Life history theory
Encyclopedia
Life history theory posits that the schedule and duration of key events in an organism's lifetime are shaped by natural selection to produce the largest possible number of surviving offspring. 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. Organisms have evolved a great variety of life histories, from Pacific salmon, which produce thousands of eggs at one time and then die, to human beings, which produce a few offspring over the course of decades. The theory depends on principles of evolutionary biology and ecology
and is widely used in other areas of science.
of an organism, and can be imagined as various investments in growth, reproduction, and survivorship.
The goal of life history theory is to understand the variation in such life history strategies. This knowledge can be used to construct models to predict what kinds of traits will be favored in different environments. Without constraints, the highest fitness would belong to a Darwinian Demon
, a hypothetical organism for whom such trade-offs do not exist. The key to life history theory is that there are limited resources available, and focusing on only a few life history characteristics is necessary.
Examples of some major life history characteristics include:
Variations in these characteristics reflect different allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions. For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time, effort, and energy available for another.
For example, birds with larger broods are unable to afford more prominent secondary sexual characteristics . Life history characteristics will, in some cases, change according to the population density, since genotypes with the highest fitness at high population densities will not have the highest fitness at low population densities. Other conditions, such as the stability of the environment, will lead to selection for certain life history traits. Experiments by Michael R. Rose
and Brian Charlesworth
showed that unstable environments selected for flies with both shorter lifespans and higher fecundity.
models the tradeoffs between reproduction, growth, and survivorship. An organism's reproductive value (RV) is defined as its expected contribution to the population through both current and future reproduction:
RV = Current Reproduction + Residual Reproductive Value (RRV)
The residual reproductive value represents an organism's future reproduction through its investment in growth and survivorship. The cost-of-reproduction hypothesis predicts that higher investment in current reproduction hinders growth and survivorship and reduces future reproduction, while investments in growth will pay off with higher fecundity (number of offspring produced) and reproductive episodes in the future. This cost-of-reproduction tradeoff influences major life history characteristics. For example, a 2009 study by J. Creighton, N. Heflin, and M. Belk on burying beetles provided "unconfounded support" for the costs of reproduction. The study found that beetles that had allocated too many resources to current reproduction also had the shortest lifespans. In their lifetimes, they also had the fewest reproductive events and offspring, reflecting how over-investment in current reproduction lowers residual reproductive value.
The related terminal investment hypothesis describes a shift to current reproduction with higher age. At early ages, RRV is typically high, and organisms should invest in growth to increase reproduction at a later age. As organisms age, this investment in growth gradually increases current reproduction. However, when an organism grows old and begins losing physiological function, mortality increases while fecundity decreases. This senescence
shifts the reproduction tradeoff towards current reproduction: the effects of aging and higher risk of death make current reproduction more favorable. The burying beetle study also supported the terminal investment hypothesis: the authors found beetles that bred later in life also had increased brood sizes, reflecting greater investment in those reproductive events.
. The central trade-off to life history theory is the number of offspring vs. the timing of reproduction. Organisms that are r-selected have a high growth rate (r) and tend to produce a high number of offspring with minimal parental care; their lifespans also tend to be shorter. R-selected organisms are suited to life in an unstable environment, because they reproduce early and abundantly and allow for a low survival rate of offspring. K-selected organisms subsist near the carrying capacity of their environment (K), produce a relatively low number of offspring over a longer span of time, and have high parental investment. They are more suited to life in a stable environment in which they can rely on a long lifespan and a low mortality rate that will allow them to reproduce multiple times with a high offspring survival rate.
