mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Commercial orchid growers are rewarded for the successful production of striking flowers. They prefer to propagate orchids by cloning the best varieties, so that they can produce large numbers of identical, superior blooms for sale. Yet there is a large but long-term potential reward for successfully breeding new varieties of orchid that in turn can be cloned in high-level production. Therefore, orchid growers devote at least some of their resources to sexual crossing experiments between individual orchids, trying to achieve the right balance between these two alternative styles of producing new orchids. In other words, the best strategy for commercial orchid production includes a mixture of reproductive modes, with most production being asexual cloning and some being sexual. Now let's think about early eukaryotes that are still reproducing like bacteria, by cloning. What if two early eukaryotes could reproduce sexually? Could sex evolve in the face of its obvious disadvantages? Suppose a eukaryote (The Clone) has been living successfully in some environment, along with other eukaryote lineages. All of them usually reproduce by cloning, although they can exchange some DNA with other cells, just as bacteria do. The Clone lives alongside its daughter cells, which were all cloned from it and are identical with it; in fact, they all are The Clone. Members of The Clone have no reason to compete with one another, because they share the same DNA. An occasional mutant differs, of course, but most mutants are not successful, and they or their clones die out. Sexual reproduction is not a good idea in these circumstances. Like an orchid company, The Clone succeeds best by continuing to clone, and so do all other eukaryotes in this environment. Suppose now that a new lineage of cells appears. Perhaps it is a successful mutant, perhaps it is a strain of cells that represents serious competition to The Clone. What should The Clone do? The Clone is either superior or inferior to the newcomer and so has perhaps a 50% chance of surviving direct competition with it if both simply continue to clone. But can The Clone increase its chances of survival in the face of new competition? I suggest that The Clone should behave like an orchid company and should devote at least some of its cells to sexual reproduction. If The Clone devotes one of its identical cells to DNA exchange with the newcomer, the offspring will contain some percentage of The Clone's DNA. If the offspring is inferior and dies out, The Clone and its competitor have each lost an equal, insignificant, investment. If the competitor remains superior, The Clone is doomed anyway, and its investment in sexual reproduction cost it nothing. If The Clone remains superior to its competitors, it continues to clone and dominate the environment. But if the offspring is superior to both, its lineage is the one that will survive. The Clone's genes are at least partly represented in the successful population, which is better than not at all. And if that happens, The Clone would try gene exchange with that successful offspring, to try to increase its genetic representation in the descendant still more. In summary, DNA exchange by a few of its individuals costs The Clone little but could very well mean the difference between genetic survival and genetic extinction. As long as there are many individuals in The Clone, the cost of these attempts is about the same as a typical insurance premium (a small fraction of the potential cost of a disaster). Each lineage of eukaryotes would have the same best strategy of sex-for-insurance, so occasional DNA exchange would be favored by all the lineages that were potential competitors. Each partner in an exchange would try to pass along as much DNA as it could, and that amount would quickly balance out to 50%. Even if a first attempt at DNA exchange did not succeed, the logic behind the attempt would remain the same, and attempts would continue. This strategy works even if the environment changes. Cells should outcompete other cells if they can; if they can't, they should reproduce sexually with rivals, if possible. This implies that sexual reproduction would have been favored as soon as it was possible, at least as an occasional strategy. The disadvantages of sexual reproduction do not count in this situation. The reasoning applies to any habitat where two or more potentially interbreeding clones of eukaryotes are competing. Competition between clones is the driving force leading to the evolution of sexual reproduction in this scenario. This reasoning is not shared by most higher eukaryotes today, where sexual reproduction is normal and groups of clones are rare. In this case, the individual is unique, and no part of it can be spared for a rather speculative insurance premium. Sexual reproduction would not arise in lineages of unique individuals, but it has persisted in evolution for other reasons; that's a different problem. © Richard Cowen. First published in /History of Life/, edition 1, 1990. Web Links on the origin of sex * News story, BBC News OnLine , May 2004. The paper is in /Science/, so it will be freely available on line in a year. Aficionados of Chapter 3 will recognize immediately that the damage to the parasite theory makes my "insurance" theory (even) more reasonable. * I wrote about orchids in my thought essay, but here's exactly the same system used by poinsettia breeders and marketers. National Geographic News, December 12, 2002 (use sex to produce new varieties, then clone like crazy). * Lecture Notes from Brown University on the evolution of sex * A paper on the origin of sex. [Dacks, J., and A. J. Roger. 1999. The first sexual lineage and the relevance of facultative sex. /Journal of Molecular Evolution/ 48: 779-7XX.] Though I am naturally pleased that they stress facultative sex, as I do, I don't see why they impose such severe conditions on their models. Dacks and Roger suggest that facultative sex was a character that the earliest eukaryotes had, and this makes perfect sense to me. * Sexual reproduction versus fast mutation. Short summary by RC of a paper by Tenaillon et al. (2000). Page last updated, June 7, 2004 Links checked August 25, 2004 Return to Chapter 3 page