BOX 3.2

Bacterial Speciation Processes

The definition of a bacterial species remains an elusive concept in microbiology in large part due to the fact that bacteria are clonal and reproduce asexually. As a result, the Biological Species Concept first espoused by Ernst Mayr (1942), which broadly states that groups of actual or potentially interbreeding natural populations can reproduce only among themselves to the exclusion of all others, is not applicable. At present, somewhat arbitrary operational definitions based on phenotypic characterizations (Bochner 1989; Mauchline and Keevil 1991) and molecular biological tools such as the association of genomic DNA in standardized DNA/ DNA hybridizations (typically >70 percent) and/or gene sequence identity of ≥97 percent for the16S ribosomal RNA (rRNA), are most commonly used to define bacterial species (Wayne et al. 1987; Stackebrandt and Goebel 1994; Konstantinidis and Tiedje 2005). The ever-increasing amount of information derived from comparative genomic analyses is providing fundamentally new insights into the question of what constitutes a bacterial species while further revealing that bacterial genomes are not static entities; many mechanisms or combinations thereof can contribute to both maintenance and change of genome structure and content over time (Ward and Fraser 2005). Among the most important forces contributing to the evolution of bacterial species are the interplay between the rates of mutation and homologous recombination (where homologous in this case denotes a substitution of some portion of a genomic sequence with a sequence of high similarity ) and the influence of biogeography in the divergence of bacterial lineages through adaptations and separation into ecological niches (Nesbø et al. 2006).

Genetic variations are the source upon which evolutionary forces such as genetic drift and natural selection act. Some mutations in DNA are spontaneous, random events that can result from several possibilities. For instance, mutations can arise by the exposure of DNA to ionizing radiation, ultraviolet radiation, and some chemicals. Normally DNA sequences are copied precisely during replication; however, errors in DNA replication can result in changes in gene sequence. In addition, a variety of recombinatory processes can occur in nature that contribute to genetic variability by joining DNA from different biological sources (Graur and Li 2000).

Basic concepts of population genetics suggest that most mutations are deleterious to the fitness of an individual (ability of the individual to survive and reproduce) and are selected against causing their removal from the population, a process referred to as negative or purifying selection. Mutations can also result in alleles (alternative forms of the gene) of similar fitness that are considered neutral mutations and are subject to loss from or fixation within a population as a result of stochastic forces. Occasionally, a mutation may produce an allele that increases the fitness of an individual. These advantageous mutations will be subjected to positive selection and will tend to be fixed more rapidly in a population than to neutral mutations (Graur and Li 2000).

ice sheet originated from the islands and continents of the temperate latitudes of the Southern Hemisphere.”

EVOLUTION OF LIFE IN SUBGLACIAL AQUATIC ENVIRONMENTS

Antarctic subglacial aquatic environments represent potentially rich and largely unexplored storehouses of genetic information. As such, a variety of biogeochemical processes could potentially be at work, resulting in unique metabolically active microbial assemblages in terms of structure and function. The age of the oldest ice in the ice sheet is 1 million years (EPICA 2004; Peplow 2006), and this has been taken to indicate the length of time of microbial isolation of Lake Vostok. Although 1 million years is not a considerable amount in terms of prokaryotic evolution, some changes



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