dollo

Dollo's law, also known as the Dollo principle, is a concept in evolutionary biology named after Belgian paleontologist Louis Dollo. Formulated in the late 19th century, it states that complex structures or traits, once lost in the course of evolution, are unlikely to re-evolve in the same form. The idea rests on the notion that a complex character depends on an intricate network of genes, regulatory elements, and developmental steps that, once degraded or repurposed, would be difficult to reassemble identically.

In its original formulation, the law implied a strict irreversibility of complex features. Modern biology treats it more as a heuristic than an absolute rule. Empirical evidence shows that organisms can regain function through different genetic or developmental routes, or that vestigial or related structures can be repurposed rather than a true re-evolution of the exact trait. As a result, apparent reversals are often explained by genetic redeployment, partial reversals, or convergent evolution that produces superficially similar outcomes via different pathways.

Dollo's law has influenced how scientists interpret the fossil record and the evolution of complex features, particularly in discussions of trait loss across lineages and the likelihood of their reappearance. It is frequently cited in debates about irreversibility, developmental constraint, and the contingency of evolutionary trajectories.

Louis Dollo (1857–1931) was a Belgian palaeontologist who studied extinct reptiles and contributed to paleontological methods and reconstruction narratives. The law bears his name, but it remains a debated and nuanced principle rather than an unassailable law.