Evolutionary change is a central, observable feature of the natural world. For the past 150 years, the theory of natural selection has served as the primary (but not exclusive) explanation for evolutionary change (Endler, 1986; Gould, 2002). Specifically, natural selection is a mechanism that explains how the constant production of novel heritable variants--through the actions of mutation, genetic recombination, and sex--differentially persist from generation to generation through nonrandom survival and reproduction. Despite an expansive terminology and empirical body of work on natural selection, biologists agree that three core ideas are necessary and sufficient for explaining evolutionary change by natural selection: (1) the presence of variation; (2) the heritability of variation; and (3) the differential survival and/or reproduction of individuals that differ in heritable traits (Endler, 1986; Nehm & Schonfeld, 2010).
Despite its centrality in the life sciences, evolutionary change by natural selection is still poorly understood by students throughout the educational hierarchy (Gregory, 2009). This poor understanding has been attributed to a wide variety of cognitive, epistemological, religious, and emotional factors, yet there are still remarkably few tools available for validly assessing students' reasoning about natural selection (Nehm, 2006; Nehm & Schonfeld, 2008). This situation is problematic because quality assessments play a central role in helping teachers foster meaningful science learning (National Research Council, 2001), and they could play a similarly important role in improving students' understanding of how natural selection may be used to explain patterns of evolutionary change.
An important recent advance in assessment of natural selection has been the finding that the knowledge and misconceptions that students show vary greatly depending upon the specific contexts in which they are assessed (Nehm & Ha, 2011). For example, some students correctly explain the evolutionary gain of traits (such as the running speed of a cheetah) as being caused by the variability of the traits, their heritability, and the differential survival of organisms that possess the traits; however, these same students seldom mention these variables (variation, heritability, and differential survival) when explaining how traits decline in phenotypic frequency (such as the evolution of flightless birds). Indeed, understanding of one type of evolutionary change is a very poor predictor of understanding the other type. Despite these evident differences in students' own understanding of what is important in explaining evolutionary change, almost all existing assessments fail to probe students' thinking across the range of contexts in which evolutionary change actually occurs. Without assessing this range of contexts, how can teachers identify those instructional strategies that yield the broadest understanding of the chief cause of evolutionary change--natural selection?
Another problem with existing assessments is that they are inflexible, and their utility as diagnostic tools can degrade over time. As an example of this inflexibility, questions from widely used assessments such as those by Bishop and Anderson (1990) and Anderson et al. (2002)--can become familiar to students after repeated exposure, and answers may even be disseminated among students. For teachers interested in understanding their students' reasoning about natural selection, we suggest that there are two fundamental problems that must be solved: (1) assessing contextual competence so that instruction can be planned accordingly and (2) having a tool that can be modified but retains inferences of validity. Here, we introduce a new diagnostic tool known as ACORNS (Assessing Contextual Reasoning about Natural Selection), provide evidence for its validity and reliability, and outline a methodology for teachers to modify the items and to use them as formative assessment tools in the classroom. …