Selection on an extreme-yet-conserved larval life-history strategy in a tapeworm.

Evolutionary stasis characterizes many phenotypes, even ones that seem suboptimal. Among tapeworms, Schistocephalus solidus and its relatives have some of the shortest developmental times in their first intermediate hosts, yet their development still seems excessively long considering they can grow faster, larger, and safer in the next hosts in their complex life cycles. I conducted 4 generations of selection on the developmental rate of S. solidus in its copepod first host, pushing a conserved-but-counterintuitive phenotype toward the limit of known tapeworm life-history strategies. Faster parasite development evolved and enabled earlier infectivity to the stickleback next host, but low heritability for infectivity moderated fitness gains. Fitness losses were more pronounced for slow-developing parasite families, irrespective of selection line, because directional selection released linked genetic variation for reduced infectivity to copepods, developmental stability, and fecundity. This deleterious variation is normally suppressed, implying development is canalized and thus under stabilizing selection. Nevertheless, faster development was not costly; fast-developing genotypes did not decrease copepod survival, even under host starvation, nor did they underperform in the next hosts, suggesting parasite stages in successive hosts are genetically decoupled. I speculate that, on longer time scales, the ultimate cost of abbreviated development is reduced size-dependent infectivity.