![]() Where p 0 and q 0 are the initial frequencies of the two alleles at a locus. ![]() Var (p) = after one generation of genetic drift for diploid organisms.Īfter many generations of genetic drift, an equilibrium will be reached. The expected variance in the frequency of an allele (call this frequency p) subject to genetic drift is: We can calculate how much genetic drift we expect to find in a population if we know the effective population size. An analysis of field populations of the wheat pathogen Mycosphaerella graminicola indicated N e of at least 70 strains per square meter (Zhan et al., 2001). N e 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. N e 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 e can also be thought of as the number of genetically distinct interbreeding individuals in a population. N e is a theoretical number that represents the number of genetically distinct individuals that contribute gametes to the next generation. N e is rarely the actual number of individuals in the population (also called N or the census size). The magnitude of genetic drift depends on N e, the effective population size, for the 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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