At the end of the ice age, the retreating ice sheet created many new lakes, some of which were colonized by sticklebacks. The presence of different predators in different lakes dictated the subsequent evolution of each isolated lake stickleback over the course of about 10,000 years. Some groups kept their spines to use against predatory fish, such as trout. Others lost their spines, perhaps to evade aquatic insect predators.
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