Evolution is often thought to take over hundreds or thousands of years. But a recent study has found a species of frog which has evolved over within about 25 years. The adaptation was spurred by salt. Researchers collected wood frog eggs from nine populations and allowed them to hatch into tadpoles. Whether populations of tadpoles from more salt-polluted waters had evolved higher salt tolerance was studied. Tadpoles collected from the saltiest wetland experienced a notably longer time until death when exposed to salt than the eight other populations collected from other areas with varying levels of salt, suggesting an evolved tolerance. Frog populations collected from ponds with low and intermediate salt concentrations exhibited similar tolerance curves over a 96-hour exposure. However, the population from a pond with the highest salt concentration exhibited a much higher tolerance. The results provide further evidence that some species of freshwater organisms can evolve tolerance to increasing salinisation, although it may only occur under relatively high concentrations and without trade-offs in growth, development, or behaviour.
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