long answer on genetic drift
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Answer:
It should now be clear that population size will affect the number of alleles present in a population. But small population sizes also introduce a random element called genetic drift into the population genetics of organisms.
Genetic drift is a process in which allele frequencies within a population change by chance alone as a result of sampling error from generation to generation. Genetic drift is a random process that can lead to large changes in populations over a short period of time. 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. Genetic drift leads to fixation of alleles or genotypes in populations. Drift increases the inbreeding coefficient and increases homozygosity as a result of removing alleles. Drift is probably common in populations that undergo regular cycles of extinction and recolonization. This may be especially important in natural ecosystems where both plants and pathogens are likely to have a patchy distribution where each patch is a small population.
Because allele frequencies do not change in any predetermined direction in this process, we also call genetic drift "random drift" or "random genetic drift." The sampling error can occur in at least three ways. We will consider these in the context of pathogen populations in plant pathosystems:
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.
A genetic bottleneck, or severe reduction in population size, 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).
A founder effect occurs when a small number of individuals, representing only a small fraction of the total genetic variation in a species, starts a new population. 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.
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Answer:
Genetic drift is a change in allele frequency in a population, due to a random selection of certain genes. Oftentimes, mutations within the DNA can have no effect on the fitness of an organism. These changes in genetics can increase or decrease in a population, simply due to chance.
Genetic drift is random changes in gene frequency that occur because of sampling error. Sampling error can be natural, or it can be manmade. Natural sampling errors are those which occur when earthquakes, floods, landslides, or other natural disasters subdivide a population and isolate small groups of organisms.
We can calculate how much genetic drift we expect to find in a population if we know the effective population size. The expected variance in the frequency of an allele (call this frequency p) subject to genetic drift is: Var (p) = after one generation of genetic drift for diploid organisms.
Random forces lead to genetic drift
These changes in relative allele frequency, called genetic drift, can either increase or decrease by chance over time. Typically, genetic drift occurs in small populations, where infrequently-occurring alleles face a greater chance of being lost.
Genetic drift can be caused by a number of chance phenomena, such as differential number of offspring left by different members of a population so that certain genes increase or decrease in number over generations independent of selection, sudden immigration or emigration of individuals in a population changing gene ...
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