Are there any known traits (in any organisms) that serve exclusively one purpose? (Counterexample: Giraffes long neck help them reach less available food AND communicate sub-sonically).

If so is that the exception rather than the rule?

This will be used for a human evolution paper, in which I would suggest that the descended hyoids in hominins and modern Homo sapiens could have serve multiple purposes, such as size exaggeration and language. I want to debate an argument that a similar descension in deer evolved, or was selected for, for only one use (i.e. size exaggeration) and that this correlates to human larynx evolution, by citing literature (if available) that demonstrates that organisms use features for multiple purposes more often than not.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – AliceD May 3 '17 at 18:46
  • $\begingroup$ if you mean a physiological trait, a butterfly wing with a fake eye is only there to look like a fake eye. when moth species turned black during the coal peaks of the industrial revolution, it was to make them hide against the black backgrounds. $\endgroup$ – com.prehensible May 4 '17 at 14:45
  • $\begingroup$ There is no rule to this, advantage is advantage however it is achieved. Slection does not care how it yields a net advantage just that it does. $\endgroup$ – John May 4 '17 at 15:02

If by trait you mean any phenotypic trait there are boatloads of enzymes which have only one function. Phosphofructokinase-1 is good only for adding another phosphate to fructose-6-phosphate.

If you want big things there are plenty of those too. The gallbladder's only function is to add bile to the intestinal contents. The quadriceps femoris has only the function of extending the leg at the knee. The abducens nerve controls the lateral rectus muscle of the eye and that is all. Beta cells in the islets of langerhans in the pancreas are only good for making insulin. You can't make them do anything else but that.

  • $\begingroup$ Actually the rectus femoris part of the quads also crosses the hip joint and thus in involved in hip flexion as well. $\endgroup$ – kmm May 4 '17 at 1:41
  • $\begingroup$ You seem to be thinking only of molecular function but even an enzyme with a single molecular function can have multiple cellular functions, be involved in various pathways and processes etc. There are actually relatively few proteins involved in one and only one function and it is very likely that they too are involved in many we just don't know about. Same for the rest of your examples: that we know of only one function does not mean there are no more. $\endgroup$ – terdon May 5 '17 at 11:24
  • $\begingroup$ @terdon: If I play the bass drum, but with the band I play many different pieces, is my function different with each piece? Or am I performing the same function in different contexts? $\endgroup$ – Willk May 5 '17 at 21:05
  • $\begingroup$ @Will that's a very, very long and complex discussion actually. The meaning of "function" in biology is very unclear. Especially when you consider the different levels one can function on (from the molecular to the cellular to the tissue or organismal, from complex systems and emergent properties down to simple chemical reactions). If a receptor kinase activates the same MAPK pathway in different tissues but results in opposite effects is it still the same function? What if the same receptor activates a different pathway? $\endgroup$ – terdon May 5 '17 at 22:40
  • $\begingroup$ In fact, PFKA1 which you mention has multiple functions: ATP binding, fructose binding, metal binding, it is involved in the metabolism of fructose and of mannose, it exists with three different types of subunits in different tissues. Is that the same function? Is it even the same protein? It all depends on how you define function and after spending 4 years doing a post doc on this sort of thing, all I can say is that it is very hard to define. $\endgroup$ – terdon May 5 '17 at 22:44

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