 Throughout the 20th and 21st centuries, biologists have struggled to detect drift empirically. In particular, they have experienced challenges in differentiating cases of drift from cases of selection. The Causal Process Account of Drift in particular can help to make sense of why this is so, as will be discussed below. Drift and selection may be different sorts of causal processes (indiscriminate and discriminate sampling, respectively), but they can produce similar outcomes. Biologists have thus struggled to identify distinctive outcomes for drift and selection, only to find underdetermination re-emerge after biologists modified or added to their assumptions about the relevant processes (for a clear characterization of this, see Dietrich and Skipper 2007). Relatedly, biologists have also disagreed over the relative prevalence of drift and selection (Beatty 1995, 1997); as noted above, these disagreements were there almost from the outset with Fisher’s response to the Hagedoorns.