What is threshold in biology

Home Education Teaching strategies Threshold concepts. Threshold concepts. What are threshold concepts? What threshold concepts have been identified for biochemistry and molecular biology? Steady state: Living organisms constitute open systems, which constantly exchange matter and energy with their surroundings; yet, net concentrations remain relatively constant over time. This dynamic, yet outwardly stable condition, is referred to as a steady state. Steady state defines the conditions of life under which chemical reactions take place in cells and organisms.

Biochemical pathway dynamics and regulation: Reactions and interactions in biological systems are dynamic and reversible. Chemical drivers result in the emergent properties observed in biological systems.

Additionally, our study treated the four dimensions of troublesome, integrative, liminality, and discursiveness weighted equally in a binary and additive way. Alternatively, one could weight dimensions differently depending on the context and the learning outcomes of the curriculum. In our study, the Outgroup of three advanced learners served as an illuminating comparator as well as an important model group for testing our approach for measuring threshold crossing.

We expected that all three advanced learners would provide strong indicators for threshold crossing i. On the basis of their interview responses, we believe the three Outgroup members existed in a liminal space different from yet proximal to the other 29 undergraduate study respondents in the Pre, Current, and Post groups.

In other words, the Outgroup members already had sufficient mastery of the threshold we were measuring and were skipping steps in logic or using an advanced shorthand to jump to a research domain or area of familiarity that they were more comfortable discussing e.

There was also hesitancy to guess or generate scenarios for explaining variation given the perceived consequences of talking outside their range of research knowledge. For example, one Outgroup respondent said, I am not an ornithologist. Or sometimes Outgroup respondents felt they needed to support their identities as researchers and provide examples to convince the interviewer that they knew what they were talking about.

For instance, YR70 said, There can be subtle variation in the way you turn on a gene. You can imagine a binding site is different between these two [birds]. Despite these issues, we felt the Outgroup served as a valuable comparator to the Pre, Current, and Post groups in the nature of their explanations. Yet we recognize that the context and proximity of the groups for comparison needs to be well defined and bounded. Ideally, this approach for measuring threshold crossing could be used in a longitudinal way, interviewing the same individuals repeatedly over time, before and after they have experienced a particular curriculum pertaining to the threshold concept of interest.

Another aim of our study was to explore the utility of the threshold concept model to inform curricular change. Disciplines are typically bounded by a curriculum e. Beginning with discursive analysis, we found that novice students first expand their variation discourse while engaged in variation-enhanced course work, and then display conformity in language and parsimony, with explanations being similar for the Post group compared with the Outgroup. A vocabulary ranging from 9 to 33 discipline-specific words Figure 3 was adequate to achieve the level of mastery defined in this study using the binary coding scheme Figure 4A for evaluating at least one form of biological variation at the genetic, cellular, or organismal level.

Even though some respondents far exceeded the nine-word benchmark e. While both NN29 Current and AJ19 Post were identified as having crossed the threshold in this study, NN29 used 33 discipline-specific words and AJ19 used 17 discipline-specific words to articulate their explanations. Furthermore, our findings complement a study on systems thinking by Dauer et al.

We recognized discomfort and conflicting or disoriented reasoning when respondents attempted to explain how genetic and cellular variation leads to phenotypic variation, with acknowledgment by students when their thinking became confused.

For some respondents, their self-doubt seemed disturbing, particularly for those in the Outgroup. Although the state of not knowing should be a common feeling for learners at all stages in the learning process, recognition of a liminal understanding summons self-dissatisfaction. Our task as educators is to make space and time for both the cognitive and affective shift and to create opportunities for points of multiple entry, as well as off-ramps and on-ramps, as students move through liminal space of a threshold concept—based curriculum.

Creating time and having patience for moving through liminal space are not only important for threshold concepts, they are essential for promoting metacognition Land et al. Traditional methods of assessment focus, by and large, on vocabulary and accuracy of understanding without recognizing important aspects such as liminality and integration, which are fundamental to learning threshold concepts.

These ideas e. At a time when risk-taking and uncertainty are discouraged in the classroom, when confirmatory investigations are the norm, and when test scores and fixed mind-sets guide educational decisions and outcomes, making time for the liminal space is most needed.

Liminal space needs to be incorporated as an intentional aspect of the curriculum, with students being encouraged to sort knowns from unknowns, take risks and dive into their uncertainties, learn to think creatively and pose questions, generate novel connections, and be comfortable with not knowing.

Instead of factual content knowledge being most prized, in a threshold concepts—focused curriculum, the capacity to tolerate uncertainty, deal with messiness and complexity, think critically, pose questions, and problem solve given ill-structured problems would be emphasized. Our results indicate that two threshold concept dimensions were particularly hard for students to achieve: 1 nontroublesome explanations and 2 integration of multiple biological scales in their explanations.

Here, we will relate our findings to these published results. Specifically, among our sample, essentialist reasoning was the most commonly overused, followed by teleological reasoning. This fits with previous work, where both majors and nonmajors most frequently endorsed teleological misconception statements but most frequently used essentialist reasoning in their written justifications Coley and Tanner, Previous literature also provides evidence that within-species variation is particularly counterintuitive.

In one study, just under half of adults held essentialist beliefs that all members of a biological species are the same Shtulman and Schulz, In another study, researchers described differences in evolutionary experts and novices in card-sorting tasks, problem-solving tasks, and interviews Nehm and Ridgway, They found that evolutionary novices were more likely to hold cognitive biases, such as teleological reasoning—which resembled that of children—while these biases were absent in experts.

