Is there a loophole in Dollo's law? A DevoEvo perspective on irreversibility (of felid dentition)

There is a longstanding interest in whether the loss of complex characters is reversible (so-called “Dollo's law”). Reevolution has been suggested for numerous traits but among the first was Kurtén, who proposed that the presence of the second lower molar (M₂) of the Eurasian lynx (Lynx lynx) was a violation of Dollo's law because all other Felids lack M₂. While an early and often cited example for the reevolution of a complex trait, Kurtén and Werdelin used an ad hoc parsimony argument to support their. Here I revisit the evidence that M₂ reevolved lynx using explicit parsimony and maximum likelihood models of character evolution and find strong evidence that Kurtén and Werdelin were correct—M₂ reevolved in E. lynx. Next, I explore the developmental mechanisms which may explain this violation of Dollo's law and suggest that the reevolution of lost complex traits may arise from the reevolution of cis-regulatory elements and protein−protein interactions, which have a longer half-life after silencing that protein coding genes. Finally, I present a developmental model to explain the reevolution M₂ in E. lynx, which suggest that the developmental programs required for the establishment of serially homologous characters may never really be lost so long as a single instance of the character remains—thus the gain and loss and regain of serially homologous characters, such mammalian molars, may be developmentally and evolutionarily “simple.”