Answer:
A small group of individuals leave a larger population and colonize a new area that is physically separated from the original population. Due either to chance or genetic drift, the genetic composition of the new colony differs from the larger population. This is an example of Random drift.
Random drift is caused by recurring small population sizes, severe reductions in population size called "bottlenecks" and founder events where a new population starts from a small number of individuals.
The process in which frequency of alleles within a population change by chance alone as a result of sampling error from generation to generation. is called Genetic drift. It is a random process which may lead to large changes in populations over a short period of time.
Explanation:
Because the frequency of allele doesn't change in any predetermined direction, we also call genetic drift "random drift" or "random genetic drift." The genetic composition error may occur due to the following conditions;
1)A small number of individuals, representing only a small parts of the total genetic variation in a species, started a new population, a founder effect occurs . A founder event occurs when one or two infected plants slip through a quarantine and introduce a disease into an area where the disease did not previously exist.
2)Small recurring population size occurs when there are not many host plants in the area to infect, or when the environment is not optimal for infection.
3) A severe reduction in population size or genetic bottleneck occurs when the plant population is removed (e.g. harvest of the crop), or when the environment changes to prevent infection of the plant or to kill the pathogen directly (e.g. periods of hot, dry weather or a deep freeze).
