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.
Page last updated, June 7, 2004
Links checked September 30, 2005
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