Developmental origins of normal and anomalous random right-left asymmetry: lateral inhibition versus developmental error in a threshold trait
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Bibliographic record
Abstract
Dramatic examples of right-left asymmetry often inspire adaptive explanations, simply because it is hard to imagine how such forms could not be functionally significant. But are conspicuous morphological asymmetries necessarily adaptive? Surprisingly, in some species where direction of asymmetry is random, asymmetry in bilaterally paired traits may arise as a developmental error in a threshold trait. When cases of asymmetry are rare within a species, they are easily recognized as developmental errors. However, as asymmetrical individuals become more common, or if the asymmetry is in a signaling trait, the temptation to advance an adaptive explanation grows, particularly if the asymmetry is not clearly maladaptive. Several models of the ontogeny of asymmetry are described for both normal and anomalous random asymmetry of bilaterally paired traits. In the absence of selection, each model predicts different expected frequencies of symmetrical and asymmetrical individuals within a species, therefore such frequency distributions can effectively test for different models of development. In normal random asymmetries – where conspicuously asymmetrical individuals predominate – lateral inhibition of one side after the other has transformed appears to be an essential step in development. In anomalous random asymmetries – where conspicuously asymmetrical individuals are relatively rare – no lateral inhibition is required. Other potentially relevant variables include: purely stochastic variation in morphogen levels, useinduced asymmetry, and local (each side independent) versus central ( e . g ., hormonal) signaling. Examples of normal and anomalous random asymmetries are reviewed for several animal groups. A closer examination of the spectacular forelimb asymmetry in empidid dance flies raises doubts about claims that the asymmetry – both its occurrence and its direction – is adaptive, even though enlargement of the forelimbs likely is. Additional studies are required to conclude that this asymmetry is truly adaptive, as opposed to the outcome of random developmental variation in a threshold trait. This dance-fly leg asymmetry illustrates nicely how alternative hypotheses need to be considered before interpreting such variation as adaptive, even in a signaling trait.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it