Why do some stickleback populations lack pelvic spines? a) In seawater, pelvic spines help fish swim faster, but not in freshwater.
b) The pelvic spines are homologous to legs in four-legged animals. Because fish don't need hind limbs to walk, many populations of fish evolved to lack pelvic spines.
c) In lakes where there are no predatory fish, there is no advantage to having pelvic spines.