Figong
Well-Known Member
Am not sure how it's a myth..
[h=2]Measuring Genetic Drift[/h] The magnitude of genetic drift depends on N[SUB]e[/SUB], the effective population size, for the population. N[SUB]e[/SUB] is rarely the actual number of individuals in the population (also called N or the census size). N[SUB]e[/SUB] is a theoretical number that represents the number of genetically distinct individuals that contribute gametes to the next generation. N[SUB]e[/SUB] can also be thought of as the number of genetically distinct interbreeding individuals in a population. N[SUB]e[/SUB] is not easy to quantify because it is affected by reproduction and breeding strategies (inbreeding, outcrossing, asexual reproduction), and is dependent on the geographical area over which a population is sampled. N[SUB]e[/SUB] is not easy to define for fungal pathogens that undergo a mixture of sexual and asexual reproduction because the absolute number of individuals can be very large while the number of different genotypes that sexually recombine can be relatively small. An analysis of field populations of the wheat pathogen Mycosphaerella graminicola indicated N[SUB]e[/SUB] of at least 70 strains per square meter (Zhan et al., 2001).
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 many generations of genetic drift, an equilibrium will be reached. At equilibrium we expect that:
Var (p) = p[SUB]0[/SUB]q[SUB]0[/SUB]
Where p[SUB]0 [/SUB]and q[SUB]0[/SUB] are the initial frequencies of the two alleles at a locus.
If p[SUB]0[/SUB]=q[SUB]0[/SUB]=0.5 and N[SUB][SIZE=-1]e[/SIZE][/SUB] = 50 then Var (p) = 0.0025
The standard deviation of (p) = (0.0025)[SUP]0.5[/SUP] = 0.05.
The standard deviation is the average absolute value of the expected difference among populations after one generation of drift and is approximately equal to the expected change in allele frequency (
The degree of change increases as the population size decreases.
If p[SUB][SIZE=-1]0[/SIZE][/SUB]=q[SUB][SIZE=-1]0[/SIZE][/SUB]=0.5 and N[SUB][SIZE=-1]e[/SIZE][/SUB] = 5 then Var (p) = 0.05
The standard deviation of (p) = (0.05)[SUP][SIZE=-1]0.5[/SIZE][/SUP] = 0.22
In this case, a population that has only five individuals is expected to experience random changes in allele frequencies of about 22% each generation.
