We therefore conclude, as have others, that the evolution of Y pestis is characterised by extensive rate variation,
probably attributable to a combination of fl uctuations in the number of bacterial replication cycles (generations) per unit time, such as between endemic and epidemic cycles, or changes in selection pressures through time, including localised adaptive evolution, or combinations of these factors. Whatever the cause of this rate variation, these data suggest that previous molecular-clock-derived estimates of the timescale of Y pestis evolution, including the date of its divergence from Y pseudotuberculosis, might be erroneous. Such extensive rate variation contrasts with studies of other bacteria such as Mycobacterium leprae in which samples collected over about 1000 years show substantial temporal structure. Importantly, without a firm timescale for Y pestis evolution we cannot exclude that earlier dated epidemics (Plague of Athens of 430 BC) or pandemics (Antonine Plague 165–180 AD) of unknown cause are potentially derived from repeated, novel emergences of Y pestis into human populations as was the case with later pandemics
We therefore conclude, as have others, that the evolution of Y pestis is characterised by extensive rate variation,
probably attributable to a combination of fl uctuations in the number of bacterial replication cycles (generations) per unit time, such as between endemic and epidemic cycles, or changes in selection pressures through time, including localised adaptive evolution, or combinations of these factors. Whatever the cause of this rate variation, these data suggest that previous molecular-clock-derived estimates of the timescale of Y pestis evolution, including the date of its divergence from Y pseudotuberculosis, might be erroneous. Such extensive rate variation contrasts with studies of other bacteria such as Mycobacterium leprae in which samples collected over about 1000 years show substantial temporal structure. Importantly, without a firm timescale for Y pestis evolution we cannot exclude that earlier dated epidemics (Plague of Athens of 430 BC) or pandemics (Antonine Plague 165–180 AD) of unknown cause are potentially derived from repeated, novel emergences of Y pestis into human populations as was the case with later pandemics
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