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How do defenders compare natural selection to gas pressure as a scientific concept?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Philosophy of biology Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle, c. 325 bce; in the collection of the Roman National Museum.

In the philosophy of biology, defenders of natural selection use the concept of gas pressure to counter the charge that selection is not a "real" force because it cannot be observed directly as a discrete entity.1

Critics often argue that if one looks for a "force" of selection, one observes only individual organisms living, reproducing, and dying, rather than a single, distinct phenomenon. Defenders argue that this criticism is overly reductionistic. They point out that in other scientific fields, it is standard practice to treat certain phenomena as discrete entities even when they are actually convenient ways of describing complex patterns of events at a higher level of generality.1

For example, if one were to examine gas pressure, one would see only individual molecules colliding with one another and with the walls of their container. Despite this, no scientist concludes that gas pressure does not exist. Similarly, defenders argue that the fact that there is nothing to see beyond the lives and deaths of individual organisms does not invalidate the existence of natural selection as a scientific concept.1