Final answer:
Frequency-dependent selection favors phenotypes that are either common or rare and can be observed in male common side-blotched lizards, as each throat-color pattern has a different reproductive strategy.
Step-by-step explanation:
Frequency-dependent selection is a type of selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection). An example of positive frequency-dependent selection can be seen in male common side-blotched lizards, where each throat-color pattern has a different reproductive strategy. Orange males are the strongest and fight other males for access to females; blue males form pair bonds with their mates; and yellow males sneak copulations. The frequency-dependent selection ensures that each phenotype has an advantage over the others, resulting in a stable balance. The abilities used to survive against natural enemies can evolve through different types of natural selections, including directional selection and frequency-dependent selection.
Directional selection favors phenotypes at one extreme end of the spectrum, a good example being the industrial melanism observed in moths during the Industrial Revolution, where the previously rare dark-colored moths became more common due to their increased camouflage against predators. On the other hand, frequency-dependent selection can favor either common phenotypes (positive frequency-dependent selection) or rare phenotypes (negative frequency-dependent selection), as observed in the male common side-blotched lizards, where the success of a male's color form depends on the frequency of the other color forms in the population. Thus, in the context of survival against natural enemies, if the enemy adapts to target a common phenotype, a rare phenotype could be favored (negative frequency-dependent selection), whereas if having a common phenotype helps deter predation, that phenotype would be favored (positive frequency-dependent selection).