One of the great challenges in science is that complexity often increases the more closely something is examined. Take for instance, speciation. Intuitively, the emergence of new species should correlate with large changes in physical characteristics, such as body shape or size. It makes sense that the more new species emerge, the greater physical changes will be in the record.

Surprisingly, evolutionary biologists have had a hard time confirming this, with past studies giving mixed results.

By analyzing more than 63,000 species, over 450 million years of evolutionary history, Chris Cooney and Gavin Thomas, both at the University of Sheffield in the UK, have found evidence that morphological evolution is correlated with the speciation rate, at least when considered on a broad enough scale. The study, published in Nature Ecology & Evolution, provides fresh support for this long-held hypothesis, and it helps to explain why researchers had struggled with such variable results.

In reviewing a number of previous studies on this subject, the pair found something not altogether uncommon in science. Different investigators used different methodologies to explore the relationship between speciation rate and morphological evolution. That made it unclear how much of the variation between studies was real, and how much was due to the different methods used.

To address the issue, the two researchers compared the performances of several methods on simulated datasets. They then picked the best performing method for their analysis.

They looked at the relationship between body size and speciation rate in five major clades of vertebrates: amphibians, birds, mammals, ray-finned fish, and scaled reptiles. For each one, they examined the relationship at both the whole-clade level and between major subclades.

The pair found a correlation at the larger whole-clade level, which supports the view that morphological evolution is positively related to the emergence of new species. But as is so often the case, things got more complex at finer scales. There, they saw considerable variation in the vertebrate tree-of-life, even finding negative correlations for some subclades. Along with the different methods used, this variation could have contributed to the mixed results of earlier studies.

While the work has gone a long way to resolving the mystery between the relationship of speciation and physical characteristics, it is also a reminder that knowledge is never complete and that while generalizations are useful, they often mask the complexity underneath.

To read more about the research, explore the paper in Nature Ecology & Evolution

Chris Cooney is a Leverhulme Early Career Research Fellow in the Department of Animal and Plant Sciences at the University of Sheffield, UK. He uses large datasets of species and their traits to address fundamental ecological and evolutionary questions about the factors shaping large-scale biodiversity patterns. He was recently awarded a NERC Independent Research Fellowship, which he will start in 2021.