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Reasoning about natural selection: diagnosing contextual competency using the ACORNS instrument.

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.

* Natural Selection Reasoning Contexts

What are the contexts for reasoning about natural selection that we would like students to recognize? One might infer that they are--at a minimum the ones addressed in curricula, and--ideally--the major contexts to which evolutionary reasoning applies. High school and college biology curricula seem to aim at a similar goal. They typically contain several different case studies of evolutionary change to illustrate aspects of natural selection and evolution. Well-known examples include Darwin's finches, bacterial resistance to antibiotics, and the evolution of horses (e.g., Campbell & Reese, 2008). Although a rationale for choosing the number and types of evolutionary case studies has never, to our knowledge, been explicitly justified or defended, a likely implicit rationale is that by exploring evolutionary change in a diversity of contexts students will progress toward an abstract conceptualization of natural selection that transcends particular cases. Such abstraction is also viewed as a central feature of knowledge transfer--that is, the ability to apply knowledge learned in one context to a different one (Barnett & Ceci, 2002; Opfer & Thompson, 2008).

How closely does students' reasoning meet the goal of an abstract conceptualization of natural selection? Recent work suggests that students fall short of this goal. Indeed, different biological contexts are associated with distinct patterns of student thinking, with student explanations very often depending on the superficial "cover stories" characteristic of evolutionary scenarios. Differences in reasoning may be revealed by comparing (Table 1): (1) within-species differences vs. between species differences; (2) the gain of traits vs. the loss of traits; (3) familiar species vs. unfamiliar species; and (4) plants vs. animals (Nehm & Ha, 2011; J. E. Opfer et al., unpubl. paper). Typically, detecting students' knowledge and misconceptions in one context will not provide evidence of competency in another context.

These findings of context-dependent learning have a number of important implications for teaching evolution. First, curricula about evolution and natural selection require much care in the choice of the "cover stories" (such as bacterial resistance to antibiotics) that are used to illustrate evolutionary change. Ideally, such examples would represent a diversity of evolutionary scenarios that could be systematically compared and contrasted. In a variety of subject areas (e.g., mathematics), choosing examples that allow systematic comparisons is known to help students identify the variables that are truly important for problem solving (for a review, see Gentner & Colhoun, 2010), and we think it quite likely that the same would be true in learning the important variables that cause evolutionary change via natural selection. Additionally, "cover stories" might be chosen to reveal and address the naive ideas that plague student reasoning. Like students' understanding of the key variables in evolutionary change, misconceptions are also context-dependent, with misconceptions triggered by some contexts being rarely elicited by other contexts (Nehm & Ha, 2011).

No biologist would doubt that targeting meaningful learning of evolution across contexts is essential for making use of evolutionary theory. From this perspective, the crux of biology education is to foster effective evolutionary reasoning across all the branches on the tree of life, not just for a few disparate "twigs." In our view, the first step toward this goal is to employ diagnostic or formative assessment instruments that provide valid and reliable evidence about student thinking across diverse examples. Without such tools, it is simply impossible to know when an instructional intervention has provided students with the tools for making use of natural selection.


To better assess students' abilities to use natural selection to explain evolutionary change, we developed a new diagnostic instrument. The ACORNS is a short-answer diagnostic test modeled after Bishop and Anderson's (1990) widely used instrument. It builds on this prior work by explicitly delineating and expanding the contextual variables central to evolutionary reasoning (Table 1). It treats different "cover stories" as unique scenarios worthy of focused instructional attention. For example, questions prompt reasoning about the evolution of trait gains in familiar animals, unfamiliar animals, familiar plants, and unfamiliar plants because we know that students' understanding displayed in one scenario might lag his or her understanding in another (Opfer & Gelman, 2010).

