Diversification Rates
Current understanding
A central question in macroevolution is whether intrinsic genomic traits — such as karyotype structure or meiotic drive — leave a detectable signature on lineage diversification rates (speciation minus extinction). Work on mammals has begun to test these hypotheses quantitatively, yielding both null results and striking rate heterogeneity depending on the trait examined.
One long-standing prediction of the chromosomal speciation hypothesis is that lineages with structurally “mismatched” karyotypes (where heterozygotes suffer meiotic costs) should diversify faster than those with matched karyotypes. A BiSSE analysis across mammals found no support for this prediction: matched and mismatched karyotypes are associated with statistically indistinguishable net diversification rates Blackmon et al. 2019, Finding 1. This constitutes an important macroevolutionary null result, suggesting that if karyotype mismatch promotes speciation, the effect is too small or too clade-specific to surface at the scale of the mammalian phylogeny.
By contrast, the rate at which meiotic drive polarity switches — a proxy for the frequency with which genomic conflict is reshuffled across the tree — varies enormously across mammalian subclades. Cetartiodactyla show the highest switching rate, with a mean waiting time of approximately 10.8 million years between transitions, while Primates show the lowest rate, with a median waiting time of approximately 90.9 million years Blackmon et al. 2019, Finding 2. This nearly order-of-magnitude difference implies that the genomic conflict landscape evolves on very different timescales in different mammalian orders, a pattern that could interact with — but apparently does not simply translate into — differences in net diversification rate.
Together, these results paint a nuanced picture: karyotype-level variation per se does not predict diversification rate differences detectable at the mammalian scale, yet the underlying meiotic drive dynamics that may generate karyotype change are themselves highly heterogeneous across clades.
Supporting evidence
- Blackmon et al. 2019, Finding 1: Karyotype morphology (matched vs. mismatched) shows no detectable effect on speciation or extinction rates under BiSSE across mammals.
- Blackmon et al. 2019, Finding 2: Meiotic drive polarity switching rates range from a mean waiting time of ~10.8 Myr in Cetartiodactyla to a median waiting time of ~90.9 Myr in Primates.
Contradictions / open disagreements
The null result for karyotype-driven diversification Blackmon et al. 2019, Finding 1 must be weighed against the known statistical limitations of BiSSE: the method is prone to false positives — and by extension potentially false negatives — on phylogenies with background rate heterogeneity. The authors themselves demonstrate elevated false-positive rates for diversification inference on the cetacean subtree in simulation. It remains possible that karyotype mismatch drives diversification within particular subclades but that the signal is diluted or confounded at the level of the full mammalian tree.
Additionally, the meiotic drive polarity switching rates Blackmon et al. 2019, Finding 2 are inferred from trees representing only 12–30% of extant species per clade, which limits the precision of these estimates and could introduce systematic bias if sampled taxa are not representative.
Tealc’s citation-neighborhood suggestions
- Studies applying BAMM, HiSSE, or MuSSE to karyotype evolution in other vertebrate groups would provide independent tests of the chromosomal speciation hypothesis at comparable phylogenetic scales.
- Empirical work linking meiotic drive intensity to speciation rates within Cetartiodactyla (the fastest-switching clade) could clarify whether the high polarity-switching rate translates into elevated diversification.