ABSTRACT
How do the changes occur and become established in populations such that specific genes survive? Whether an allele becomes established at high frequency or disappears depends on many factors, one being the fitness of the allele for the organisms in a particular environment. The process differs somewhat in sexual eukaryotes compared to asexual organisms. Among the latter organisms, once a mutant gene arises, it either has a detrimental effect on the organism, an advantageous effect on the organism, or a neutral effect on the organism. If it is detrimental, it may not survive in the population unless it is linked to a gene that is beneficial (Figures 8.1 and 11.1), or it may survive at low levels if its negative characteristics are mild. If it is advantageous, then it has a higher probability of being inherited by its progeny, because the organisms that possess the allele have a higher probability of surviving to a stage where they can produce daughter cells or progeny. Eventually, the allele might become the most frequent allele in that population. If it is a neutral allele, then it can go in either direction, and its frequency may change due to random genetic drift. It may persist for some time within the population, but it is equally likely that it will disappear. All of this is dependent on the size of the initial population, random events, and the comparative rates of cell division and cell death in that environment (Figure 8.1). In sexual eukaryotes, their nuclei usually contain at least two copies of each gene, and each might be a different allele. Therefore, the various alleles have a higher probability of remaining in the population, even if an individual allele is detrimental (especially when recessive), because it might be paired with an allele that is advantageous for that organism. Additionally, crossovers can alter the linkage to the detrimental allele, which can sometimes eliminate it prior to reproduction to produce the next generation. Therefore, in these eukaryotes, there is some buffering for those genomes that contain detrimental alleles, while in haploid eukarya, bacteria, and archaea, natural selection is much more immediate, because there is little to mitigate the effects of detrimental alleles.
