Predators are not the only cause of death of a prey. Random events can cause prey to die. Just being at the wrong spot at the wrong moment. Random events such as fire, illness or just random mutations can cause a change in the variation with long-term consequences. In large populations random events are likely to have no impact on the direction of the evolution of a species, especially when random events affect only a few individuals at a time. Exceptions are big events, such as a meteor hitting the Earth and causing the extinction of the dinosaurs. But for small populations random events can cause a change in the direction of evolution. We call this genetic drift and it is a different mechanism than natural selection. Such a genetic drift can be caused when a sub population gets isolated on an island, lake or small terrestrial habitat.
The model Bug Hunt drift allows the study of genetic drift. Instead of picking slow bugs because they are easier to catch, you can click on the button to pick one of the bugs at random. We see that over time, after clicking many random bugs, one of the bug types start to dominate (Figure 7). This is not the slowest or fastest bug, but bug type with speed 4. Genetic drift can lead to new variations of a species that do not have a fitness benefit.
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Figure 7: Distribution of types of bugs over time for a typical simulation with random removal.*