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The evolution of multicellularity has occurred independently at least 25 times in many different lineages (Bonner 1998, 2000). It likely arose just once in animals (King 2004) and multiple times in bacteria, plants (including land plants and several algae clades), fungi and bacteria (Bonner 2000, Kirk 1998). Multicellularity has also likely been secondarily lost in some lineages (Bonner 2000, Medina et al. 2003). While multicellular organisms such as animals are multicellular throughout most of their life cycle, organisms in other lineages transition between unicellular and multicellular states in response to environmental stimuli (Grosberg and Strathmann 2007). The latter type of multicellularity is present in algae, bacteria, myxobacteria myxomycetes and cellular slime molds (Branda et al. 2001, Lurling and Van Donk 2000, Bonner 1998, Kaiser 2001). == Steps in the evolution of multicellularity == The first step in the evolution of multicellularity was likely the formation of small clusters of cells via incomplete separation between mother and daughter cells during division or via the aggregation of genetically distinct individuals (Kirk 2005, Ratcliff et al. 2012). Once simple clusters form, selection acting between clusters must be greater than selection acting within clusters for the new, multicellular phenotype to be evolutionarily stable (Michod 2005). The evolution of multiple different types of cells, a hallmark of complex multicellularity, is only likely to occur when the evolution of reproductively sterile cells increases colony-level fitness (Willensdorfer 2009). 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「Multicellular evolution」の詳細全文を読む スポンサード リンク
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