**Figure 4 | Taxon-wide positional concentration of mature-miRNA k-mer
profiles.** Positional concentration was defined as one minus the normalized
Shannon entropy of a motif’s frequency profile across start positions 0–19;
larger values therefore indicate a less uniform positional distribution.
**a,** Observed-minus-null concentration effects for all 16 dinucleotides
under the position-wise, length-stratified null. CG and UA are highlighted;
the label at the right of each taxon identifies its largest positive
dinucleotide effect. **b,** CG and UA concentration effects under the
within-sequence null (circles) and position-wise, length-stratified null
(diamonds). Symbols are connected because both null comparisons use the same
observed positional profile. **c,** Percentage of the complete dinucleotide
(n = 16) and trinucleotide (n = 64) repertoires showing significantly greater
positional concentration or significantly greater positional uniformity under
the position-wise null. **d,** Descriptive cross-taxon median and interquartile
range of concentration effects for trinucleotides containing CG or UA under
the position-wise null. Values at right give the number of taxa in which the
individual context was significant. Filled symbols indicate
Benjamini–Hochberg-adjusted q ≤ 0.05 and open symbols q > 0.05 where
individual taxon-level results are displayed. Statistics are based on 10,000
permutations per null model.

The within-sequence null independently shuffles each mature miRNA and
preserves its length and mononucleotide composition. The position-wise,
length-stratified null independently permutes each nucleotide column among
sequences of the same length, preserving length-specific nucleotide counts at
every position while disrupting adjacent-base associations.

**Interpretive boundary.** A positive concentration effect means that a motif
is more unevenly distributed across positions than predicted by the specified
null. It does not by itself identify the underlying mechanism. Cross-taxon
medians and interquartile ranges in panel d are descriptive summaries rather
than a phylogenetic meta-analysis.
