Positive Frequency-dependent Selection
Where negative frequency-dependent selection gives an advantage to rare phenotypes, positive frequency-dependent selection gives an advantage to common phenotypes. In the between-species analogue, this is equivalent to an Allee effect, in which if a species is too rare, it may decline to extinction. This means that new alleles can have a difficult time invading a population, since they don't experience significant benefit until they become common.
This has been proposed as a difficulty in the evolution of aposematic (or warning) coloration in toxic or distasteful organisms. The presumed advantage of the aposematic coloration is that predators have learned to avoid potential prey with that color pattern. One example is the mimicry complex between the mimic, scarlet kingsnake (Lampropeltis elapsoides), and the model, eastern coral snake (Micrurus fulvius). A study conducted by Harper and Pfennig found that in locations where the model and mimic were found in deep sympatry, the scarlet kingsnake was quite phenotypically variable due to relaxed selection. But when the pattern is rare, the predator population does not become 'educated', so the pattern brings no benefit. The same study conducted by Harper and Pfennig found that the scarlet kingsnake was much less phenotypically variable on the allopatry/sympatry border of the model and mimic, likely due to increased selection since the eastern coral snake is rare, but present, on this border. Therefore the warning color is only advantageous once it has become common. Warning coloration, if it involves more than one species, is known as Müllerian mimicry, a form of convergent evolution where multiple species share the same advantageous pattern.
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