The Hardy-Weinberg principle assumes a very large population. Population size matters because it controls how much allele frequencies are affected by chance.In every generation, only some individuals reproduce, and each parent passes on a random half of its alleles. In a large population these chance effects average out, so allele frequencies stay close to those of the previous generation. In a small population, chance events (such as which individuals happen to survive a storm or find a mate) have a much bigger effect, and allele frequencies can change a lot from one generation to the next.A useful comparison is tossing a coin. Toss it 1000 times and you will get very close to 50% heads. Toss it 10 times and getting 7 heads, or only 3, would not be surprising. Small samples stray further from the expected ratio.
Small populations also tend to have a smaller gene pool with lower genetic diversity: fewer different alleles are present, and a rare allele can be lost completely if the few individuals carrying it fail to breed. Lower diversity means the population has less variation for natural selection to act on, so it is less able to adapt if the environment changes, for example when a new disease appears.Large populations usually keep more alleles and are more stable. This is one reason conservation programmes try to keep endangered populations as large as possible.
Small populations also tend to have a smaller gene pool with lower genetic diversity: fewer different alleles are present, and a rare allele can be lost completely if the few individuals carrying it fail to breed. Lower diversity means the population has less variation for natural selection to act on, so it is less able to adapt if the environment changes, for example when a new disease appears.Large populations usually keep more alleles and are more stable. This is one reason conservation programmes try to keep endangered populations as large as possible.