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