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Life expectancy is a statistical measure of the average time an organism is expected to live, based on the year of its birth, its current age, and other demographic factors like sex. The most commonly used measure is life expectancy at birth (LEB), which can be defined in two ways. Cohort LEB is the mean length of life of a birth cohort and can be computed only for cohorts born so long ago that all their members have died. Period LEB is the mean length of life of a hypothetical cohort assumed to be exposed, from birth through death, to the mortality rates observed at a given year.
Menopause, also known as the climacteric, is the time in women's lives when menstrual periods stop permanently, and they are no longer able to bear children. Menopause usually occurs between the age of 48 and 52. Medical professionals often define menopause as having occurred when a woman has not had any menstrual bleeding for a year. It may also be defined by a decrease in hormone production by the ovaries. In those who have had surgery to remove their uterus but still have ovaries, menopause may be considered to have occurred at the time of the surgery or when their hormone levels fell. Following the removal of the uterus, symptoms typically occur earlier, at an average of 45 years of age.
Senescence or biologicalaging is the gradual deterioration of functional characteristics in living organisms. The word senescence can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence involves an increase in death rates and/or a decrease in fecundity with increasing age, at least in the latter part of an organism's life cycle.
Life extension is the concept of extending the human lifespan, either modestly through improvements in medicine or dramatically by increasing the maximum lifespan beyond its generally-settled limit of 125 years.
Biological immortality is a state in which the rate of mortality from senescence is stable or decreasing, thus decoupling it from chronological age. Various unicellular and multicellular species, including some vertebrates, achieve this state either throughout their existence or after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury, poison, disease, lack of available resources, or changes to environment.
Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic gene may have an effect on several traits simultaneously, due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.
The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking. It builds on the previously postulated "mother hypothesis" which states that as mothers age, the costs of reproducing become greater, and energy devoted to those activities would be better spent helping her offspring in their reproductive efforts. It suggests that by redirecting their energy onto those of their offspring, grandmothers can better ensure the survival of their genes through younger generations. By providing sustenance and support to their kin, grandmothers not only ensure that their genetic interests are met, but they also enhance their social networks which could translate into better immediate resource acquisition. This effect could extend past kin into larger community networks and benefit wider group fitness.
The age of onset is the age at which an individual acquires, develops, or first experiences a condition or symptoms of a disease or disorder. For instance, the general age of onset for the spinal disease scoliosis is "10-15 years old," meaning that most people develop scoliosis when they are of an age between ten and fifteen years.
Life history theory is an analytical framework designed to study the diversity of life history strategies used by different organisms throughout the world, as well as the causes and results of the variation in their life cycles. It is a theory of biological evolution that seeks to explain aspects of organisms' anatomy and behavior by reference to the way that their life histories—including their reproductive development and behaviors, post-reproductive behaviors, and lifespan —have been shaped by natural selection. A life history strategy is the "age- and stage-specific patterns" and timing of events that make up an organism's life, such as birth, weaning, maturation, death, etc. These events, notably juvenile development, age of sexual maturity, first reproduction, number of offspring and level of parental investment, senescence and death, depend on the physical and ecological environment of the organism.
Enquiry into the evolution of ageing, or aging, aims to explain why a detrimental process such as ageing would evolve, and why there is so much variability in the lifespans of organisms. The classical theories of evolution suggest that environmental factors, such as predation, accidents, disease, starvation, ensure that most organisms living in natural settings will not live until old age, and so there will be very little pressure to conserve genetic changes that increase longevity. Natural selection will instead strongly favor genes which ensure early maturation and rapid reproduction, and the selection for genetic traits which promote molecular and cellular self-maintenance will decline with age for most organisms.
The paternal age effect is the statistical relationship between the father's age at conception and biological effects on the child. Such effects can relate to birthweight, congenital disorders, life expectancy and psychological outcomes. A 2017 review found that while severe health effects are associated with higher paternal age, the total increase in problems caused by paternal age is low. While paternal age has increased since 1960–1970, this is not seen as a major public health concern.
Caretaker genes encode products that stabilize the genome. Fundamentally, mutations in caretaker genes lead to genomic instability. Tumor cells arise from two distinct classes of genomic instability: mutational instability arising from changes in the nucleotide sequence of DNA and chromosomal instability arising from improper rearrangement of chromosomes.
Ageing (BE) or aging (AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. Furthermore, ageing connotes a biological and social construct. It is usually associated with dynamic changes in the biological, psychological, physiological, environmental, behavioral and social processes. In the broader sense, ageing can refer to single cells within an organism which have ceased dividing or to the population of a species.
The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls for more than one phenotypic trait in an organism. Antagonistic pleiotropy is when one gene controls for more than one trait, where at least one of these traits is beneficial to the organism's fitness early on in life and at least one is detrimental to the organism's fitness later on due to a decline in the force of natural selection. The theme of G.C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs.
