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16 Interspecies Hybrids Sonya E. Vickers and Donald I. Williamson Since Darwin, evolution has been presented as a matter of gradual variations accumulating in branching lineages that eventually speciate. Here, however, Williamson and Vickers show that evolution was sometimes radically inventive, producing striking new animal forms in a single generation by means of fertilization across would-be species boundaries. Examples include butterflies and starfish and their larvae. Here we outline an idea that could radically alter the way we understand animal evolution. Fertile sex—egg-sperm fertilization—occurred (and still does) between members of entirely different species, even different phyla. The rare but fertile hybrids generated striking new animal life forms in a geological instant. Neo-Darwinian evolutionary theory ascribes gradual changes to mutations (alterations in genetic material). The forms of larvae and their distribution in the animal kingdom suggest a radically different mode of evolution that involves sudden changes beyond the gradual accumulation of mutations. We call this “larval transfer” or “origin of larvae by hybridogenesis .” The hypothesis is that larvae, or the genes that specify them, have been transferred from one animal group to another by cross-species, cross-genera, and even cross-phyla egg-sperm fertilizations. Larvae, immature stages in the life histories of many animals, may differ dramatically from the adults that develop from them. We are familiar with a few larvae, including the caterpillar larva that spins a chrysalis from which an adult butterfly emerges. But many other types of larvae exist, and most of them live in the sea. When a larva differs entirely from the adult into which it develops (holometaboly), we claim that larval transfer occurred in the history of that lineage. Most of the evidence comes from larvae of clams, starfish, and sea urchins, species for which eggs and sperm cast into the sea may merge 184 Chapter 16 in fertilization. Darwin assumed, as do most people today, that larvae and adults begin as a single individual., since they develop from the same egg. The young then are assumed to gradually become more and more different from the adults into which they will eventually develop. This is the core of the “same-stock theory.” But no; we explain that the larval forms of animals and the mature adults into which they eventually develop have different ancestors. The larva began as an adult that developed from a fertile egg in one lineage and interbred with an adult from an entirely different animal lineage. Most such anomalous matings didn’t even produce offspring, and most of the offspring they did produce failed to survive to sexual maturity. In rare cases of hybrid survival, the genomes of the hybrid would no doubt be so incompatible that gene expression would lead to lethality and hybrids would tend to be selected against. But a hybrid would require survival of only a few individuals, under strong natural selection, to survive. Some hybrids, in fact, left many viable descendants that survived as doubled and even tripled or quadrupled genomes. How did two or more disparate animal beings in the same body at the same time ensure the indefinite continued survival of the hybrids? By expression of the combined genome in sequence rather than concurrently. The larval genome is envisioned to express itself first in the sequence; then the second, third, or fourth genome would express the adult morphology. Metamorphosis from larva to adult is a legacy of the shift from one to another of unlike ancestors. Patterns of Change At the time of Darwin, conventional religious beliefs held that the living forms then alive were unchanged over time. Pine trees had always been pine trees, and starfish had always been starfish. (See plate X.) However, the continued exploration of fossils and the discovery that domesticated animals had changed by selective breeding seeded speculation that life forms had evolved through time. When Darwin (1859) argued that this was indeed the case, his idea was rejected by many. The Darwinian view of life’s history is a tree pattern. The bottom of the central trunk represents the common ancestor. The branches denote subsequent diversity, with gradual change in a lineage. The top of the tree represents the present. Branches that fail to reach the top represent extinct life. Because more than 99 percent of all past life on Earth is estimated to be extinct, most branches fail to reach the top. The main [18.118.227.69] Project MUSE (2024-04-26 10...

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