Broadly true. It's standard stuff. Think about Charles Darwin and his writing about the unusual diversity of the species on the Galapagos islands, which are small and isolated. There are numerous other examples, e.g. pygmy species on Mediterranean islands (now extinct).

When population numbers within a species are large, interbreeding will tend to dilute genetic mutations within the total population. When populations are small, e.g. due to physical isolation or temporal isolation, mutations are more likely to spread and become established. This is increased further when selective pressure is applied.