In bird species, nest parasitism, in which female birds lay eggs in other individuals' nests, is often observed. If the host is the same species as that of parasite, it is called 'intraspecific nest parasitism', Otherwise it is called 'interspecific nest parasitism'. About 1% of all bird species have been observed to be interspecific parasites, although the percentage seems to be much higher in recent reports. The evolutionary process of nest parasitism is an important issue, although it is difficult to study in the field or the laboratory. Therefore, the evolution of parasitism has been studied by mathematical analysis, which is very useful for clarifying of the evolutionary processes and conditions of nest parasitism. In this paper, factors affecting the evolution of parasitism are discussed, and mathematical studies of the relationship between these factors and the evolutionary processes are reviewed.

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　種間交雑や遺伝子浸透という事象は，進化研究においてこれまであまり重視されてこなかった．新しい系統群が生じるのは，祖先集団が二つに分岐する過程が主であり，一旦分化を始めた二集団が再び融合するような現象は例外的に扱われてきたためである．しかし近年の分子生物学的手法の発展，とりわけミトコンドリアDNAに限らず様々な核DNAの遺伝子領域が分析対象となってきたことにより，生物集団間に生じる交雑や遺伝子浸透の重要性が再評価されている．本稿は，鳥類を対象にした最近の研究事例について，集団間に種間交雑や遺伝子浸透が生じた際に，（1）二系統の分化の程度が拡大する場合，（2）二系統が一つに融合する場合，（3）交雑により新たな系統が形成される場合，（4）系統が維持されつつ遺伝子浸透が生じる場合に分けて紹介し，鳥類の進化や保全における種間交雑や遺伝子浸透の重要性を概説する．

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