Fossils don’t lie ... Do they?

By Rachel Berkowitz

In the 1960s Walter Fitch and Emanuel Margoliash discovered how to reconstruct evolutionary trees from protein and DNA sequences, a pioneering step that advanced the field of molecular phylogenetics. Until now, their technique for determining how one species relates to another and how relationships within groups of species have changed over millennia has gone undisputed.

Historically, evolutionary relationships have, at the most basic level, been established by morphology: the form and structure of organisms and their component parts. Morphological data sets draw from the fossil record; today, scientists also analyze species’ characteristics using methods such as high-resolution x-ray computed tomography.

Discrepancies in the evolutionary position of a species as defined by morphological versus molecular studies have usually been attributed to inadequacies in past morphological studies or to misinterpretation of how genomes evolve. But a recent morphological analysis of lizards by Jacques Gauthier of Yale University and his colleagues “argues persuasively that we should reconsider” whether DNA evidence is always superior for constructing an evolutionary history, according to a Nature perspectives letter by Harvard University’s John Losos and his colleagues.

The world’s 9000 species of lizard are divided into Iguania (iguanas and chameleons) and Scleroglossa (monitor lizards, geckos, and also snakes). The classical view is that iguanians exhibit many characteristics of their ancestral predecessors, while scleroglossans exhibit subsequent changes.

But recent molecular analyses by John Wiens of Stony Brook University and his colleagues suggest that iguanians are newer organisms, having evolved with late-arriving snakes and monitor lizards. Similarities with their ancestral predecessors, this research suggests, are a result of their having re-evolved many ancestral characteristics shared with more distant relatives, characteristics not displayed in current scleroglossans.

How can the conclusions of the molecular study, which suggest the relatively recent arrival of iguanians, be reconciled with Gauthier’s massive morphological data set of 192 species of extinct and extant lizards, which suggests that iguanians are much older?

One possible explanation for the conflict between morphological and molecular data sets is convergent evolution, which could explain why iguanians possess characteristics typical of long-ago ancestors. That explanation, however, is unlikely, since characteristics that molecular data suggest have been lost by iguanians come from functionally different parts of the anatomy. Further, iguanian lifestyles are so diverse that it is difficult to justify how convergent evolution could be adaptive to an overall iguanian lifestyle.

Another possible explanation is that the molecular data provide a false signal. Natural selection occurs at the molecular level too, and examples of molecular convergence do indeed confound species relationships. Further, differential selection of DNA bases could produce biased patterns and skew the analysis.

Molecular data imply shared characteristics due to shared evolution, but say little about the functional link between structure and lifestyle. But morphology implies molecular evolution patterns that have yet to be explained.

This discrepancy calls into question both the veracity of the fossil record when no molecular evidence exists and the possibility of extrapolating species’ lifestyles from molecular evidence alone.

Gauthier’s study demonstrates the importance of both morphological and fossil investigation for understanding variation in lizard species. Surely other animals exhibit hitherto unrecorded variation. The problem creates a rich future for collaborations between molecular biologists and paleontologists.