A severe fire kills 90% of a population of squirrels in a forest. The remaining squirrels evolve into a population quite distinct from the original. In another forest, a bacterial infection kills 90% of the squirrels in the forest and the remaining squirrels also evolve into a distinct population. The first instance is considered genetic drift, and the second is natural selection. Explain the difference
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Natural selection is based on traits, and the immunity to a bacterial infection can be considered a genetic trait. The squirrels survived due to their bodies' abilities to fight off the infection, so they had superior immune systems. These "better" squirrels were able to reproduce and create "better" offspring. In terms of the fire, being able to live through a fire isn't directly genetic, and is mostly based on "luck." It's possible that said fire didn't even impact some of the squirrels, so that wouldn't be direct natural selection.
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The first instance of the fire is genetic drift because the survival of the squirrels did not depend on their genetic makeup. It was just a fire.
In the second instance, however, 10% of the squirrel population had gene(s) that made them resistant to the bacteria, and that is why they survived. This is natural selection.
In the second instance, however, 10% of the squirrel population had gene(s) that made them resistant to the bacteria, and that is why they survived. This is natural selection.