Life history theory
Life history theory is an analytical framework[1] 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.[2] 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, life span and post-reproductive behavior—have been shaped by natural selection. A life history strategy is the "age- and stage-specific patterns"[2] and timing of events that make up an organism's life, such as birth, weaning, maturation, death, etc.[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.[5] In order to study these topics, life history strategies must be identified, and then models are constructed to study their effects. Finally, predictions about the importance and role of the strategies are made,[6] and scientists use these predictions to understand how evolution affects the ordering and length of life history events in an organism's life, particularly the lifespan and period of reproduction.[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.[8] It also uses measures of evolutionary fitness to determine if organisms are able to maximize or optimize this fitness,[9] by allocating resources to a range of different demands throughout the organism's life.[1] It serves as a method to investigate further the "many layers of complexity of organisms and their worlds".[10]
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, who 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. 
Brief history of field
Life history theory is seen as a branch of evolutionary ecology[2] and is used in a variety of different fields. Beginning in the 1950s, mathematical analysis became an important aspect of research regarding LHT.[11] There are two main focuses that have developed over time: genetic and phenotypic,[10] but there has been a recent movement towards combining these two approaches.[11] 
Darwinian fitness
In the context of evolution, fitness is determined by the number of descendents an organism produces over the course of its life. The main elements are survivorship and reproductive rate.[5] This means that the organism's traits and gene are carried on into future generations, and contribute to evolutionary "success". 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.[5] In life history theory, evolution works on the life stages of organisms and creates adaptation—this process establishes fitness
Life history traits
There are seven traits that are traditionally recognized as important in life history theory.[4] The trait that is seen as the most important for any given organism is the one where a change in that trait creates the most significant difference in that organism's level of fitness. In this sense, an organism's fitness is determined by its changing life history traits.[6] The way in which evolutionary forces act on these life history traits serves to limit the genetic variability and heritability of the life history strategies,[4] although there are still large varieties that exist in the world
Life history strategies
Combinations of these life history traits and life events create the life history strategies. 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.[13] These types of strategies are defined by the body size of the fish, age at maturation, high or low survivorship, and the type of environment they are found in. So, a fish with a small body size, a late age of maturation, and low survivorship, found in a seasonal environment, would be classified as having a periodic life strategy.[13] The type of behaviors taking place during life events can also define life history strategies. For example, an exploitative life history strategy would be one where an organism benefits by using more resources than others, or by taking these resources from other organisms
Life history characteristics
Life history characteristics are traits that affect the life table 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 favoured 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:
Age at first reproductive event
Reproductive lifespan and ageing
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.[15] 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.[16] 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 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.[17]
Biological tradeoffs also appear to characterize the life histories of viruses, including bacteriophages.
Reproductive value and costs of reproduction
Reproductive value 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:[19]
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[20] 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.[21] 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.
r/K selection theory
Further information: 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 theory. 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.[23]
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,[24] 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 reproduces at ten years old and breeds every other year during its 40-year lifespan.[25]
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
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