Some organisms that are very r-selected are semelparous, only reproducing once before they die. Semelparous organisms may be short-lived, like annual crops. However, some semelparous organisms are relatively long-lived, such as the African flowering plant Lobelia telekii
which spends up to several decades growing an inflorescence that blooms only once before the plant dies, or the periodical cicada which spends 17 years as a larva before emerging as an adult. Organisms with longer lifespans are usually iteroparous, reproducing more than once in a lifetime. However, iteroparous organisms can be more r-selected than K-selected, such as a sparrow
, which gives birth to several chicks per year but lives only a few years, as compared to a wandering albatross
, which first gives birth at ten years old and breeds every other year during its 40 year lifespan.
r-selected organisms usually:
K-selected organisms usually:
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
development, age of sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
, first reproduction, number of offspring and level of parental investment
Parental investment
In evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...
, senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
and death, depend on the physical and ecological environment of the organism. Organisms have evolved a great variety of life histories, from Pacific salmon, which produce thousands of eggs at one time and then die, to human beings, which produce a few offspring over the course of decades. The theory depends on principles of evolutionary biology and ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
and is widely used in other areas of science.
Life history characteristics
Life history characteristics are traits that affect the life tableLife table
In actuarial science, a life table is a table which shows, for each age, what the probability is that a person of that age will die before his or her next birthday...
of an organism, and can be imagined as various investments in growth, reproduction, and survivorship.
The goal of life history theory is to understand the variation in such life history strategies. This knowledge can be used to construct models to predict what kinds of traits will be favored in different environments. Without constraints, the highest fitness would belong to a Darwinian Demon
Darwinian Demon
A Darwinian Demon is a hypothetical organism that can maximize all aspects of fitness simultaneously and would exist if the evolution of species was entirely unconstrained. Such organisms would reproduce directly after being born, produce infinitely many offspring, and live indefinitely...
, a hypothetical organism for whom such trade-offs do not exist. The key to life history theory is that there are limited resources available, and focusing on only a few life history characteristics is necessary.
Examples of some major life history characteristics include:
- Age at first reproductive event
- Reproductive lifespan and aging
- Number and size of offspring
Variations in these characteristics reflect different allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions. For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time, effort, and energy available for another.
For example, birds with larger broods are unable to afford more prominent secondary sexual characteristics . Life history characteristics will, in some cases, change according to the population density, since genotypes with the highest fitness at high population densities will not have the highest fitness at low population densities. Other conditions, such as the stability of the environment, will lead to selection for certain life history traits. Experiments by Michael R. Rose
Michael R. Rose
Michael R. Rose is a Professor in the Department of Ecology and Evolutionary Biology at the University of California, Irvine. His advisor was Brian Charlesworth. His main area of work has been the evolution of aging. In 1991, he published Evolutionary Biology of Aging exploring a view of the...
and Brian Charlesworth
Brian Charlesworth
Professor Brian Charlesworth FRS is a British evolutionary biologist at the University of Edinburgh, and editor of Biology Letters....
showed that unstable environments selected for flies with both shorter lifespans and higher fecundity.
Reproductive value and costs of reproduction
Reproductive valueReproductive value
Reproductive value is a term used in social psychology to describe the level of attractiveness of women. The reproduction value theory introduces a mechanism that men unintentionally use when "searching" for a partner. The level of the RV is decided by several things, including:* Genetic...
models the tradeoffs between reproduction, growth, and survivorship. An organism's reproductive value (RV) is defined as its expected contribution to the population through both current and future reproduction:
RV = Current Reproduction + Residual Reproductive Value (RRV)
The residual reproductive value represents an organism's future reproduction through its investment in growth and survivorship. The cost-of-reproduction hypothesis predicts that higher investment in current reproduction hinders growth and survivorship and reduces future reproduction, while investments in growth will pay off with higher fecundity (number of offspring produced) and reproductive episodes in the future. This cost-of-reproduction tradeoff influences major life history characteristics. For example, a 2009 study by J. Creighton, N. Heflin, and M. Belk on burying beetles provided "unconfounded support" for the costs of reproduction. The study found that beetles that had allocated too many resources to current reproduction also had the shortest lifespans. In their lifetimes, they also had the fewest reproductive events and offspring, reflecting how over-investment in current reproduction lowers residual reproductive value.