Furthermore, unlike prior tests, the ACORNS standardizes taxon and trait familiarity among items so that these effects are not conflated with other factors. In novice learners, for example, reasoning about the dodder's haustoria is typically different from reasoning about penguin wings, whereas for experts it is not. Thus, multiple versions of the instrument may be assembled to examine particular reasoning patterns: between-species gain vs. loss (standardizing by animals of comparable familiarity); within-species differences for familiar vs. unfamiliar taxa/ traits (standardizing by animals or plants); and so on. Such flexibility allows teachers to tailor the ACORNS to their own, unique curricula.

A final aspect of ACORNS is that it prompts students to formulate their explanations of evolutionary change from the standpoint of a biologist ("How would biologists explain..."). Some assessments are vague in regard to the vantage point from which a response is to be conceptualized, as well as the audience that the response is intended for. Students' informal explanations are likely to be quite different from their explanations employing academic discourse. Thus, in the ACORNS, students are explicitly prompted to reason and write using scientific language in their responses. In this way, we can separate students' scientific explanatory abilities from their personal beliefs.

While Bishop and Anderson's (1990) test and a modified version known as the ORI (Nehm & Reilly, 2007) have both been shown to produce valid inferences about evolutionary thinking, we also examined aspects of the validity and reliability of our new derivative items. Many different models might be used to establish inferences about the validity and reliability of diagnostic test scores (AERA, NCME & APA, 1999); we employed convergent testing to explore validity, and response consistency to examine reliability (Furr & Bacharach, 2008). Specifically, for convergent validity evidence, we compared 28 undergraduate students' (mean age 19.8; 60% female; 80% White non-Hispanic) performance on three different measures of evolutionary knowledge and misconceptions: (1) clinical interview scores derived from >10 hours of oral questioning (mean 19 minutes/student); (2) CINS multiple-choice test scores (Anderson et al., 2002); and (3) ACORNS short-answer test scores. The overall purpose of this work was to make sure that the ACORNS produced meaningful results that could be trusted by biology teachers.

* Methods

The 20 multiple-choice items on the CINS were tallied as correct or incorrect (20-point maximum). Interview performance was scored on a scale from -1 to +1, based on the overall magnitude of scientifically accurate or inaccurate responses (mirroring the methods of Nehm & Schonfeld, 2008). Interview questions included two ACORNS items, a CINS item, and two novel isomorphic items (see Appendices 1 and 2). ACORNS short-answer responses were tallied for the number of scientific key concepts (e.g., variation, heredity, differential survival) as well as naive ideas (needs, goals, use and disuse, etc.) using the scoring rubrics of Nehm et al. (2010). Reliabilities for the CINS and ACORNS were calculated using Cronbach's alpha (Ary et al., 2002).

* Results

Our detailed analyses were used to determine how well the ACORNS exposed student thinking about patterns of evolutionary change by natural selection. Interview inter-rater reliabilities (using blind scoring) were 75%, and all scoring differences were resolved via deliberation between the raters. Kappa inter-rater reliabilities for ACORNS essay scoring were >0.80, discrepancies of which were also resolved via deliberation. No scoring reliability measures were needed for the CINS, as answers were either right or wrong relative to the answer key. Overall, different raters generated very similar assessment scores, which gives us confidence in scoring consistency.

Does ACORNS validly capture the thinking patterns of students? To answer this question, we compared performance on the ACORNS test to scores derived from an oral interview (considered the "gold standard" in education research) and the multiple-choice CINS test (Figure 1). The strong and statistically significant agreement between clinical interview scores and students' ACORNS scores supports validity inferences (Figure 1). Reliabilities, measured using Cronbach's alpha, were also robust and statistically significant (Key Concept Alpha = 0.77; Misconception Alpha = 0.67; CINS Alpha = 0.75). Interestingly, although the number of naive ideas captured using the ACORNS was significantly and meaningfully associated with naive idea frequencies captured in clinical interviews with students, this was not found to be the case using CINS scores (Figure 1). Given that prior studies have also noted related problems with the CINS (Nehm & Schonfeld, 2008; Battisti et al., 2010), this result is not surprising. Overall, our results indicated that ACORNS scores served as valid and reliable proxies for students' reasoning abilities across different evolutionary contexts.

