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I always thought of the efficiency of natural selection in the context of molecular evolution. I.e. that linked selection and smaller population size cause less efficient selection. It took me a while before I even noticed that people also say the efficacy of selection.

When researching the answer myself, I found that older papers talk more often about efficacy (like this one). I also found that besides the natural-selection context, it seems efficiency of selection is also used in the context of breeding (like here). However, you can certainly find cases when used in the context of natural selection. Finally, there are about 4x more papers that contain the words "efficiency" and "natural selection" compared to "efficacy" and "natural selection".

So, is effective and efficient natural selection the same thing? Is "efficacy of selection" and "efficiency of selection" be used interchangeably?

-- EDIT --

I am interested in the use of these two words in the context of natural selection only. Unlike in other contexts, in natural selection dictionary definitions of the two words do not make much sense.

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    $\begingroup$ I think it is a case of Humpty Dumpty in Alice in Wonderland. The words meant exactly what the authors chose them to mean, which unfortunately we cannot tell, because there is no authority that gives these English words precise definitions in biology. As I keep saying, biology is not physics. $\endgroup$ – David Apr 22 at 17:11
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    $\begingroup$ I think that in this case, they are used synonymously, even though they do indeed have a subtle difference in definition. This is a common synonymy in biology, many researchers are not native speakers and there exist many native speakers who do not appreciate the difference between the two enough to use it with precision. For instance, here you see it used more or less interchangeably. Effectiveness, efficacy, efficiency... $\endgroup$ – S Pr Apr 23 at 12:23
  • $\begingroup$ @SPr Thanks, that's already a nearly complete answer. Would you mind also elaborating a bit about the subtle difference in this context? (also if you post it as an answer, I would be happy to accept it) $\endgroup$ – Kamil S Jaron Apr 23 at 14:46
  • $\begingroup$ As a reader, when I read effective selection, it means it works, and is successful to a high degree. For example, 70% ethanol is effective at sterilizing your work bench (negative selection). It does the job well when you wipe several times with it. Conversely, when I read efficient selection, I read it as selection that is accomplished with the least amount of wasted time or effort. Paraformaldehyde or gamma ray radiation are more more efficient at sterilization (they work more stringently and the performance is better), but they are obviously not as practicable. Unsafe and costly. $\endgroup$ – S Pr Apr 26 at 9:08
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You have a word problem here, not really biology.

Efficacy means "power to produce the desired result/effect" Effective means "producing the desired result", stressing the ability to produce that result Efficiency means "degree of being efficient" Efficient in turn means "ability to produce desired result with little waste"

Efficacy and efficiency are not interchangeable as they have different meanings, albeit a subtle difference. You could on the other hand us efficacy and effective as synonyms, though properly the synonym of efficacy is effectivness.

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    $\begingroup$ Appreciate the effort but I did google the definitions of the words before I asked. The problem I have is that there is no "desire" in evolution and it's hard to guess what "little waste" would mean in this context. So I think the question "What is the difference in efficacy of natural selection and efficiency of natural selection?" remains unanswered. $\endgroup$ – Kamil S Jaron Apr 22 at 10:23

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