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Why are small populations more vulnerable to chance fluctuations than large populations?

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Line graph showing world population growth from 1700 to 2000 with a steep rise after 1950, followed by three projection scenarios for 2000 to 2100: high, medium, and low, reaching between 7 and 14 billion. — population Graph of the world's estimated human population from 1700 until 2000, with population projections extending until 2100.

Small populations are more vulnerable to chance fluctuations than large ones primarily due to the effects of genetic drift and sampling error.12

In any population, gene frequencies can change from one generation to the next by pure chance. Because small populations have a limited number of reproducing individuals, they are highly susceptible to these statistical accidents. This can be compared to flipping a coin: while a large number of tosses will likely result in an even split of heads and tails, a very small number of tosses can easily result in an unrepresentative outcome, such as all heads.3

In a biological context, this means that rare alleles can be lost entirely from a small population within a few generations, reducing the genetic variation necessary for a species to adapt to environmental changes, diseases, or predators. Furthermore, small populations face demographic risks; for example, a breeding pair might produce offspring of only one sex by chance, leading to the immediate cessation of reproduction and potential extinction. Large populations are more stable because any specific allele is typically carried by enough individuals to ensure its transmission to the next generation.1342