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I often hear that natural selection, or simply, selection is a "force".

e.g.:

the forces of directional selection on correlated life history characters, and an adaptive topography

Source: Lande, R. (1982). A quantitative genetic theory of life history evolution. Ecology, 63(3), 607–615.

because, as is becoming increasingly appreciated, long-term studies (> one year) are necessary to understand the selective forces affecting territoriality (e.g. see MacLean and Seastedt 1979).

Source: Millington, S. J., & Grant, P. R. (1983). Feeding ecology and territoriality of the Cactus Finch Geospiza scandens on Isla Daphne Major, Galápagos. Oecologia, 58(1), 76–83. http://doi.org/10.1007/BF00384545

See also this one:

Selection is a vector having intensity and direction.

Source: Simpson, G. G. 1944. Tempo and mode in evolution. Columbia University Press, New York.

But I've also heard that selection is not a force and that should be thought as a "process" taking the analogy of a chemical reaction.

Natural selection is, in a sense, a "force" that can "act" on gene frequencies or trait value distributions, and this is the most common sense in which "act" and "force" are used. But this is only a vague and most improper analogy with physics. [...] The use of "force" in describing natural selection is an instance of confusion between the change in an object and the changes in relationships among objects, and can also be an artifact of typological (class) thinking. 

Source: Endler, J. A. (1986). Natural Selection in the Wild. Princeton University Press.

It also seems that the way we can calculate selection (using the famous Lande and Arnold 1983 paper), using a vector field, which can be thought as a field of forces. Is there a conclusion to this debate?

Is selection a process or a force?

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    $\begingroup$ I'm voting to close this question as off-topic because it is about linguistics, not biology. $\endgroup$
    – David
    Nov 7, 2017 at 8:26
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    $\begingroup$ If the debate was only about semantics and not biology, then we would reconsider the term "natural selection" which is not the purpose of this question. If you want to talk about the semantics, see Darwin's letter when he says that "I suppose natural selection was bad term; but to change it now, I think, would make confusion worse confounded. Nor can I think of better; Natural preservation would not imply a preservation of particular varieties & would seem a truism; & would not bring man's & nature's selection under one point of view." My point is a clarification on a biological concept. $\endgroup$
    – M. Beausoleil
    Nov 7, 2017 at 13:23
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    $\begingroup$ You may think your point is about biology but the people who answered all discuss semantics. Like many questions about evolution it is unsuitable for SE Biology in that it doesn't have a real scientific answer, and the intension in posing the question is more to initiate a discussion than to answer a problem. Tests of concept. How will you decide which answer is correct? How will accepting an answer as correct alter your future research or thinking? $\endgroup$
    – David
    Nov 7, 2017 at 13:39
  • $\begingroup$ Artificial preferential selection of your clothes in the morning is not a force. the force is the weather. You need to read background texts. A force is not a selection. A force is a pressure and it has encourages selection. $\endgroup$
    – bandybabboon
    Nov 8, 2017 at 5:44

2 Answers 2

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The main difficulty in this question is to define the terms process and force. As such it is as much a question of linguistic than a biology question.

Process

merriam-webster list a number of definitions. Below each definition I am answering wether NS would fit this definition.

progress, advance

No, NS is not a progress

something going on

Well... I guess, yes!

a natural phenomenon marked by gradual changes that lead toward a particular result.

Yes!

a continuing natural or biological activity or function

Yes!

a series of actions or operations conducing to an end

Not sure, how action is defined here...

the whole course of proceedings in a legal action

Nope!

the summons, mandate, or writ used by a court to compel the appearance of the defendant in a legal action or compliance with its orders

Nope!

a prominent or projecting part of an organism or organic structure a bone process a nerve cell process

Nope!

conk

What?

Force

From dictionnary.com, a force can be defined as

physical power or strength possessed by a living being

Nope!

strength or power exerted upon an object; physical coercion

Nope!

strength; energy; power; intensity

Hum.... no!

In physics a force is typically defined as (from wiki)

[..] a force is any interaction that, when unopposed, will change the motion of an object.

Then, no!

Conclusion

Really it is all a matter of what you want to call a process or a force. We know how NS is defined. The rest of the work is pure terminology about what you like to call a process and a force. There is no debate here. There is only people using different definition and allowing themselves to use some terms with a loose definition.

Process

I am personally quite happy calling NS a process following certain of the above definitions.

Force

I am personally not 100% happy about calling NS a force because the term 'force' already has a definition in science under which NS is not a force. Many people call NS a force because it leads to a directional effect on allele frequency. If you vision, the allele frequency as a position and moving through allele frequencies a movement, then NS is a force acting on the object that is the allele frequency. However, this is just an analogy.

Note by the way that diffusion equations can be used to describe change in allele frequency due to genetic drift (diffusion term) and NS (drift term) just like they are used to describe movement of particles. Btw, the fact that genetic drift matches the diffusion term and NS matches the drift term in diffusion equation makes me think that "genetic drift" would better be named "genetic diffusion".

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  • $\begingroup$ Interesting. I would also call natural selection a process that, in order to work, has to fill 4 main criteria. I find it funny that 99.9% of the articles in biology talking about selection, call it a "force". I'm not sure why people prefer this way to define natural selection. $\endgroup$
    – M. Beausoleil
    Nov 7, 2017 at 13:21
  • $\begingroup$ I'm curious about the way you would write a sentence such as "selection is pushing the distribution" or "selection acts against" or anything similar to this. Basically, based on a definition of selection, what would be the preferred way of talking about it? Is selection described as a force since Lande and Arnorld's paper on selection gradients? $\endgroup$
    – M. Beausoleil
    Oct 10, 2018 at 20:48
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Natural selection is a selection. Selection is an act of selecting and also the result of the act: a particular choice; choice of a particular individual or individuals; a number of selected individuals. What is selected? That which survives. What survives? That which reproduces itself. Process is the fact of going on or being carried on, as an action, or a series of actions or events. Selection is a process. Natural selection is a natural process.

Natural selection is natural. Natural pertains to nature. Nature is phenomenal. All phenomena are motions. Power is the ability to put things in motion. Phenomena are powerful motions. Force is an attribute of (powerful) motion. Force has a magnitude and direction, but no goal to reach. There are many forces acting on the individual's ability to survive. To survive is the individual's goal; if not, its chances to survive decrease. What survives? That which is strong enough to reproduce. Natural selection is a powerful selection.

Natural selection is a powerful process.

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