Population size and allele frequency

Does it really matter whether a species has low or high levels of genetic variation? Biologists, conservationists, environmentalists, and informed citizens all worry about the impact of environmental change on the ecosphere. Although organisms cannot plan for environmental change, the more variation that exists in a population, the better prepared that population will be to adapt to change when it does occur. Note that the level of genetic variation within a population is dynamic: It reflects an ever-changing balance between processes, both random and nonrandom, which remove variation. Sometimes, the latter can overwhelm the former, leading to low levels of variation that cannot be reconstituted over ecological time scales. Researchers understand that variation arises through mutation and recombination, and they also know that natural selection can remove variation from a population. Moreover, scientists are well aware of the fact that real-life populations are not infinite, as the Hardy-Weinberg model requires us to assume. Together, these factors lead to a relentless loss of variation, a process referred to as genetic drift. Genetic drift is the reason why we worry about African cheetahs and other species that exist in small populations. Drift is more pronounced in such populations, because smaller populations have less variation and, therefore, a lower ability to respond favorably — that is, adapt — to changing conditions. Thus, it’s not just the number of cheetahs that worries us—it’s also the decreased variation in those cheetahs. Effective Population Size A line graph shows the relationship between the effective mating population and the number of females present in the population. NF, the number of females, is shown on the X-axis. NE, the effective mating population, is shown on the vertical Y-axis. The graph exhibits a downward-opening parabola curve. The height of the line increases as NF increases from 0 to 500. In this half of the graph, less than half of the effective mating population is female. When NF reaches 500, exactly half of the effective mating population is female. In contrast, the height of the line decreases as NF increases from 500 to 1000, indicating that more than half of the effective mating population is female. Figure 4: The relationship between Ne and Nf in a population of 1000 mating individuals. © 2008 Nature Education All rights reserved. View Terms of Use Because most populations are large, it seems fair to ask whether genetic drift is really all that important. It’s true that most populations are large, but they don’t necessary act large. Thus, the rate of genetic drift is not really proportional to census population size (Nc). Rather, it’s proportional to something more abstract — specifically, the effective population size (Ne). In an ideal population of sexually reproducing individuals , Ne will equal Nc. An “ideal“ population has the following characteristics, and most deviations will decrease the effective population size: There are equal numbers of males and females, all of whom are able to reproduce. All individuals are equally likely to produce offspring, and the number of offspring that each produces varies no more than expected by chance. Mating is random. The number of breeding individuals is constant from one generation to the next. Essentially, anything that increases the variance among individuals in reproductive success (above sampling variance) will reduce Ne (the size of an ideal population that experiences genetic drift at the rate of the population in question). For example, consider the effect of unequal numbers of mating males and females. In an ideal population, all males and all females would have an equal chance of mating. However, in situations in which one sex outnumbers the other, an individual’s chance to mate is now affected by its sex, even if all individuals within each sex have an equal chance to mate. In this situation, effective population size can be predicted by the formula Ne = 4NmNf/(Nm Nf), where Nm is the number of males and Nf is the number of females. Figure 4 shows the relationship between Ne and Nf in a population of 1,000 mating individuals. In an ideal population, all individuals have an equal opportunity to pass on their genes. In real life, however, this is rarely the case, and Ne is particularly sensitive to unequal numbers of males and females in the population. #dnaMolecule #molecularBiology #GeneStructure #gene #phenotype #mRNA #protein #codons #Promoter #rnaSplicing #gregorMendel #Haploid #Heterozygous #genomes #Genetics101 #transcription #GeneticsLecture #geneExpression #AllelicFrequencies #genes #genome #geneticDrift #BloodType #eukaryotes #explain #prokaryotes #Cancer #alleles #nucleicAcids #mitosis #genetics #tRNA #Isochromosome #dominant