* Implications for Biology Teachers

Our findings have a number of important implications for biology teachers that we highlight below.

(1) Use ACORNS to assess understanding across multiple contexts. The overarching implication of our work for biology teachers is that contexts or "cover stories" are significant factors in the teaching and learning of natural selection, and the ACORNS test may be used to expose students' context-specific reasoning patterns. Not all naive ideas and not all reasoning patterns will be exposed using one "cover story"; curriculum and instruction must be modified to address this fact. The ACORNS may be used as a diagnostic test before a unit on natural selection and thereby help align instruction with students' learning needs (National Research Council, 2001). Lessons about natural selection must not solely use examples of trait gains in familiar organisms (such as antibiotic resistance), but must also discuss cases of unfamiliar animals and plants, trait loss, etc. (Table 1).

(2) Direct students' attention to how the same explanatory variables apply across the different items in ACORNS. In teaching natural selection, it is helpful to include explicit comparisons across "cover stories" in order to help students identify the variables that are truly important for problem solving. By explicitly and systematically comparing the evolution of Darwin's finches to the evolution of antibiotic-resistant bacteria, it should be easier for students to see how the factors of variability, heritability, and differential survival operate to explain evolutionary change by natural selection. When insight is acquired across different examples that share few superficial features, it should help students use these same explanatory variables across different contexts.

(3) Explicitly address student misconceptions. Given that students' misconceptions (such as "use and disuse") often coexist with students' use of accurate key concepts (Nehm & Ha, 2011), teachers need to make a special effort to combat these misconceptions. Teaching that variation, heritability, and differential survival are necessary for evolutionary change isn't enough--these variables are also sufficient for explaining evolutionary change via natural selection. By contrast, the "need" for a trait (a common student explanation) is neither necessary nor sufficient for evolutionary change.

(4) Using ACORNS as a base, develop a battery of worked examples for assessment and instruction. For obvious reasons, test questions must be changed from time to time. The same is likely true of instructional materials, which should include both familiar and less familiar examples. The reason for this is that highly familiar explanations for repeatedly presented examples (e.g., peppered moths) are often memorized by students but not understood. To estimate how familiar students already are with particular species and traits, we recommend using GoogleLabs Books Ngram Viewer (GoogleLabs, 2011) or Google Ranks. In our study, the frequency of species and traits in Ngram, as well as ranks in Google searches, followed general patterns of taxon and trait familiarity that one would anticipate (Figure 2). This approach may be a useful starting point for building and attempting to standardize a new battery of taxon/ trait combinations for instruction and assessment. Additional studies are being conducted to expand upon our compendium of taxon/trait combinations that are realistic and of comparable familiarity and difficulty (see Appendix 2).

One potential drawback of using the ACORNS is that it requires scoring students' written responses, which is more time consuming and requires more training and expertise in evolution than scoring a multiple-choice test (Nehm & Haertig, 2012). Nevertheless, a series of studies exploring automated computer scoring of written responses to ORI and ACORNS items have shown great promise (Nehm & Haertig, 2012; Nehm et al., 2012). Progress on these efforts can be followed online at

DOI: 10.1525/abt.2012.74.2.6

Appendix 1. Selected quotations from students representing each
of the three possible interview scores and their corresponding
CINS score.

Interview   Selected Quotations from   Selected Quotations from   CINS
Score         Interview Responses          ACORNS Responses       Score

1           "In this ancestral         "In a population of         20
            species of possum there    ancestral, tendril-less
            would have to be a         grapes some of the
            population, and within     individuals had a
            that population, the       tendril-like structure.
            ones that had tails,       This structure might
            some would either have a   have allowed those
            mutation that would give   grapes to grow taller
            a smaller or almost no     and have better support
            tail. And so that          and were able to
            variant of opossum would   reproduce more. Because
            exist in the population.   they were more fit to
            And some sort of an        have more offspring, the
            environmental or even a    trait for the tendril
            sexual pressure would be   structure became more
            placed on those possums    frequent in the
            that would favor the       population. After time
            ones with shorter or no    and generations that
            tails. So in the next      trait was more and more
            generations, the possums   frequent, and some
            that, I guess didn't       individuals had even
            have tails were more       more effective tendril
            favored, so they had       structure, which became
            more offspring, so they    more frequent.
            were more fit, and the     Eventually fully formed
            frequency of those genes   tendrils such as we see
            would increase each        in the current
            generation until the       population were present
            population, I guess,       in all individuals of
            didn't have tails at       the new species."

