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A common misconception is that evolution always requires natural selection. However, this is not always the case.

Genetic drift is one way evolution can happen without natural selection. It is defined as a change in the allele frequency of a population because of chance.

To understand this idea, imagine a small giraffe population with two coat color alleles, tan and brown, represented by two different colored marbles. At the start, both alleles are equally common.

When these giraffes reproduce, the alleles passed to the next generation are chosen randomly. This causes some generations to inherit more tan alleles and fewer brown alleles, or the opposite.

Over time, these shifts can cause one color to become much more common or cause the other color to disappear entirely.

Genetic drift has a stronger effect in small populations. Two well-known examples are the bottleneck effect and the founder effect.

The bottleneck effect happens when a population is suddenly reduced in size, such as after a natural disaster or overhunting.

Imagine a bottle filled with marbles of different colors that represent different alleles. If the bottle is turned upside down, only a few marbles randomly fall