The runaway process is very sensitive to initial conditions and random events. Runaway's initial direction depends on the female preferences and male traits that happen to exist in a population. Runaway's progress depends on several kinds of random genetic events such as sexual recombination, which mixes genes randomly every time two parents produce offspring, and the evolutionary process called genetic drift, which eliminates some genes by chance in small populations, as a result of an effect called "sampling error." Because runaway is a positive-feedback process, its sensitivity to initial conditions and random events gets amplified over evolutionary time. These effects make runaway's outcome quite unpredictable. It never happens the same way twice.
Runaway's unpredictability is apparent if you look at the diversity of sexual ornamentation in closely related species. Of a dozen species of bowerbirds, no two construct the same style of courtship nest. Of three hundred species of primate, no two have the same facial hair color and style. These differences cannot be explained as adaptations to different environments—they are the capricious outcomes of sexual selection.
Computer simulations confirm runaway's unpredictability. In the early 1990s when we were psychology graduate students at Stanford, Peter Todd and I spent months running simulations of runaway sexual selection. We would run the same program, repeatedly, while just changing the initial conditions slightly, or changing the random numbers used by the computer to simulate random events like mutation. The results were quite capricious. Two populations can start out very similar to each other, and evolve slightly different sexual preferences, which lead their sexual ornaments to evolve in slightly different directions, which reinforce their sexual preferences, and so forth. The populations end up in opposite corners of the range of possibilities, sprouting different sexual ornaments, with different sexual preferences. And if you run the same simulation again, with just slightly different random numbers influencing mutations, the populations will evolve in yet another set of directions. A population will often split apart spontaneously into two clusters that are reproductively isolated, creating two distinct species. If you went out for a coffee while running a simulation and came back ten minutes later, the population would usually have moved where you least expected it—not through the physical space of its simulated habitat, but through the abstract space of possible ornament designs.
Suppose you take a dozen species of ape that lived in social groups in Africa about ten million years ago. Think of these species as nearby clusters in the space of all possible sexual ornaments and courtship behaviors. Now turn runaway sexual selection loose in each species. One species might develop a runaway preference for large muscles, and turn into gorillas. Another species might develop a runaway preference for constant sex, and turn into bonobos (previously known as "pygmy chimpanzees"). A third species might develop a runaway preference for creative intelligence, and turn into us.
Depending on your philosophy of science, runaway's unpredictability could be seen as a strength or a weakness. It is a strength if you are looking for an evolutionary process that can explain why two closely related species take dramatically different evolutionary routes. It is a weakness if you expect evolution to be predictable and deterministic, able to explain exactly why one ape species evolved creative intelligence while another did not. Of course, if you think that our mental evolution was driven entirely by natural selection for survival abilities, a fairly deterministic attitude is appropriate. But if you accept that mental evolution could have been influenced by runaway sexual selection, which produces unpredictable divergence, then you can't expect it to be predictable or deterministic.
If our evolution was driven by an unpredictable process like runaway, we should not expect a precise answer to questions like "Why did we, rather than chimpanzees, evolve creative intelligence and language?", or "Why are we the first articulately conscious species on Earth?" It would be like a lottery winner asking why she won. However, we can still ask, "What are the adaptive functions of human creative intelligence, language, and morality?", and "Did these capacities evolve through survival selection, sexual selection, or something else?" Given an adaptation, we can still try to explain why it evolved to have the features and functions it does. We just might not be able to explain why it evolved exactly when and where it did, in the lineage that it did, rather than in other lineages.
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