One potential mechanism for nonclassical resistance is evolving traits that give rise to higher within‐host parasite densities; this may offer protection against drugs by increasing the likelihood that some (genetically identical) parasites survive treatment (White, 1998). Experimental rodent malaria infections confirm that more virulent parasite strains, with faster within‐host replication, survive better in drug‐treated hosts (Schneider, Chan, Reece, & Read, 2008; Schneider et al., 2012). But within‐host densities are at least in part governed by a resource allocation trade‐off in malaria and other sexually reproducing parasites: achieving higher within‐host densities comes at the cost of producing fewer specialized sexual stages (gametocytes) that are required for transmission (Carter et al., 2013; Pollitt et al., 2011), as a parasite in a given infected host cell can follow only one of the two developmental routes. Transmission investment—by convention referred to as the conversion rate—varies plastically within artificial culture, increasing as conditions become more crowded (Bruce, Alano, Duthie, & Carter, 1990). While conversion rate can change plastically in response to changing environmental conditions, data suggest that there is parasite genetic variation for patterns of conversion (Pollitt et al., 2011; Birget, Repton, O’Donnel, Schneider, & Reece, 2017) and that this variation can be selected upon (reviewed in Bousema & Drakeley, 2011). It is well known, for example, that serial passage and culture experiments, which by their nature select for faster within‐host replication, result in reduced transmission investment (Dearsly, Sinden, & Self, 1990; Sinha et al., 2014; reviewed in Carter et al., 2013). Similarly, artificial selection for attenuation in a related parasite, Eimeria, resulted in indirect selection for earlier investment in transmission, which translated into a substantial reduction in total transmission potential (McDonald & Shirley, 2009). Therefore, conversion rates represent an evolvable parasite trait essential to transmission, and the challenge is to explore if and how drug treatment might alter parasite strategies.