Final answer:
The most likely outcome of a sexual arms race, where a behavior benefits one sex but is harmful to the other, is the development of sexual dimorphism due to different selection pressures, exemplified by traits that signal genetic fitness despite potential handicaps.
Step-by-step explanation:
When a behavior is in the best interest of one sex but detrimental to the other, it can set up a scenario known as a sexual arms race. The most likely outcome of this situation is the evolution of sexual dimorphism, where males and females of a species exhibit different characteristics, particularly secondary sexual characteristics. This is a result of the different selection pressures they experience.
For example, the peacock's opulent tail is favored by sexual selection because it signals to potential mates that the male carries good genes and is capable of surviving despite the handicap of the tail. This concept is known as the handicap principle and is part of a broader set of theories related to sexual selection, including the good genes hypothesis and Fisherian runaway selection.