Life history theory
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.
The theory was developed in the 1950s and is used to answer questions about topics such as organism size, age of maturation, number of offspring, life span, and many others. 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, and these predictions are used 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. Life history theory draws on an evolutionary foundation, and studies the effects of natural selection on organisms, both throughout their lifetime and across generations. It also uses measures of evolutionary fitness to determine if organisms are able to maximize or optimize this fitness, by allocating resources to a range of different demands throughout the organism's life. It serves as a method to investigate further the "many layers of complexity of organisms and their worlds".
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 and is used in a variety of different fields. Beginning in the 1950s, mathematical analysis became an important aspect of research regarding LHT. There are two main focuses that have developed over time: genetic and phenotypic, but there has been a recent movement towards combining these two approaches.Life cycle
All organisms follow a specific sequence in their development, 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.The major events in this life cycle are usually shaped by the demographic qualities of the organism. Some are more obvious shifts than others, and may be marked by physical changes—for example, teeth erupting in young children. Some events may have little variation between individuals in a species, such as length of gestation, but other events may show a lot of variation between individuals, such as age at first reproduction.
Life cycles can be divided into two major stages: growth and reproduction. These two cannot take place at the same time, so once reproduction has begun, growth usually ends. This shift is important because it can also affect other aspects of an organism's life, such as the organization of its group or its social interactions.
Each species has its own pattern and timing for these events, often known as its ontogeny, and the variety produced by this is what LHT studies. Evolution then works upon these stages to ensure that an organism adapts to its environment. 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. All of these are defined in a specific biological way, which is not necessarily the same as the way that they are commonly used.
Darwinian fitness
In the context of evolution, fitness is determined by how the organism is represented in the future. Genetically, a fit allele outcompetes its rivals over generations. Often, as a shorthand for natural selection, researchers only assess the number of descendants an organism produces over the course of its life. Then, the main elements are survivorship and reproductive rate. This means that the organism's traits and genes are carried on into the next generation, and are presumed to contribute to evolutionary "success". The process of adaptation contributes to this "success" by impacting rates of survival and reproduction, which in turn establishes an organism's level of Darwinian fitness. In life history theory, evolution works on the life stages of particular species but is also discussed for a single organism's functional, lifetime adaptation. In both cases, researchers assume adaptation—processes that establish fitness.Traits
There are seven traits that are traditionally recognized as important in life history theory:- size at birth
- growth pattern
- age and size at maturity
- number, size, and sex ratio of offspring
- age- and size-specific reproductive investments
- age- and size-specific mortality schedules
- length of life
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. 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. 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. 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.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
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 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.
Biological tradeoffs also appear to characterize the life histories of viruses, including bacteriophages.
Reproductive value and costs of reproduction
models the tradeoffs between reproduction, growth, and survivorship. An organism's reproductive value is defined as its expected contribution to the population through both current and future reproduction: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 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.