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SYMMETRY EXCEPTION. “An asymmetrically dividing bacterium has recently been found to show senescence. Remarkably, however, even the symmetrically dividing E. coli ages: it shows subcellular mother-offspring asymmetry, delineating age classes upon which selection can act to produce senescence.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

SYMMETRY. “More important than this lack of a clear germ line/soma distinction, however, is the fact that prokaryotes, protozoans, algae, and symmetrically dividing unicells, do not have clearly delineated age classes. In symmetrically dividing unicells, for example, individuals should not age because parent and offspring are phenotypically indistinguishable – it is impossible to determine old from young, and age is thus invisible to selection. By the same logic, aging should exist in asymmetrically reproducing organisms where aging parents are phenotypically distinct from offspring.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

SOMA VERSUS GERM LINE. “For a long time it was thought that bacteria do not age. Indeed, one of Williams' strongest assertions about the evolution of aging was that only organisms with a separation of germ line and soma should age. In such organisms, the germ line is maintained indefinitely, but the aging soma is “disposable” after fulfilling its reproductive role. Bacteria, by contrast, do not exhibit a clear delineation into germ line and soma, and should therefore be immortal.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

DEFENSES. “Species that are well protected from predators – for example, those that have a shell, can fly, or are poisonous – tend to live longer than related, less well-protected species.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

THE BALANCE. “The evolution of lifespan can be viewed as a balance between selection for increased reproductive success and the factors that increase the intrinsic age-dependent components of mortality.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

THE EQUATION. “These lifespan promoting effects of selection are balanced by those that tend to increase adult mortality relative to juvenile mortality. Consequently, if extrinsic, environmentally imposed adult mortality is high, selection becomes weak, thereby allowing the evolution of higher levels of intrinsic mortality (i.e., aging).” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

REPLICOMETER. “Telomeric shortening appears to be the replicometer that determines the number of times that a normal cell is able to divide. Once a threshold number of telomeric (TTAGGG) repeats is reached, downstream events presumably are triggered that signal the cessation of DNA replication. Wright and Shay have offered an alternative explanation of how telomere shortening acts as a replicometer.” https://lnkd.in/esfPTAR View in LinkedIn

LIFESPAN AND BREEDING. “A longer lifespan normally implies increased reproductive success, and factors such as low adult mortality (permitting more reproductive events per lifetime), high juvenile mortality (making it necessary for adults to reproductively compensate for such loss), and high variation in juvenile mortality from one bout of reproduction to the next (increasing uncertainty in reproductive success and requiring reproductive compensation as well) therefore all tend to lengthen reproductive lifespan.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn

MITOTIC CLOCK. “Hayflick first described the limited replicative capacity of normal human fibroblasts more than 30 years ago. Since then, numerous other somatic cell types, including epithelial cells, endothelial cells, myoblasts, astrocytes, and lymphocytes, have also shown evidence of a mitotic clock which limits their division capacity.” https://lnkd.in/dFjeJA3 View in LinkedIn

RANGE OF LONGEVITY. “Different species vary dramatically in how long they life. The dome-shelled Galápagos giant tortoise (Geochelone elephantopus) can reach an age of about 180 years, whereas some mayfly species (belonging to the insect order Ephemeroptera) die after about 30 minutes. Even older than giant tortoises are certain trees, such as the yew (Taxus baccata), with some specimens between 4,000 and 5,000 years old. A few other organisms, such as freshwater polyps of the genus Hydra, are thought to age at a negligible rate or to be even potentially immortal, although this is still somewhat controversial.” http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151 View in LinkedIn