What is the difference between effective and efficient selection?

Question

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.

Answer 1

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.

Answer 2

The distinction made by @bob1 makes perfect sense:

• Effective selection applies to the situation where selection may or may not work: e.g., in a small population subject to the random drift, the selection may be ineffective in the sense that deleterious alleles may fix with nearly the same probability as beneficial ones, the outcome thus nearly independent on the selection. On the other hand, in a sufficiently large population the outcome is largely determined by selection, i.e., the selection is efficient, as suggested in the first reference given in the OP:

Deleterious alleles can reach high frequency in small populations because of random fluctuations in allele frequency. This may lead, over time, to reduced average fitness. In this sense, selection is more “effective” in larger populations.

• Efficient selection refers to the cost of the selection, as suggested by the second OP reference:

A new index of selection efficiency, i.e. the ratio of the probability of achieving the desired genetic gain in a target population to the cost expended for that gain, is introduced to define the optimum selection procedures that give the most opportunities for producing new commercial varieties.

The cost here may mean very different things - the actual financial cost to the breeder, the energy expenditure by organisms, the number of generations necessary for achieving the desired outcome. Thus, selection may be effective in the sense that we do arrive at the desired resultat, but inefficient in the sense, e.g., that it takes very long time to arrive at this resultat.

Brief googling for efficient selection confirms that this is often the sense in which the term is used.

Nevertheless, the third reference in the OP essentially speaks of efficiency in the same sense as effectiveness/efficacy, which is probably just poor English usage (I myself was not quite clear about the difference before reading @bob1's answer).