im not sure about this, please help explain :( will mark brainliest

A small group of birds became isolated from a large population when deforestation occurred for the development of new buildings. Over the course of a few generations, the gene pool of the small group became drastically different from the original population. Which of the following best describes this occurrence?

A selective process caused an increase in the population's genetic variation.
The founder effect caused the chance event that altered the allele frequency.
Bottleneck effect led to over-representation of certain alleles in the new gene pool.
Gene flow led to an increase in the gene pool's allele frequencies.