The related terminal investment hypothesis describes a shift to current reproduction with higher age. At early ages, RRV is typically high, and organisms should invest in growth to increase reproduction at a later age. As organisms age, this investment in growth gradually increases current reproduction. However, when an organism grows old and begins losing physiological function, mortality increases while fecundity decreases. This senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
shifts the reproduction tradeoff towards current reproduction: the effects of aging and higher risk of death make current reproduction more favorable. The burying beetle study also supported the terminal investment hypothesis: the authors found beetles that bred later in life also had increased brood sizes, reflecting greater investment in those reproductive events.
r/K selection theory
The selection pressures that determine the reproductive strategy, and therefore much of the life history, of an organism can be understood in terms of r/K selection theoryR/K selection theory
In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...
. The central trade-off to life history theory is the number of offspring vs. the timing of reproduction. Organisms that are r-selected have a high growth rate (r) and tend to produce a high number of offspring with minimal parental care; their lifespans also tend to be shorter. R-selected organisms are suited to life in an unstable environment, because they reproduce early and abundantly and allow for a low survival rate of offspring. K-selected organisms subsist near the carrying capacity of their environment (K), produce a relatively low number of offspring over a longer span of time, and have high parental investment. They are more suited to life in a stable environment in which they can rely on a long lifespan and a low mortality rate that will allow them to reproduce multiple times with a high offspring survival rate.
Some organisms that are very r-selected are semelparous, only reproducing once before they die. Semelparous organisms may be short-lived, like annual crops. However, some semelparous organisms are relatively long-lived, such as the African flowering plant Lobelia telekii
Lobelia telekii
Lobelia telekii is a species of flowering plant in the bellflower family, Campanulaceae, that is found only in the alpine zones of Mount Kenya, Mount Elgon, and the Aberdare Mountains of East Africa. It lives at high altitudes on well-drained sloped hillsides...
which spends up to several decades growing an inflorescence that blooms only once before the plant dies, or the periodical cicada which spends 17 years as a larva before emerging as an adult. Organisms with longer lifespans are usually iteroparous, reproducing more than once in a lifetime. However, iteroparous organisms can be more r-selected than K-selected, such as a sparrow
Sparrow
The sparrows are a family of small passerine birds, Passeridae. They are also known as true sparrows, or Old World sparrows, names also used for a genus of the family, Passer...
, which gives birth to several chicks per year but lives only a few years, as compared to a wandering albatross
Wandering Albatross
The Wandering Albatross, Snowy Albatross or White-winged Albatross, Diomedea exulans, is a large seabird from the family Diomedeidae, which has a circumpolar range in the Southern Ocean. It was the first species of albatross to be described, and was long considered the same species as the Tristan...
, which first gives birth at ten years old and breeds every other year during its 40 year lifespan.
r-selected organisms usually:
- mature rapidly and have an early age of first reproduction
- have a relatively short lifespan
- have a large number of offspring at a time, and few reproductive events, or are semelparous
- have a high mortality rate and a low offspring survival rate
- have minimal parental care/investment
K-selected organisms usually:
- mature more slowly and have a later age of first reproduction
- have a longer lifespan
- have few offspring at a time and more reproductive events spread out over a longer span of time
- have a low mortality rate and a high offspring survival rate
- have high parental investment
Determinants of Life History
Many factors can determine the evolution of an organism's life history, especially the unpredictability of the environment. Organisms that live in a very unpredictable environment—one in which resources, hazards, and competitors may fluctuate rapidly—selects for organisms that produce more offspring earlier in their lives, because it is never certain whether they will survive to reproduce again. Mortality rate may be the best indicator of a species' life history: organisms with high mortality rate—the usual result of an unpredictable environment—typically mature earlier than those species with low mortality rates, and give birth to more offspring at a time. A highly unpredictable environment can also lead to plasticity, in which individual organisms can shift along the spectrum of r-selected vs. K-selected life histories to suit the environment.Perspectives
Life history theory has provided new perspectives in understanding many aspects of human reproductive behavior, such as the relationship between poverty and fertility. A number of statistical predictions have been confirmed by social data and there is a large body of scientific literature from studies in experimental animal models, and naturalistic studies among many organisms.See also
- Behavioral ecologyBehavioral ecologyBehavioral ecology, or ethoecology, is the study of the ecological and evolutionary basis for animal behavior, and the roles of behavior in enabling an animal to adapt to its environment...
