Final answer:
Stickleback fish do not automatically lose their pelvic spines in freshwater. This is a characteristic that has evolved in some, but not all, freshwater populations due to environmental factors and selective pressures over time.
Step-by-step explanation:
The statement that stickleback fish lose their pelvic spines whenever they are in fresh water is not entirely accurate. The pelvic spine structure of stickleback fish can indeed vary between populations that live in different environments. The three-spined stickleback (Gasterosteus aculeatus) evolved from saltwater to freshwater and is observed to have varying pelvic spine structures dependent on their environment and evolutionary pressures. In some freshwater populations, there is a tendency to lose or reduce these spines due to different selective pressures compared to their saltwater relatives. This trait modification showcases evolutionary adaptation based on environmental factors and predation risks. However, the loss of pelvic spines is not an automatic response to being in freshwater but rather a result of long-term evolutionary changes across different populations.