Final answer:
Due to pollution decreasing visibility in their habitat, the fish species that have recently undergone sympatric speciation may experience a cessation of sexual selection based on color, as color can no longer be effectively used for mate choice, possibly resulting in increased hybridization or different mate selection traits becoming more prominent.
Step-by-step explanation:
When the water in the lake becomes cloudy due to pollution, and the female fish can no longer easily distinguish the color of the male fish, sexual selection based on color may no longer be effective. It is likely that in this scenario, females may start using other cues or signals to choose a partner, if such alternative traits are available and can be detected in the conditions of reduced visibility. Over time, this shift in selective pressure could lead to changes in the ways that mates are chosen, potentially affecting the genetic makeup of the population.
If the fish species were strictly reliant on color for mate selection and no other traits can compensate for the lack of visibility, then there may be an increase in hybridization between the two species, possibly leading to a reduction in reproductive isolation. This scenario does not align well with any of the selection options provided, such as stabilizing selection, disruptive selection, directional selection, as these refer to shifting or homogenizing allele frequencies for certain traits within a population. However, since the question has a focus on sexual selection, it suggests that the answer they are looking for might be the cessation of sexual selection based on coloration.