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4.11: Curbing runaway selection

Sexual selection can lead to what has been termed (but is not really) runaway selection. For example, the more prominent the peacock male's tail the more likely he will find a mate even though larger and larger tails may also have significant negative effects. All of which is to say that there will be both positive and negative selection for tail size, which will be influenced by the overall probability that a particular male mates successfully. Selection does not ever really run away, but settles down when the positive (in terms of sexual success) and negative (in turns of various costs) of a trait come to equal each other. Sufficient numbers of male peacocks emerge as reproductively successful even if many males are handicapped by their tails and fall prey to predators. For another example, consider the evolution of extremely large antlers associated with male dominance and mate accessibility, such as occurred in Megaloceros giganteous. These antlers could also act to inhibit the animal’s ability to move through heavily wooded areas. In a stable environment, the costs of generating antlers and benefits of effective sexual advertising would be expected to balance out; selection would produce an optimal solution. But if the environment changes, pre-existing behavior and phenotypes could act to limit an organism’s ability to adapt or to adapt fast enough to avoid extinction. In the end, as with all adaptations, there is a balance between the positive effects of a trait, which lead to increased reproductive success, and their negative effects, which can also influence survival. The optimal form of a trait may not be stable over time, particularly if the environment is changing.

Summary: Social and ecological interactions apply to all organisms, from bacteria to humans. They serve as a counter-balance to the common caricature of evolution as a ruthless and never ceasing competition between organisms. This hyper-competitive view, often known as the struggle for existence or Social Darwinism, was not supported by Darwin or by scientifically-established evolutionary mechanisms, but rather by a number of pundits who used it to justify various political (that is, inherently non-scientific) positions, particularly arguing against social programs that helped the poor (often characterized as the unfit) at the “expense” of the wealthy. Assuming that certain organisms were inherently less fit, and that they could be identified, this view of the world gave rise to Eugenics, the view that genetically inferior people should be killed, removed, or sterilized, before their "bad" traits overwhelmed a particular culture. Eugenics was a particularly influential ideology in the United States during the early part of the 20th century and inspired the genocidal programs of the Nazis in Germany. What is particularly odd about this evolutionary perspective is that it is actually anti-evolutionary, since if the unfit really were unfit, they could not possibly take over a population. In addition, it completely ignores the deeply social (cooperative) aspect of the human species.

Questions to answer & ponder

•What does it mean to cheat, in terms of sexual selection - is the "cheating" organism actually being consciously deceptive?

•Why do the different sexes (of the same species) often display different traits?

•If the two sexes appear phenotypically identical, what might you conclude (at least tentatively) about their reproductive behaviors?

•What types of "cheating" behaviors do females use with males? What about males with females?

•What are the costs involved when a male tries to monopolize multiple females? What are the advantages?

•What limits runaway selection?

•Is Devendra Singh right about mating budgets?

•Is the schooling or herd behavior seen in various types of animals (such as fish and cows) a homologous or an analogous trait?


  • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.