Random unidirectional change in allele frequencies that occurs by chance in all populations and especially in small populations is known as

Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations by chance. Its effects are strongest in small populations. E.g., Bottleneck effect and Founder effect.

Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny.

A mutation is a change in DNA.

In the case of migration and interbreeding, genes/alleles are added to the new population and these are lost from the old population. There would be a gene flow if this gene migration, happens multiple times.