The Assessment for Learning disposition has been long established in the literature as a desirable attribute for teachers. To use the biological metaphor of adaptation, assessment for learning has been argued to be a key base pair on the teacher genome. We argue that the selection of the correct genotype for teachers is not enough. What is needed is empirical confirmation that these genotypes are expressed in the appropriate phenotypes, or teacher practices. The data in this study were generated from interviews that explored the phenotype, or practices, of six teachers who self-selected for the favoured genotype using the Teacher Assessment for Learning Literacy Tool. The findings indicate that the Assessment for Learning genotype was not always expressed in the phenotype, or practices of these six teachers. The selective environmental pressure of the Teacher Assessment for Learning Literacy Tool was not enough to activate plasticity in all the teachers. The implications are that there may need to be a combination of environmental pressures in the form of teacher professional learning interventions using the Teacher Adaptive Practice scale in concert with the Teacher Assessment for Learning Literacy Tool as well as an internal mechanism like a teacher selection tool that discriminates between rigidity and plasticity in a teacher's disposition.