<P> "The fact that independently observing humans see much the same species in nature does not show that species are real rather than nominal categories . The most it shows is that all human brains are wired up with a similar perceptual cluster statistic (Ridley, 1993). On this view we (humans) might have been' wired' differently and different species might now be wired differently from us, so that no one wiring can be said to be' true' or veridical ." </P> <P> Another position of realism is that natural kinds are demarcated by the world itself by having a unique property that is shared by all the members of a species, and none outside the group . In other words, a natural kind possesses an essential or intrinsic feature ("essence") that is self - individuating and non-arbitrary . This notion has been heavily criticized as essentialist, but modern realists have argued that while biological natural kinds have essences, these need not be fixed and are prone to change through speciation . According to Mayr (1957) reproductive isolation or interbreeding "supplies an objective yardstick, a completely non-arbitrary criterion" and "describing a presence or absence relationship makes this species concept non-arbitrary". The BSC defines species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups". From this perspective, each species is based on a property (reproductive isolation) that is shared by all the organisms in the species that objectively distinguishes them . </P> <P> Some philosophical variants of nominalism propose that species are just names that people have assigned to groups of creatures but where the lines between species get drawn does not reflect any fundamental underlying biological cut - off point . In this view, the kinds of things that people have given names to, do not reflect any underlying reality . It then follows that species do not exist outside the mind, because species are just named abstractions . If species are not real, then it would not be sensible to talk about "the origin of a species" or the "evolution of a species". As recently at least as the 1950s, some authors adopted this view and wrote of species as not being real . </P> <P> A counterpoint to the nominalist views in regard to species, was raised by Michael Ghiselin who argued that an individual species is not a type, but rather an actual individual, an actual entity . This idea comes from thinking of a species as an evolving dynamic population . If viewed as an entity, a species would exist regardless of whether or not people have observed it and whether or not it has been given a name . </P>

Who determines whether the two organisms of a species will be exactly similar or not