<P> Some authors have argued that flagella cannot have evolved because they can only function properly when all proteins are in place . In other words, the flagellar apparatus is "irreducibly complex". This has long been debunked, because many proteins can be deleted or mutated and the flagellum still works, though sometimes at reduced efficiency . In addition, the composition of flagella is surprisingly diverse across bacteria, with many proteins only found in some species, but not others . Hence, the flagellar apparatus is clearly very flexible in evolutionary terms and perfectly able to lose or gain protein components . For instance, a number of mutations have been found that increase the motility of E. coli . Additional evidence for the evolution of bacterial flagella includes the existence of vestigial flagella, intermediate forms of flagella and patterns of similarities among flagellar protein sequences, including the observation that almost all of the core flagellar proteins have known homologies with non-flagellar proteins . Furthermore, several processes have been identified as playing important roles in flagellar evolution, including self - assembly of simple repeating subunits, gene duplication with subsequent divergence, recruitment of elements from other systems (' molecular bricolage') and recombination . </P> <P> Different species of bacteria have different numbers and arrangements of flagella . </P> <Ul> <Li> Monotrichous bacteria have a single flagellum (e.g., Vibrio cholerae). </Li> <Li> Lophotrichous bacteria have multiple flagella located at the same spot on the bacterial surfaces which act in concert to drive the bacteria in a single direction . In many cases, the bases of multiple flagella are surrounded by a specialized region of the cell membrane, the so - called polar organelle . </Li> <Li> Amphitrichous bacteria have a single flagellum on each of two opposite ends (only one flagellum operates at a time, allowing the bacterium to reverse course rapidly by switching which flagellum is active). </Li> <Li> Peritrichous bacteria have flagella projecting in all directions (e.g., E. coli). </Li> </Ul> <Li> Monotrichous bacteria have a single flagellum (e.g., Vibrio cholerae). </Li>

What are the four main types of flagellar arrangement
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