- Darwinian DemonDarwinian DemonA Darwinian Demon is a hypothetical organism that can maximize all aspects of fitness simultaneously and would exist if the evolution of species was entirely unconstrained. Such organisms would reproduce directly after being born, produce infinitely many offspring, and live indefinitely...
- Evolutionary developmental psychologyEvolutionary developmental psychologyEvolutionary developmental psychology, , is the application of the basic principles of Darwinian evolution, particularly natural selection, to explain contemporary human development...
- Evolutionary physiologyEvolutionary physiologyEvolutionary physiology is the study of physiological evolution, which is to say, the manner in which the functional characteristics of individuals in a population of organisms have responded to selection across multiple generations during the history of the population.It is a subdiscipline of both...
- Human behavioral ecologyHuman behavioral ecologyHuman behavioral ecology or human evolutionary ecology applies the principles of evolutionary theory and optimization to the study of human behavioral and cultural diversity. HBE examines the adaptive design of traits, behaviors, and life histories of humans in an ecological context...
- Parental investmentParental investmentIn evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...
- Mating effort
- Parental effort
- Somatic effortSomatic effortSomatic effort refers to the total investments of an organism in its own development, differentiation, and maintenance which consequently increases its reproductive potential....
- Age determination in woody plantsDendrochronologyDendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
- Age determination in herbaceous plantsHerbchronologyHerbchronology is the analysis of annual growth rings in the secondary root xylem of perennial herbaceous plants. While leaves and stems of perennial herbs die down at the end of the growing season the root often persists for many years or even the entire life...
Further reading
- (peer-reviewed) Nature Education Knowledge entry on Semelparity
- Ellis, B.J. (2004). Timing of pubertal maturation in girls: an integrated life history approach. Psychological Bulletin. 130:920-58.
- Kaplan, H., K. Hill, J. Lancaster, and A.M. Hurtado. (2000). The Evolution of intelligence and the Human life history. Evolutionary Anthropology, 9(4): 156-184..
- Quinlan, R.J. (2007). Human parental effort and environmental risk. Proceedings of the Royal Society B: Biological Sciences, 274(1606):121-125.
- Vigil, J. M., Geary, D. C., & Byrd-Craven, J. (2005). A life history assessment of early childhood sexual abuse in women. Developmental Psychology, 41, 553-561.
- Walker, R., Gurven, M., Hill, K., Migliano, A., Chagnon, N., Djurovic, G., Hames, R., Hurtado, AM, Kaplan, H., Oliver, W., de Souza, R., Valeggia, C., Yamauchi, T. (2006). Growth rates, developmental markers and life histories in 21 small-scale societies. American Journal of Human Biology 18:295-311.
- Derek A. Roff (2007). Contributions of genomics to life-history theory. Nature Reviews Genetics 8, 116-125.
- Freeman, Scott and Herron, Jon C. 2007. Evolutionary Analysis 4th Ed: Aging and Other Life History Characteristics. 485-86, 514, 516.
- Kaplan, H.S., and AJ Robson, 2002. The emergence of humans: The coevolution of intelligence and longevity with intergenerational transfers. PNAS 99: 10221-10226.
- Kaplan, H.S., Lancaster, J.B., & Robson, 2003. Embodied Capital and the Evolutionary Economics Of the Human Lifespan. In: Lifespan: Evolutionary, Ecology and Demographic Perspectives, J.R. Carey & S. Tuljapakur (eds.) Population and Development Review 29, Supplement 2003, Pp. 152–182.