Practice question

A fish transferred to a new aquarium may gradually alter ion transport, ventilation, metabolic rate, stress-hormone levels, and behaviour in response to changed temperature, salinity, oxygen, light, or water chemistry. Such within-lifetime, usually reversible adjustment exemplifies acclimatization when it occurs under ordinary environmental exposure; in strictly controlled experimental terminology it may be called acclimation. No generational genetic change is required. The evolution of lungs in amphibian ancestors, by contrast, involved heritable changes filtered by natural selection over many generations. Dense fur in polar bears is likewise a population-level adaptation, although an individual mammal may seasonally alter its coat. Hibernation is a programmed dormancy strategy that can be triggered by seasonal cues; it is not itself synonymous with becoming adjusted to a new aquarium environment. Proper aquarium transfer often requires gradual temperature and water-chemistry equalization because physiological compensation has limits. A sudden large shift can exceed tolerance and cause osmotic shock or respiratory stress before adjustment occurs. The fish example therefore highlights phenotypic plasticity: one genotype can produce different physiological states under different environmental conditions without becoming a new evolutionary lineage.