[Paleopsych] SW: Mortality and Lifespan
checker at panix.com
Mon May 23 19:48:57 UTC 2005
Evolutionary Biology: Mortality and Lifespan
The following points are made by Peter A. Abrams (Nature 2004
1) Recent work(1) has involved an investigation of one of the main
factors that influence the evolution of an organism's lifespan. That
factor is the risk of dying that a population faces as a result of
environmental conditions (e.g., predation). The study subjects were
guppies, small tropical fish that are widely used in evolutionary
studies, and the authors have provided the first experimental support
for the prediction that a higher environmental risk of mortality can
select for inherently longer-lived organisms.
2) Guppies from the lower reaches of several rivers in Trinidad are
subject to much higher rates of predation than those in the upper
parts of the same rivers, where waterfalls block access by larger
fish. In predator-free lab experiments, Reznick et al(1) found that
guppies from the high-predation segments of two of the rivers lived up
to 35% longer than those from low-predation segments of the same
watercourse. In addition, the guppies from high-predation sites had a
40% longer reproductive span and reproduced at a higher rate. So a
background of higher mortality under natural conditions has apparently
led to the evolution of both a longer lifespan and a longer
3) Some history is required to see why this observation is surprising.
Environmentally caused ("extrinsic") mortality has long been
recognized as a key factor determining how natural selection molds
"intrinsic" mortality -- the death rate that a population would have
under some standardized, generally benign, set of environmental
conditions. Although evolution should favor lower intrinsic mortality
(and a longer intrinsic lifespan) when all else is equal, many
organisms face a trade-off between higher levels of reproduction or
lower levels of intrinsic mortality. One of the main reasons that
senescence occurs is because repair is costly: resources that are
devoted to maintaining an organism are not available for reproduction.
In the 1950s, Peter Medawar(2) and George Williams(3) pointed out that
high extrinsic mortality could favor shorter intrinsic lifespan. Why,
they reasoned, should an organism invest in costly repair that will
probably only ensure that it is in prime physical condition when its
life ends? Higher extrinsic mortality should favor low investment in
repair, and thus a high intrinsic mortality and a short intrinsic
4) But this reasoning did not take account of two further factors. One
is that higher extrinsic mortality also slows the rate of population
growth, and more slowly growing populations are expected to evolve to
have lower rates of intrinsic mortality and a longer lifespan(4,5).
The other factor is the interaction between extrinsic mortality
factors and physiological repair or maintenance(5). If predators can
be evaded by fast, but not by slow prey, greater predation risk should
select for greater maintenance of the body systems essential for fast
movement. This higher level of repair would then prolong intrinsic
1. Reznick, D. N., Bryant, M. J., Roff, D., Ghalambor, C. K. &
Ghalambor, D. E. Nature 431, 1095-1099 (2004)
2. Medawar, P. B. An unsolved problem in Biology (Lewis, London)
3. Williams, G. C. Evolution 11, 398-411 (1957)
4. Charlesworth, B. A. Evolution in Age-Structured Populations
(Cambridge Univ. Press, 1980)
5. Abrams, P. A. Evolution 47, 877-887 (1993)
SIGNALS FROM THE REPRODUCTIVE SYSTEM REGULATE THE LIFESPAN OF C.
The following points are made by H. Hsin and C. Kenyon (Nature 1999
1) Understanding how the ageing process is regulated is a fascinating
and fundamental problem in biology. The authors demonstrate that
signals from the reproductive system influence the lifespan of the
nematode Caenorhabditis elegans. If the cells that give rise to the
germ line are killed with a laser microbeam, the lifespan of the
animal is extended.
2) The authors suggest their findings indicate that germline signals
act by modulating the activity of an insulin/IGF-1 (insulin-like
growth factor) pathway that is known to regulate the ageing of this
organism. Mutants with reduced activity of the insulin/IGF-1-receptor
homologue DAF-2 have been shown to live twice as long as normal, and
their longevity requires the activity of DAF- 16, a member of the
forkhead/winged-helix family of transcriptional regulators.
3) The authors find that in order for germline ablation to extend
lifespan, DAF-16 is required, as well as a putative nuclear hormone
receptor, DAF-12. In addition, the findings suggest that signals from
the somatic gonad also influence ageing, and that this effect requires
4) The authors suggest that together their findings imply that the C.
elegans insulin/IGF-1 system integrates multiple signals to define the
animal's rate of ageing. The authors suggest this study demonstrates
an inherent relationship between the reproductive state of this animal
and its lifespan, and may have implications for the co-evolution of
reproductive capability and longevity.
