Final answer:
The observed sexual dimorphism in phalaropes and spotted sandpipers can be explained by sexual selection, where the normally duller females compete for the brightly colored males, which is an inverse of more common patterns. Female competition could influence the evolution of female displays and male selectivity in mates.
Step-by-step explanation:
The concept of sexual selection provides an explanation for the observed patterns of sexual dimorphism. In the case of phalaropes, the brightly colored females and dull colored males suggest that it is the females who are competing for the males, a reverse of the more commonly seen pattern where males compete for females.
This could lead to the evolution of elaborate female displays and more choosy males. When considering the spotted sandpipers, their behavior of fighting for resource-rich territories suggests a mating system where females compete for the best territories to attract males, indicating female competition as a type of sexual selection.
Typically, where males are the more decorated sex due to sexual selection, they are also often involved in elaborate courtship displays. There are two possibilities here: either males with courtship rituals assist with parental care, or they do not and instead invest their energy in attractive displays.
If males assist with parental care, they have less energy to develop elaborate plumage and displays. If they do not assist, they might use their energy to develop more attractive displays and traits to outcompete other males visually. However, individual species can vary greatly in terms of mating systems and parental care strategies.