The loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population is termed

The founder effect is the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony. The new population may be very different from the original population, both in terms of its genotypes and phenotypes. The original drifted population are called founders. 

The bottleneck effect refers to the genetic drift that occurs when the size of a population is severely reduced due to natural calamities. 

Gene flow refers to the transfer of genetic material from one population to another through interbreeding. 

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