Zhang and Wiens reviewed 373 studies comparing morphological and genetic species delimitation across all major vertebrate groups. Every morphology-based species contains, on average, approximately two genetically distinct species. The ratio holds for mammals, birds, and fish.
The number isn't the interesting part. A ratio of 1.7 or 2.3 wouldn't change the argument. What's interesting is that the ratio is stable. You'd expect it to vary dramatically across groups — birds, with their convergent plumage evolution, should hide more cryptic species than mammals with their diverse body plans. Fish, with their enormous diversity and limited morphological characters, should be worst of all. But the ratio is approximately 2:1 across all of them. The undercounting is systematic, not random.
This means the resolution limit is in the method, not the organisms. Morphological taxonomy resolves species at a particular grain size, and that grain size is approximately twice the genetic species grain size, regardless of which taxonomic group you're examining. The tool has a fixed resolving power that is half of what the actual diversity requires. Every vertebrate taxonomist in history was working with approximately half the picture, and the proportion of the picture that was missing was constant.
The conservation implications are immediate and disturbing. If you're managing a species that is actually two species, you might be protecting the common one and ignoring the rare one. Worse, you might be managing them as a single interbreeding population when they're actually reproductively isolated. The population genetics you're using assume gene flow that doesn't exist.
Mitochondrial DNA, which evolves faster than nuclear DNA, is the primary tool for detecting these splits. The study found that mitochondrial estimates are not systematically inflated relative to nuclear estimates — an important methodological validation, since critics of DNA barcoding have long argued that mtDNA overestimates species counts due to its faster evolution rate and matrilineal inheritance.
The deeper pattern: every measurement system has a resolution limit, and the resolution limit determines what you can count. Below the limit, things that are different look the same. A morphological species is not the same entity as a genetic species — it's a coarser-grained measurement of the same underlying diversity. The 2:1 ratio is a calibration factor between two measurement systems, and it happens to be remarkably constant. Whether this tells us more about the consistency of morphological constraints across vertebrates, or about the consistency of genetic divergence rates, is an open question.