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The species problem is a mixture of difficult related questions that often come up when biologists define the word "species". There are a wide range of approaches to defining how we identify species and how species function in nature; each approach is known as a species concept. The number and types of species concepts which exist are constantly changing, but there are at least 26 recognized species concepts. Biological reality means that a definition that works well for some organisms (e.g., birds) will be useless for others (e.g., bacteria) particularly due to concerns regarding if the species reproduces asexually or sexually. The scientific study of the species problem has been called microtaxonomy. One common, but sometimes difficult, question is how best to decide which species an organism belongs to, because reproductively isolated groups may not be readily recognizable, and cryptic species may be present. There is a continuum from total reproductive isolation to panmixis. Populations can move forward or backwards along the interbreeding continuum, at any point meeting or failing the criteria for different species concepts. Thus, it is difficult to define reproductive isolation. Some separately evolving groups may continue to interbreed to some extent, or begin interbreeding again after a period of isolation, and it can be a difficult matter to discover whether this hybridization affects the long-term genetic make-up of the groups. Many of the debates on species touch on philosophical issues, such as nominalism and realism, as well as on issues of language and cognition. The current meaning of the phrase "species problem" is quite different from what Charles Darwin and others meant by it during the 19th and early 20th centuries. For Darwin, the species problem was the question of how new species arose: speciation. == Confusion on the meaning of "''Species''" == Species is one of several ranks in the hierarchical system of scientific classification, called ''taxonomic ranks''. Even though it is not disputed that ''species'' is a taxonomic rank, this does not prevent disagreements when particular species are discussed. In the case of the Baltimore oriole (''Icterus galbula'') and Bullock's oriole (''I. bullockii''), two similar species of birds have sometimes in the past been considered to be one single species, the northern oriole (''I. galbula''). Currently, biologists agree that these are actually two separate species, but in the past this was not the case. Disagreements surround what the best criteria are for identifying new species. In 1942, Ernst Mayr wrote that, because biologists have different ways of identifying species, they actually have different species concepts. Mayr listed five different species concepts, and since then many more have been added. and the .〔 〕 The question of which species concept is ''best'' has occupied many printed pages and many hours of discussion.〔 The debates are philosophical in nature. One common disagreement is over whether a species should be defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species an organism belongs to. For many species, all of the individuals that fit the defining criteria also make up a single evolving unit, but it might not be known whether that is the case. These two different ways of thinking about species, as a category or as an evolving population, may be quite different from each other. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Species problem」の詳細全文を読む スポンサード リンク
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