0           "I think this question     "Tendrils help to anchor    15
            is kind of similar to      a plant to a branch or
            the last question except   post and aid in it's
            the teeth part is easier   horizontal and vertical
            to understand because      growth. A plant with
            that's something that's    stem tendrils is less
            used with humans as well   likely to be damaged by
            for food consumption. So   wind or displaced by
            a snail that didn't have   animals. Tendrils could
            teeth and now it has       have increased the
            teeth, it's descendent     survival rate in grape
            has teeth ... there might  species."
            have been ... does that
            mean that all of ...       "Generally petals serve
            when we say that like      as an attraction device
            this is the ancestor       for pollinators like
            does that mean that        birds and insects. A
            population is the          lilly without petals is
            ancestor or that single    probably in existence
            organism is the            because it now utilizes
            ancestor? ... I don't      wind to aid in
            know, I'm having a really  pollination. It is
            hard time, I'm sorry ...   possible that animal
            maybe the teeth in the     pollinators were not
            ancestral species          helping plant reproduce
            weren't used so they       as well as the wind
            kind of like faded out     pollinated (no petal)
            in the population's        type."
            genetics. But then
            something happened, and
            the few individuals that
            had teeth ... I mean ...
            I don't know ... were
            the ones that evolved
            into this new species
            that had teeth."

-1          "So I suppose the snail    "Biologists would           10
            with teeth needed it       explain this evolution
            maybe for eating           by saying that grapes
            purposes or, you know,     lacked tendrils and
            as protection from         selective pressures in
            predators. So overtime     which it under went
            the snail that didn't      basically forced them to
            have teeth needed to       gain tendrils in order
            find something that        to survive." "The living
            could, you know, maybe     snail species currently
            it no longer had           has teeth because it
            something that it can      needed another mechanism
            eat without teeth so it    to fight of predators
            needed to evolve teeth     possibly or a new
            in order to eat or like    mechanism for finding/
            I said to fight off        eating food."
            predators, so I guess in
            that way it would need
            teeth in the long run.
            So over time things
            change, so we went from,
            you know, not having
            legs to having legs
            because we needed them,
            you know as time
            progresses, years went
            on, or a large amount of
            time, the snail needed
            teeth as a means for

Appendix 2. ACORNS items and formats.

Examples          Item Formats

Gain              How would biologists explain how a living (Taxon)
                  species with (Trait) evolved from an ancestral
                  (Taxon) species that lacked (Trait)?

Loss              How would biologists explain how a living (Taxon)
                  species lacking (Trait) evolved from an ancestral
                  (Taxon) species that had (Trait)?

Within species    How would biologists explain how some individuals of
                  (Taxon) with (Trait) originated within a population
                  of (Taxon) species that lacked (Trait)?

Between species   How would biologists explain how a species of
                  (Taxon) with (Trait) evolved from an ancestral
                  (Taxon) species that lacked (Trait)?

Familiar taxa +   * Bacteria, antibiotic resistance
traits for use    * Cactus, spine
in each item      * Cheetah, speed
                  * Elm, winged seed
                  * Fish, fins
                  * Fly, wing
                  * Grape, tendrils
                  * Lily, petals
                  * Locust, DDT resistance
                  * Mouse, claws
                  * Oak, nut
                  * Opossum, tail
                  * Penguin, flightless
                  * Rose, thorns
                  * Salamander, eyesight
                  * Snail, foot
                  * Snail, poison
                  * Snail, teeth

