Answer:
Sudden change in environments that alters gene frequency of a population
Explanation:
The bottleneck effect is a genetic drift that happens when the size of a population is suddenly reduced for at least one generation, this can be triggered by a natural disaster. This sudden change leads to a decrease in genetic variation, as the population decreases the frequency of different alleles changes leading to a completely new population that may or may not be able to adapt. In the image, I added you can observe how the bottleneck effect works.
Considering this information we can conclude that the correct answer is "sudden change in environments that alters gene frequency of a population".
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