September 3, 2009
Before Charles Darwin, there was Jean-Baptiste Lamarck, the French naturalist who proposed that an organism could pass to its offspring characteristics that it acquired during its lifetime. The classic example is the idea that giraffes got their long necks by gradually stretching them over successive generations in response to the need to reach food high in the trees. Darwin’s theory—which held, in contrast, that giraffes with the longest necks were more likely to survive and reproduce—eventually won out, though Lamarckism persisted well into the 20th century (particularly in the Soviet Union, where it was revived as Lysenkoism).
One proponent of Lamarckism in the 1920s was Austrian biologist Paul Kammerer, who undertook a series of experiments on amphibians, including the midwife toad. These toads are special because they copulate on land and then the male keeps the eggs out of the water by carrying them around, on land, stuck to his own legs.
By placing the toads in an arid, hot environment, Kammerer induced the toads to mate in the water. Under these conditions, the toads simply deposited the eggs into the water—the male did not carry them—and only a few hatched into tadpoles. But later generations who grew up under normal conditions preferred to copulate in the water, and some males developed a trait called “nuptial pads” on their forelimbs (black spots that are used for gripping females and are common on water-dwelling toads). Kammerer believed that this was evidence that Larmarckian evolution was real.
In 1926, however, a herpetologist determined that the nuptial pads on the only specimen remaining from Kammerer’s experiment were simply black spots created by injections of India ink. And six weeks after the herpetologist’s paper appeared in Nature, Kammerer killed himself.
Kammerer denied injecting the frog, but his experiments were never repeated and he is often held up as an example of Lamarckian fraud. Nothing was ever proven, though, and nuptial pads have since been found in a wild midwife frog, proving they are a possible trait. Now, in a new paper, University of Chile biologist Alexander Vargas argues that Kammerer’s experiments produced intriguing evidence of epigenetics, in which a gene’s expression can change but not its underlying sequence, years before scientists discovered this non-Mendelian form of inheritance.
In Kammerer’s time, traits were thought to be inherited in a strict Mendelian fashion, in which genes obey statistical laws. We now know that genetics are far messier; the DNA sequence of a gene is only one part of the picture. For instance, with DNA methylation, a methyl group attaches to DNA resulting in less expression of the gene. Environmental factors can influence DNA methylation, and this can look something like Lamarckian evolution.
Vargas argues that moving the toad eggs from land to water changed their environment, and that change could have caused alterations in gene methylation. And epigenetic mechanisms are now known to influence some of the features that became altered in Kammerer’s toads, such as adult body size and egg size. “Rather than committing fraud,” Vargas writes, “it seems that Kammerer had the misfortune of stumbling upon non-Mendelian inheritance at a time in which Mendelian genetics itself was just becoming well accepted.”
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