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Larvae, the immature forms of many animals, are distinct from adult forms by definition. In many life histories—caterpillars and the trochophore larvae of clams and sea snails are examples—larvae and adults bear no resemblance to each other. Biologist Williamson has proposed that larvae are juvenile forms acquired through hybridization—the fusing of two genomes, one of which is now expressed early in an animal's life, the other late. This hypothesis, which goes against traditional thinking that branches on the evolutionary tree cannot fuse to form chimeric species, is one of several possible solutions to open questions about the evolution of larvae. Although an experiment did not yield convincing DNA evidence, the hypothesis is consistent with certain patterns seen in the distribution of genes across species. Along with other evidence of cross-species hybridization, it implies a pattern of evolution that looks more like a network than like Darwin's tree of life.
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