Negligible senescence is a term coined by biogerontologist Caleb Finch to denote organisms that do not exhibit evidence of biological aging (senescence), such as measurable reductions in their reproductive capability, measurable functional decline, or rising death rates with age. There are many species where scientists have seen no increase in mortality after maturity. This may mean that the lifespan of the organism is so long that researchers' subjects have not yet lived up to the time when a measure of the species' longevity can be made. Turtles, for example, were once thought to lack senescence, but more extensive observations have found evidence of decreasing fitness with age.
The guppy, also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish and one of the most popular freshwater aquarium fish species. It is a member of the family Poeciliidae and, like almost all American members of the family, is live-bearing. Guppies originate from northeast South America, but have been introduced to many environments and are now found all over the world. They are highly adaptable and thrive in many different environmental and ecological conditions. Male guppies, which are smaller than females, have ornamental caudal and dorsal fins. Wild guppies generally feed on a variety of food sources, including benthic algae and aquatic insect larvae. Guppies are used as a model organism in the fields of ecology, evolution, and behavioural studies.
The patriarch hypothesis is a hypothesis that explains the occurrence of menopause in human females and how a long post-fertile period could confer an evolutionary advantage. It is an alternative theory to the grandmother hypothesis which tends to ignore male benefits of continued spermatogenesis and their roles in assistance.
The reproductive-cell cycle theory posits that the hormones that regulate reproduction act in an antagonistic pleiotrophic manner to control aging via cell cycle signaling; promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence. Rather than seeing aging as a loss of functionality as we get older, this theory defines aging as any change in an organism over time, as evidenced by the fact that if all chemical reactions in the body were stopped, no change, and thus no aging, would occur. Since the most important change in an organism through time is the chemical reactions that result in a single cell developing into a multicellular organism, whatever controls these chemical reactions that regulate cell growth, development, and death, is believed to control aging. The theory argues that these cellular changes are directed by reproductive hormones of the hypothalamic-pituitary-gonadal axis. Receptors for reproductive hormones have been found to be present in all tissues of the body. Thus, HPG axis hormones normally promote growth and development of the organism early in life in order to achieve reproduction. Hormones levels then begin to change in men around age 30 and more abruptly in women when they reach menopause, around age 50. When the HPG axis becomes unbalanced, cellular growth and development is dysregulated, and cell death and dysfunction can occur, both of which can initiate senescence, the accumulated damage to cells, tissues, and organs that occurs with the passage of time and that is associated with functional loss during aging.
The disposable soma theory of aging states that organisms age due to an evolutionary trade-off between growth, reproduction, and DNA repair maintenance. Formulated by Thomas Kirkwood, the disposable soma theory explains that an organism only has a limited amount of resources that it can allocate to its various cellular processes. Therefore, a greater investment in growth and reproduction would result in reduced investment in DNA repair maintenance, leading to increased cellular damage, shortened telomeres, accumulation of mutations, compromised stem cells, and ultimately, senescence. Although many models, both animal and human, have appeared to support this theory, parts of it are still controversial. Specifically, while the evolutionary trade-off between growth and aging has been well established, the relationship between reproduction and aging is still without scientific consensus, and the cellular mechanisms largely undiscovered.
The mutation accumulation theory of ageing was first proposed by Peter Medawar in 1952 as an evolutionary explanation for biological ageing and the associated decline in fitness that accompanies it. Medawar used the term 'senescence' to refer to this process. The theory explains that, in the case where harmful mutations are only expressed later in life, when reproduction has ceased and future survival is increasingly unlikely, then these mutations are likely to be unknowingly passed on to future generations. In this situation the force of natural selection will be weak, and so insufficient to consistently eliminate these mutations. Medawar posited that over time these mutations would accumulate due to genetic drift and lead to the evolution of what is now referred to as ageing.
References
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- ↑ Dańko MJ, Burger O, Kozłowski J (2017). "Density-dependence interacts with extrinsic mortality in shaping life histories". PLOS ONE. 12 (10): e0186661. Bibcode:2017PLoSO..1286661D. doi:10.1371/journal.pone.0186661. PMC 5648222 . PMID 29049399.
- ↑ Carnes BA (April 2004). "Darwinian bodies in a Lamarkian world". The Gerontologist. 44 (2): 274–279. doi: 10.1093/geront/44.2.274 .
- ↑ Blell M (2017-09-29). "Grandmother Hypothesis, Grandmother Effect, and Residence Patterns". The International Encyclopedia of Anthropology. John Wiley & Sons, Ltd. pp. 1–5. doi:10.1002/9781118924396.wbiea2162. ISBN 9780470657225.
{{cite book}}: Missing or empty|title=(help) - 1 2 Quinlan RJ (2010-06-01). "Extrinsic Mortality Effects on Reproductive Strategies in a Caribbean Community". Human Nature. 21 (2): 124–139. doi:10.1007/s12110-010-9085-1. S2CID 39977514.