Chromosome number optima
Current understanding
Across animals, chromosome numbers are not uniformly distributed; certain counts appear far more frequently than expected by chance, suggesting that selection, drift, or both may impose soft “optima” on karyotype evolution. In beetles (Coleoptera) — the most species-rich animal order — this non-uniformity is striking and differs sharply between the two major suborders.
In Polyphaga, autosome counts range from 1 to 35 but cluster tightly around a single mode of nine autosomes, a count shared by roughly 29% of all recorded Polyphaga species. This degree of concentration is remarkable for a group spanning hundreds of thousands of species. In Adephaga the picture is more complex: the mean autosome count is higher (≈15.6 vs. ≈10.6) and the distribution is bimodal, with peaks at 11 and 18 autosomes that together account for about 43% of Adephaga records. Drift drives the evolution 2024, Finding 1
These patterns raise two non-exclusive hypotheses. First, the modal counts may reflect genuine fitness optima — chromosome numbers at which meiotic fidelity, recombination landscape, or gene-dosage balance is optimised. Second, the modes may be statistical artefacts of phylogenetic inertia: if the ancestral karyotype of a large, rapidly diversifying clade happened to sit near a particular number, that count will dominate the dataset even without ongoing selection favouring it. Disentangling these explanations requires phylogenetically controlled analyses that can separate within-lineage evolution from among-lineage sampling.
A practical complication is uneven taxonomic sampling. Within Coleoptera, family-level variance in chromosome number correlates positively with the number of records available for that family (r = 0.41), meaning that poorly sampled families contribute disproportionately little to estimates of modal counts and may harbor karyotypic diversity that would shift or broaden the apparent optima. Drift drives the evolution 2024, Finding 1
Supporting evidence
- Adephaga bimodality and Polyphaga unimodality in beetles: Autosome counts in Adephaga peak at both 11 and 18; in Polyphaga a single sharp peak at 9 accounts for nearly a third of all species. Drift drives the evolution 2024, Finding 1
Contradictions / open disagreements
No direct contradictions exist among the currently folded-in findings. However, there is internal tension between the interpretation that modal autosome counts reflect adaptive optima and the observation that sampling bias (r = 0.41 between family record count and variance) could generate apparent modes artifactually. Future phylogenetically informed analyses will need to adjudicate between these explanations.
Tealc’s citation-neighborhood suggestions
- Studies modelling chromosome number evolution under birth–death or Markov frameworks (e.g., ChromEvol, ChromoSSE) would provide a direct test of whether the observed modes exceed those expected under neutral karyotype drift.
- Comparative work on meiotic drive and fusion/fission rates in Adephaga vs. Polyphaga could explain the mechanistic basis for their different distributional shapes.