This is slightly different from our results, as we found that our Post and Outgroup respondents often demonstrated overly intuitive reasoning in their explanations. However, it is unclear whether this was due to fundamental misunderstanding or a cognitive shorthand to explain variation. This is despite our emphasis on conveying the importance of precise language within the courses. Additionally, we found that respondents seemed to ritualize Mendelian thinking to the point where they tried to apply monohybrid crosses and Punnett square analysis to polygenic traits, even though our curriculum goes to some length to delineate that discrete traits are influenced predominantly by a single gene and continuous traits are influenced by a large number of genes.

Consistent with previous research, these results suggest that variation within species is particularly challenging to accurately explain. Using a threshold concepts model, we also observed that a majority of respondents failed to integrate biological scales in their explanations of how variation changed over one generation.

Similar to our results, Speth et al. Notably, all of the members of our Outgroup demonstrated sufficient mastery with explanations that integrated phenotypic variation at the organismal level with two or more other scales of variation genetic, population, environmental, etc.

Together, our results suggest that, while students are quick to acquire discipline-specific terminology of variation, it takes them considerably longer to develop conceptual genetics knowledge and to integrate the concept of variation among biological scales in order to provide well-reasoned, accurate explanations. The threshold concept model was useful in helping us bring together information, much of which is supported by previous studies, and combine these dimensions toward a deep understanding of variation within species.

Understanding of evolution more generally is even more fraught as a threshold concept, because it involves multiple thresholds converging e. While our analysis has focused on the former, recent work suggests that randomness is a measurable threshold concept that preservice teachers in particular struggle to achieve competence in Fiedler et al.

In addition to the potential oversimplicity of coding already described, a major remaining limitation to the current study was its cross-sectional design. While efficient, this design was limited in its ability to measure transformation in a single student over time, which would have required a long-term longitudinal study.

Future experimentation may aim to use these data to design more directed questions in several contexts that can be given to many students over time with greater efficiency. The main implication for instruction that we have taken away from studying the threshold concepts framework is the importance of deliberately making time in the curriculum for students to be in a liminal space and for us as instructors to empathize with the challenge of being uncertain.

Particularly, our data suggest that it is particularly challenging for learners to explain variation accurately i. So, we find it especially important to implement curricular tasks that focus on the accuracy and integration of variation in a highly formative way. Below, we suggest two specific examples from the literature that may satisfy these parameters: In a lecture-based setting, students can generate models that follow molecular origins to evolutionary outcomes so called gene-to-evolution models , which they iteratively revise throughout the semester with the use of feedback from peers and instructors Dauer et al.

This provides students with the task of integrating biological scales related to variation but also reveals inaccuracies in their thinking. And, doing so in an iterative, formative way, rich with feedback, allows students the opportunity to accept their uncertainty and learn from it. In a lab-based setting, students can give ungraded feedback presentations on research plans related to variation in which they receive input from their instructors and peers on their plans to observe, explain, predict, and measure variation, similar to a research lab meeting Batzli et al.

Through this context, students are provided with feedback about the accuracy i. Further, because all students give feedback presentations in the same period, they share the experience of a liminal space together. No matter the specific implementation plan, we find it especially important that we as instructors create an environment where uncertainty is accepted and treated as a useful step toward deep understanding.

In summary, through our analysis, we found evidence for threshold concept dimensions i. On the basis of our rubrics, we found that only the integrative, liminal, and troublesome dimensions discriminated among different cross-sectional groups aim 2. The discursive dimension was achieved by the majority of respondents in all cross-sectional groups, while adequate accuracy i.

While trying to detect threshold crossing aim 3 , we realized that, by definition, the liminal space and the threshold are hard to define, which also makes the exact point of threshold crossing hard to define.

While we believed that we detected threshold crossing in our study, we realize that our approach and interpretation may be overly simplistic. Instead, we conclude that pinpointing the exact moment of threshold crossing is not as important as identifying the lessons learned from the threshold concept framework as applied to curricular design.

We are grateful for interview transcription by Maddie Batzli and many conversations with colleagues, including Michelle Harris, Emily Jobe, and Robin Forbes-Lorman, whose insights and questions have been invaluable.

Walck-Shannon et al. This article is distributed by The American Society for Cell Biology under license from the author s. It is available to the public under an Attribution—Noncommercial—Share Alike 3.

Louis, MO Search for more papers by this author. Add to favorites Download Citations Track Citations. View article. First- and second-stage coding schemes for each threshold concept dimension a Dimension First-stage coding Second-stage coding Discursive Respondents earned 1 point for each type of variation that was described using discipline-specific words.

All occurrences were summed with equal weight: Oscillating between more than one answer Self-reported mimicry Self-reported discomfort or uncertainty Note that a tentativeness in accepting a new assertion without further data Halmo et al. McCartney et al. Batzli et al. The only different part would be maybe some of the proteins within the beak cells that I guess are coding. But like DNA is universal, but like maybe different … different restrictors on segments of DNA would allow for certain expression [in beaks].

I would argue that they have the same genes. But I think the alleles are different. Or, it might be you have alleles that control whether other alleles can be expressed, and control how many spots you have, what color the spots are, how big the spots are, where the spots are, all of that stuff. I would expect the genes to be variations within like the nucleotide sequence of the DNA. You know like a transcription factor could cause the expression of that gene and lead to more mRNA, leading to proteins that are expressed in the cell to give those pigments ….

And so whatever gene is signaling how much beak grow, there is probably more in the larger bird than the smaller bird. With development, you have a transcription factor that is turning on a gene that expresses white pigment. So differences in transcription factor abundance between cells. They generally have the same genome, and the same genes, but different alleles. However, there are copy number variations and other um insertions and deletions that can lead to inner individual variation in gene number.

Also, I think that environment at different times in different seasons, in different ages, in different sexes, can lead to huge phenotypic effects.

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