AGING, LIFESPAN, AND SENESCENCE
Notes by ScienceWeek:
Our knowledge of the basis of senescence of cells, tissues, and
organisms (including humans) has entered a new phase in recent decades
because of the new vistas opened by molecular biology. Model systems
have started to provide insights, and one important approach has been
the identification of genes that determine the lifespan of an
organism. The very existence of genes that when mutated can extend
lifespan suggests to many researchers that one or a few processes may
be critical in aging, and that a slowing of these processes may slow
The following points are made by L. Guarente et al (Proc. Nat. Acad.
Sci. 1998 95:11034):
1) In the budding yeast Saccharomyces cerevisiae, aging results from
the asymmetry of cell division, which produces a large mother cell and
a small daughter cell arising from the bud. Much of the macromolecular
composition of the daughter cell is newly synthesized, whereas the
composition of the mother cell grows older with each cell division. It
has been shown that mother cells of this yeast species divide a
relatively fixed number of times, and exhibit a slowing of the cell
cycle, cell enlargement, and sterility. Analysis of *ribosomal DNA in
old cells reveals an accumulation of *extrachromosomal ribosomal DNA
of discrete sizes, apparently representing a cumulative fragmentation
of chromosomal ribosomal DNA. The authors suggest it will be of great
interest to assess the generality of this process as an aging
2) In Caenorhabditis elegans, the *neurosecretory system regulates
whether animals enter the reproductive life cycle or arrest
development at a primitive *diapause stage. Developmental arrest is
apparently induced by a *pheromone and involves behavioral and
morphological changes in many tissues of the animal, with the lifespan
becoming 4 to 8 times longer than that of the normal 3-week lifespan
of fully developed animals. Declines in pheromone concentration induce
recovery to reproductive adults with normal metabolism and lifespan.
Genes that regulate the function of the C. elegans diapause and the
neuroendocrine aging pathway have been identified, and at least one of
these genes codes for an *insulin-like receptor apparently involved in
metabolism. The authors suggest that if the association of longevity
and diapause is general, it is possible that *polymorphisms in the
human insulin receptor-signaling pathway genes and related gene
*homologues may underlie genetic variation in human longevity.
3) In plants, there is a large range of lifespans in the various plant
kingdoms. Certain tree species live for well over a century, whereas
other plants complete their life cycle in a few weeks. The "yellowing"
of leaves is often referred to in the plant literature as leaf
senescence or the "senescence syndrome" -- referring to the process by
which nutrients are mobilized from the dying leaf to other parts of
the plant to support their growth. The senescence syndrome is
characterized by distinct cellular and molecular changes, with the
chloroplast the first part of the cell to undergo disassembly
(producing the "yellowing"). In many plant species, certain hormones
can either enhance or delay senescence. Although the genes that are
expressed during the plant senescence syndrome (as well as ways to
manipulate such senescence) have been identified, much remains to be
done to understand the molecular basis of aging in plants. For
example, nothing is known about the signal transduction pathways that
lead to altered gene expression during senescence, or how plant
hormones such as *cytokinin influence senescence. But there are now
many tools to explore this process. The authors conclude: "It remains
to be seen whether common mechanisms link the aging process in diverse
Proc. Nat. Acad. Sci. http://www.pnas.org
Notes by ScienceWeek:
ribosomal DNA: A ribosome (not to be confused with riboZYME) is a
small particle, a complex of various ribonucleic acid component
subunits and proteins that functions as the site of protein synthesis.
The term "ribosomal DNA" refers to the gene or genes that code for the
RNA in ribosomes. In other words, the term "ribosomal DNA" does not
refer to any DNA in ribosomes (there is no DNA in ribosomes).
extrachromosomal: In general, this refers to anything outside of
chromosomes, and in this case to DNA fragments unincorporated into
neurosecretory system: In general, all neural systems contain both
neurons that themselves secrete chemical messengers and neurons that
signal special secretory cells to secrete chemical messengers. A
neurosecretory pathway is a delineated signaling system that involves
such a resultant secretion.
diapause: In general, this refers to any programmed period of
suspended development in invertebrates.
pheromone: In general, a chemical substance which, when released into
an animal's surroundings, influences the development or behavior of
other individuals of the same species.
insulin: A protein hormone that promotes uptake by body cells of free
glucose and/or amino acids, depending on target cell type.
polymorphisms: A genetic polymorphism is a naturally occurring
variation in the normal nucleotide sequence of the genome within
individuals in a population. Variations are denoted as polymorphisms
only if they cannot be accounted for by recurrent mutation and occur
with a frequency of at least about 1 percent.
homologues: In general, the term "homologous" means having the same
structure. But the term has special uses in genetics and evolution
cytokinin: A group of plant growth substances. They are chemically
identified as derivatives of the purine base adenine. They stimulate
cell division and determine the course of differentiation. They work
synergistically with other plant hormones called "auxins".
More information about the paleopsych