiotic union. Testing the widely held hypothesis that lichen-forming fungi are a closely knit, peripheral group of fungi, a team of scientists compared DNA sequences from 10 lichen-forming and 65 non-lichen-forming fungus species. These scientists classified the lichen forming fungi into five kinds that were more closely related to kinds of non-lichen-forming fungi than to one another. The closest relatives of the lichen-forming kinds included some fungi that benefit, and others that cause disease to, associated organisms. Therefore, lichen-forming fungi probably evolved from various species whose effects on associated organisms ranged from benign to malignant. Furthermore, lichen-forming fungi seem to have given rise to other fungi that act as harmful parasites on lichens themselves. These considerations undermine the long-standing hypothesis that symbiotic relationships normally evolve in an orderly progression from those that are harmful to one partner to those that are mutually beneficial.
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以上解析由 考满分老师提供。
