Species of animals that are cryptically colored, so as to match their backgrounds, are often polymorphic, occurring in multiple distinctive forms. This phenotypic diversity was thought to be promoted by frequency-dependent predation, in which more abundant variants are attacked disproportionately often, but the hypothesis had never before been explicitly tested. Using blue jays searching for digital moths on computer monitors, we conducted the first well-controlled experiments on the effects of visual predators on morph stability, prey crypticity, and phenotypic variance.

In our initial study using virtual moth populations (Bond & Kamil 1998), the frequency of each prey type in the virtual population is determined by the relative numbers of that type surviving from prior sessions. We observed populations of three moth types to converge on a stable equilibrium in which each one occurred at a characteristic abundance. These results showed that predators do more than simply concentrate on more common prey. They must also be responding to population reversals, switching to alternative prey types when the abundance of common ones declines. Our results demonstrated that the detection of cryptic prey does entail apostatic selection, and that such selection can serve to maintain prey polymorphism.This was the first direct demonstration of the dynamic relationship between searching image, apostatic selection, and prey population stability.

In our second study (Bond & Kamil 2002), moth phenotypes evolved via a genetic algorithm in which individuals detected by the jays were much less likely to reproduce. Jays often failed to detect atypical cryptic moths, confirming frequency-dependent selection and suggesting the use of searching images, which enhance the detection of common prey. Over successive generations, the moths evolved to become significantly harder to detect, and they showed significantly greater phenotypic variance than non-selected or frequency-independent selected controls. This is apparent even in a cursory, visual inspection of the prey populations.

Parental Generation: Below is a sample of moths from the parental generation on uniform (left) and cryptic (right) backgrounds. Note that there is moderate phenotypic variance, but that all moths are at least generally similar in appearance, and that they are all fairly easy to detect on the experimental background.

 

Experimental Results: The next sample of moths were taken from the 100th generation of one of the experimental selection runs. Note that phenotypic diversity has greatly increased, and that at least some of the individuals are much more difficult to detect on the cryptic background.

 

Non-selected Controls: These moths came from one of the 100th generation of non-selected controls. The phenotypic diversity is very high, and many of the moths are actually quite conspicuous, much more so than in the experimental populations that were actually exposed to jay predation.

Frequency-independent Selected Controls: These are a sample of moths resulting from 100 generations of frequency-independent selection for crypticity alone. Note that they are both more uniform in appearance than the moths from the experimental runs and more cryptic. Because the birds are searching for particular features in the moth phenotypes, they readily overlook disparate individuals, even when they are actually relatively conspicuous. Thus, there is a limit to how cryptic moths will become under jay predation. The frequency-independent controls are not distracted by differences in prey appearance and therefore select for more cryptic and more uniform moth populations.

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