[3] 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.
The theory was developed in the 1950s[4] and is used to answer questions about topics such as organism size, age of maturation, number of offspring, life span, and many others.
[7] Life history theory draws on an evolutionary foundation, and studies the effects of natural selection on organisms, both throughout their lifetime and across generations.
[11] All organisms follow a specific sequence in their development,[9] beginning with gestation and ending with death, which is known as the life cycle.
Events in between usually include birth, childhood, maturation, reproduction, and senescence, and together these comprise the life history strategy of that organism.
[5] For example, a human, between being born and reaching adulthood, will pass through an assortment of life stages, which include: birth, infancy, weaning, childhood and growth, adolescence, sexual maturation, and reproduction.
Often, as a shorthand for natural selection, researchers only assess the number of descendants an organism produces over the course of its life.
The process of adaptation contributes to this "success" by impacting rates of survival and reproduction,[2] which in turn establishes an organism's level of Darwinian fitness.
As an example, Winemiller and Rose, as cited by Lartillot & Delsuc, propose three types of life history strategies in the fish they study: opportunistic, periodic, and equilibrium.
A fish with a large body size, a late age of maturation, and low survivorship, found in a seasonal environment, would be classified as having a periodic life strategy.
Experiments by Michael R. Rose and Brian Charlesworth showed that unstable environments select for flies with both shorter lifespans and higher fecundity—in unreliable conditions, it is better for an organism to breed early and abundantly than waste resources promoting its own survival.
For example, a 2009 study by J. Creighton, N. Heflin, and M. Belk on burying beetles provided "unconfounded support" for the costs of reproduction.
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.
An essential component of studying life history strategies is identifying the trade-offs[26] that take place for any given organism.
Instead of focusing on one trait and looking at how it changed, scientists are looking at these trade-offs as part of a larger system, with complex inputs and outcomes.
[6] This base assumption, that over the course of its life span an organism is aiming for optimal energy use,[7] then allows scientists to test other predictions.
Models of resource allocation have been developed and used to study problems such as parental involvement, the length of the learning period for children, and other developmental issues.
This is because when offspring value is low, yet food is abundant, building stores to breed from allows these organisms to achieve higher rates of reproduction than they otherwise would have.
It affects the levels of genetic variability by serving as a source of variation and integration of fitness traits.
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.
[33] In studying humans, life history theory is used in many ways, including in biology, psychology, economics, anthropology, and other fields.
[8][36] Cooperative breeding and the grandmothering hypothesis have been proposed as the reasons that humans continue to live for many years after they are no longer capable of reproducing.
[7] The change in brain size may have been the result of a dietary shift—towards higher quality and difficult to obtain food sources[36]—or may have been driven by the social requirements of group living, which promoted sharing and provisioning.
[39][40][41] In investigating life history frameworks for explaining reproductive strategy development, empirical studies have identified issues with a psychometric approach, but tentatively supported predicted links between early stress, accelerated puberty, insecure attachment, unrestricted sociosexuality and relationship dissatisfaction.
[42] Life history theory has provided new perspectives in understanding many aspects of human reproductive behavior, such as the relationship between poverty and fertility.
This criticism argues that the total amount of predation threat faced by the young has the same effective protection need effect no matter if it comes in the form of a long childhood and far between the natural enemies or a short childhood and closely spaced natural enemies, as different life speeds are subjectively the same thing for the animals and only outwardly looks different.
This criticism argues that the longer the time the child needed parental investment relative to the lifespans of the species, the higher the percentage of children born would still need parental care when the female was no longer fertile or dramatically reduced in her fertility.
[49][50][page needed] There are also criticisms of size and organ trade-offs, including criticism of the claim of a trade-off between body size and longevity that cites the observation of longer lifespans in larger species, as well as criticism of the claim that big brains promoted sociality citing primate studies in which monkeys with large portions of their brains surgically removed remained socially functioning though their technical problem solving deteriorated in flexibility, computer simulations of chimpanzee social interaction showing that it requires no complex cognition, and cases of socially functioning humans with microcephalic brain sizes.
[51][page needed][52] 52) Marco Del Giudice "Evolutionary psychopathology: a unified approach", Oxford university Press, 2018