AFor more than a century, evolutionary biologists adhered strictly to Dollo's Law of Irreversibility, formulated in 1893 by Belgian paleontologist Louis Dollo. The principle stated that once an organism has lost a complex anatomical feature during evolution, it cannot regain that identical feature in future generations. Evolution was regarded as a strictly one-way street: structures lost in the evolutionary past were gone forever.
BHowever, opposing views on evolutionary throwbacks emerged in other disciplines. In criminology, Cesare Lombroso argued in the late nineteenth century that criminals were atavistic throwbacks to primitive ancestral humans. While Lombroso's criminological theories were swiftly discredited as pseudoscience, the biological question remained: could long-dormant ancestral traits ever genuinely reappear?
CIn recent decades, modern geneticists and paleontologists have discovered startling examples of evolutionary throwbacks that directly contradict Dollo's Law. Humpback whales have occasionally been documented with external hind legs complete with femurs and digits. Similarly, domestic chickens have been genetically coaxed into developing alligator-like teeth. Such examples have forced biologists to ask how long-lost physical traits could suddenly re-emerge after millions of years of absence.
DBiologists caution that superficial similarities must not be confused with true evolutionary reversals. For instance, both sharks (fish) and killer whales (mammals) possess dorsal fins and streamlined torpedo bodies, but this is a classic example of convergent evolution – unrelated species independently developing similar adaptations to navigate aquatic environments – rather than an evolutionary throwback.
EGroundbreaking research by evolutionary biologist Gunter Wagner provided a persuasive explanation for authentic atavisms. Wagner demonstrated that when a physical trait disappears, the underlying genetic developmental programming does not necessarily vanish immediately. Silent genes can persist dormant in an organism's genome for millions of years, preserved by functioning in other developmental roles. If a future mutation reactivates the dormant genetic circuit, the ancient trait can spontaneously reappear.
FThe realization that evolution can run backwards under certain conditions fundamentally alters our understanding of evolutionary potential. Organisms carry an internal genetic museum of their ancient ancestry, retaining hidden blueprints that nature can occasionally dust off and redeploy.
Match each statement with the correct person or concept, A–D.
Options:
A Dollo's Law
B Cesare Lombroso
C Gunter Wagner
D Convergent evolution
Complete each sentence with the correct ending, A–G, below.
List of Endings:
A led to the question of how long-lost traits could suddenly reappear.
B exemplify the independent occurrence of a particular feature in different species.
C prove that Darwin's theory of natural selection is fundamentally flawed.
D the continued survival of silent genetic information.
E resulted in immediate industrial application across agriculture.
F rejected the possibility of evolution being reversible.
G Dollo's findings and the pseudoscientific convictions held by Lombroso.
Do the following statements agree with the claims of the writer? Write YES, NO, or NOT GIVEN.
37. Humpback whales with hind legs have been bred deliberately in research aquariums.
38. Dollo's Law was considered an indisputable biological principle for over a century.
39. Cesare Lombroso's theories on atavism remain widely accepted in modern forensic medicine.
40. Re-activating dormant genes can enable modern animals to thrive in extreme polar climates.