What Is Drift? A Response to Millstein, Skipper, and Dietrich

Abstract

Organisms are born; they die; they mate.These events change the make-up of organism-ensembles.Organisms enter an ensemble when they are born and exit when they die; by mating, organisms determine what kind of individual enters by a subsequent birth.This is how births, deaths and matings influence the make-up of organism-ensembles.Since evolution consists of changes to the make-up of organismensembles, let us call these events individual-level selection events or ILSEs.(The significance of the term 'selection' will become evident in a moment.)There are other kinds of ILSE -e.g., migration -but we need not worry about them here.Organism-ensembles also change.For instance, suppose that we have an ensemble of 100 organisms, evenly divided into two kinds, K and K′.An organism of kind K (call it Kris) is killed by a predator; in the meanwhile, an organism of kind K′ (call it Kristie) is born.If there are no other births or deaths, there are now 49 organisms of kind K, and 51 of kind K′.Kind K′ has proportionately increased relative to kind K.This change of ensemble-wide proportions is an ensemble-level selection event or ELSE.That is, it is a change involving an ensemble of individual organisms.What is the cause of this ELSE?Nothing other than that the predator killed Kris, and that Kristie became pregnant.That is: nothing other than ILSEs.After Kris's death and Kristie's birth -these events are both ILSEs -the ensemble-proportions change as a merely mathematical consequence.To put it in another way, the ensemble-level change is mere bookkeeping (cf.Walsh 2004); the causes of change are to be found at the individual level.It is because Kris's death and Kristie's birth result in changes in ensemble-level proportions that I call them individual-level selection events.

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