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Increasing human activity means that populations are more susceptible to sudden environmental change. However, we have an inadequate understanding of the long-term effects of human-induced change on natural populations. This thesis focuses on the long-term effects of exposure to high environmental radiation levels resulting from the Chernobyl disaster. The accident caused both abrupt environmental changes to populations from high acute exposure from the initial radionuclide release and long-term chronic exposure from radionuclides that persist in contemporary populations. This work tests radiation effects by combining laboratory and field approaches using Daphnia, a freshwater crustacean, as a model organism. I first examined the fitness of Chernobyl Daphnia populations sampled from eight lakes with different radiation levels and found that variation across populations was not due to dose rate. Assessment of population genetics showed that genetic diversity increased with dose rate, indicative of mutation accumulation. However, gene flow between populations reduced population structure, which could explain why no phenotypic differences were observed between populations. I then tested radiation effects under controlled laboratory conditions. This required the characterisation of the irradiation facility at Stirling University. Testing under continuous radiation exposure revealed a reduction in Daphnia survival across generations, consistent with mutation accumulation. Assessment of reproductive fitness revealed that inferior lineages were selectively removed from the experiment, stripping variation in reproductive effects. This thesis demonstrates that ionising radiation negatively impacts individual Daphnia lineages in the laboratory at dose rates relevant to highly contaminated areas in the Chernobyl Exclusion Zone (350 µGy h-1 in the laboratory, < ~180 µGy h-1 estimated field dose rate to Daphnia), and also found that genetic diversity was higher in wild populations experiencing higher dose rates. However, my field research also uncovered evidence that is consistent with the idea that the negative radiation effects are masked by selection from other ecological pressures.