Mutation and Recombination

Mutation is a sudden, heritable change in the genetic material, most often appearing as an alteration of a single gene by replacement, duplication, or deletion of a number of DNA base pairs. Mutation can sometimes alter the structure or number of genes or entire chromosomes. Most mutations are harmful, but some are advantageous. Mutations occur naturally at low rates (10-5 to 10-6 per gene per generation). Mutation is considered to be the major factor limiting the speed of evolution. Isolated populations tend to accumulate different mutations diverging genetically. This is illustrated in Figure 1A, which describes the dynamics of genetic divergence by mutation between two hypothetical populations that were identical genetically before their physical separation. It is assumed that a new mutation is fixed in one of the subpopulations with a small probability, 0.001 per generation. The graph gives the number of mutations that differ between the populations as a function of time. Note that the number of genetic differences ("genetic distance") increases approximately linearly with the time since separation; thus genetic distance can serve as a proxy for time (a "molecular clock"). This property allows one to calculate the time to a common ancestor on the basis of genetic distance.

This technique is widely used for reconstructing phylogenetic relationships between different groups of organisms.

Recombination is the exchange of genes between paternal and maternal chromosomes that occurs when reproductive cells are formed. Recombination results in offspring that have a combination of genes (characteristics) different from that of their parents. Recombination can potentially produce an enormous variety of new genotypes. This is illustrated in Figure 1B using a hypothetical example. It is assumed that one diploid parental organism, P1, has alleles A in ten specific loci (locus is a position on a chromosome occupied by a gene; loci is the plural form of locus). Because each diploid organism has two copies of each gene (inherited from its father and mother), the overall number of A-alleles is twenty. Another parental organism, P2, has A-alleles in these

Figure 2

Effects of Random Genetic Drift on Allele Frequencies in an Asexual Diploid Population

Generation number

Different lines describe the changes in the frequency of allele A in five simulated populations that had the same initial frequency (equal to 0.4). The frequency of A quickly reaches 1 (in populations 1, 2, and 3) or zero (in populations 4 and 5) meaning the disappearance from the population of allele a or A respectively.

Generation number

Different lines describe the changes in the frequency of allele A in five simulated populations that had the same initial frequency (equal to 0.4). The frequency of A quickly reaches 1 (in populations 1, 2, and 3) or zero (in populations 4 and 5) meaning the disappearance from the population of allele a or A respectively.

loci. Thus the overall number of A-alleles here is zero. Their offspring, which we denote as F1, will all have an intermediate number of A-alleles (which is ten). Figure 1B describes the distribution of the number of A-alleles among the offspring of F1 organisms, assuming that these organisms mated among themselves and that the loci are unlinked. This distribution includes genotypes with many different numbers of A-alleles at relatively high frequencies.

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