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I'm thinking in particular of wings on birds that would - I'm guessing - have to progress through stages during which they confer no particular advantage. Or is it that all evolved features must have followed a path of incremental benefit therefore imposing a fundamental limit on what evolution through natural selection can "achieve"? (features that are not beneficial while gradually appearing cannot be evolved)

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I thought i'd mention a thing about recombination, not as an answer since it's too short: two individuals may posses radically different sets of alleles if they have been part of lineages that have not had much contact (e.g. tribesmen of africa vs. urban dwellers) once they mate, alleles that have never met before may co-operate in a way that could lead to drastically novel features. Features that are due to the combination of alleles already existing but whose phenotypic output is entirely novel. – hello_there_andy Mar 21 '14 at 21:20
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Features can evolve (or change already present features) which have no negative effect under the current conditions. You can very well have neutral changes which have no purpose. They can then prove positive later (or in a different environment). Changes are usually small and take place over very long time periods.

An example would be bacteria which mutated one of their enzymes for a energy pathway. Before the mutation the enzyme could only metabolize nutrient A, after it, it is a bit less specific for A, but can also metabolize B. As long as the living conditions are so, that only A is present, this will not change anything. If the conditions are so, that A and B are present and A gets limited, these cells will have a profound advantage over cells, which can only utilize A.

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I see. So wings appeared through neutral changes in biped reptiles without going through detrimental stages? And more generally, the "path" of features never goes through detrimental stages - only neutral and beneficial? – schmop Mar 21 '14 at 21:12

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