Unfamiliar        * Dodder, haustoria
and unknown       * Labiatae, Pulegone
taxa + traits     * Prosimian, tarsi
for use in each   * Shrew, incisors
item              * Suricata, pollex


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ROSS H. NEHM is Associate Professor in the School of Teaching and Learning and the Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, 1945 N. High St., Columbus, OH 43210; e-mail: ELIZABETH P. BEGGROW ( and MINSU HA ( are Ph.D. students in the School of Teaching and Learning, The Ohio State University. JOHN E. OPFER is Associate Professor of Psychology, The Ohio State University, 1835 Neil Avenue, Columbus, OH 43210; e-mail:

Table 1. Differences in students' reasoning about natural selection.

 Reasoning                                       ACORNS Items for
  Contexts         Reasoning Patterns          Revealing Reasoning

Immunity/      Immunity and resistance      (A) How would biologists
resistance     are unique reasoning         explain how a living bed
vs. other      contexts that elicit naive   bug species with
trait          ideas about "adapting"       resistance to a pesticide
changes        similarly to adjustment or   evolved from an ancestral
               acclimation, whereas other   bed bug species that
               trait-change scenarios do    lacked resistance to the
               not elicit the same types    same pesticide? (B) How
               of concepts or naive         would biologists explain
               ideas.                       how a living mosquito
                                            species resistant to DDT
                                            evolved from an ancestral
                                            mosquito species that
                                            lacked resistance to DDT?

Within vs.     For the same taxon (e.g.,    (A) How would biologists
between        birds), within a species,    explain how a species of
species        biological concepts such     flightless birds evolved
differences    as mutation, sex,            from an ancestral bird
               recombination, and           species that could fly?
               heredity are commonly used   (B) How would biologists
               by students to explain       explain how some
               biological differences. By   individuals of flightless
               contrast, naive ideas are    birds originated within a
               much more prevalent in       population of bird species
               between-species              that could fly?
               explanations (e.g., in

Gains vs.      For the same taxon/trait     (A) How would biologists
losses of      (e.g., rose thorns),         explain how a living rose
traits         students are typically       species with thorns
               much more adept at using     evolved from an ancestral
               scientific ideas to          rose species that lacked
               explain the gain of          thorns? (B) How would
               thorns, whereas              biologists explain how a
               significantly more naive     living rose species
               ideas are invoked in         lacking thorns evolved
               situations involving the     from an ancestral rose
               loss of, for example, rose   species that had thorns?

Animals vs.    The types of naive ideas     (A) How would biologists
plants         used to explain              explain how a living mouse
               evolutionary change are      species with claws evolved
               typically different          from an ancestral mouse
               between animals and          species that lacked claws?
               plants. Intentional and      (B) How would biologists
               "use and disuse"             explain how a living lily
               explanations are more        species without petals
               common for animal items,     evolved from an ancestral
               whereas teleological         lily species that had
               explanations are more        petals?
               common for plant items.
               Overall, plant evolution
               appears to be more
               difficult for students,
               perhaps because plants are
               often less familiar to
               students (see below).

Familiar vs.   Students demonstrating       (A) Dodder, a plant
unfamiliar     competency in evolutionary   species, have haustoria.
taxa/traits    reasoning using familiar     How would biologists
               taxa/traits often have       explain how the dodder
               difficulty abstracting       species with haustoria
               their thinking to            evolved from the ancestral
               unfamiliar cases (e.g.,      species that lack
               dodder haustoria).           haustoria? (B) How would
               Students often believe it    biologists explain how a
               is not possible to solve     living Suricata species
               the problem without          that lacks a pollox
               knowing how the trait        evolved from an ancestral
               functions, which likely      Suricata species that had
               indicates the absence of     a pollox?
               an abstract model of
               natural selection.
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Title Annotation:Assessing Contextual Reasoning about Natural Selection
Author:Nehm, Ross H.; Beggrow, Elizabeth P.; Opfer, John E.; Ha, Minsu
Publication:The American Biology Teacher
Article Type:Report
Geographic Code:1USA
Date:Feb 1, 2012
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