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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 (length of time alive)—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 (e.g., length of juvenile period) 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
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.
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, although there are still large varieties that exist in the world.
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
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 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
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:
- 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.
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.
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 reproduces 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
Variation
Variation
is a major part of what LHT studies, because every organism has its own
life history strategy. Differences between strategies can be minimal or
great.
For example, one organism may have a single offspring while another may
have hundreds. Some species may live for only a few hours, and some may
live for decades. Some may reproduce dozens of times throughout their
lifespan, and others may only reproduce one or twice.
Trade-offs
An essential component of studying life history strategies is identifying the trade-offs
that take place for any given organism. Energy use in life history
strategies is regulated by thermodynamics and the conservation of
energy, and the "inherent scarcity of resources",
so not all traits or tasks can be invested in at the same time. Thus,
organisms must choose between tasks, such as growth, reproduction, and
survival,
prioritizing some and not others. For example, there is a trade-off
between maximizing body size and maximizing lifespan, and between
maximizing offspring size and maximizing offspring number. This is also sometimes seen as a choice between quantity and quality of offspring. These choices are the trade-offs that life history theory studies.
One significant trade off is between somatic effort (towards growth and maintenance of the body) and reproductive effort (towards producing offspring).
Since an organism cannot put energy towards doing these simultaneously,
many organisms have a period where energy is put just toward growth,
followed by a period where energy is focused on reproduction, creating a
separation of the two in the life cycle.
Thus, the end of the period of growth marks the beginning of the period
of reproduction. Another fundamental trade-off associated with
reproduction is between mating effort and parenting effort. If an organism is focused on raising its offspring, it cannot devote that energy to pursuing a mate.
An important trade-off in the dedication of resources to breeding
has to do with predation risk: organisms that have to deal with an
increased risk of predation often invest less in breeding. This is
because it is not worth as much to invest a lot in breeding when the
benefit of such investment is uncertain.
These trade-offs, once identified, can then be put into models
that estimate their effects on different life history strategies and
answer questions about the selection pressures that exist on different
life events.
Over time, there has been a shift in how these models are constructed.
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.
Constraints
The
idea of constraints is closely linked to the idea of trade-offs
discussed above. Because organisms have a finite amount of energy, the
process of trade-offs acts as a natural limit on the organism's
adaptations and potential for fitness. This occurs in populations as
well. These limits can be physical, developmental, or historical, and they are imposed by the existing traits of the organism.
Optimal life-history strategies
Populations
can adapt and thereby achieve an "optimal" life history strategy that
allows the highest level of fitness possible (fitness maximization).
There are several methods from which to approach the study of
optimality, including energetic and demographic. Achieving optimal
fitness also encompasses multiple generations, because the optimal use
of energy includes both the parents and the offspring. For example,
"optimal investment in offspring is where the decrease in total number
of offspring is equaled by the increase of the number who survive".
Optimality is important for the study of life history theory
because it serves as the basis for many of the models used, which work
from the assumption that natural selection, as it works on a life history traits, is moving towards the most optimal group of traits and use of energy. This base assumption, that over the course of its life span an organism is aiming for optimal energy use,
then allows scientists to test other predictions. However, actually
gaining this optimal life history strategy cannot be guaranteed for any
organism.
Allocation of resources
An
organism's allocation of resources ties into several other important
concepts, such as trade-offs and optimality. The best possible
allocation of resources is what allows an organism to achieve an optimal
life history strategy and obtain the maximum level of fitness,
and making the best possible choices about how to allocate energy to
various trade-offs contributes to this. 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.
The allocation of resources also plays a role in variation, because the
different resource allocations by different species create the variety
of life history strategies.
Capital and income breeding
The division of capital and income breeding focuses on how organisms use resources to finance breeding, and how they time it. In capital breeders, resources collected before breeding are used to pay for it, and they breed once they reach a body-condition threshold, which decreases as the season progresses. Income breeders, on the other hand, breed using resources that are generated concurrently with breeding, and time that using the rate of change in body-condition relative to multiple fixed thresholds.
This distinction, though, is not necessarily a dichotomy; instead, it
is a spectrum, with pure capital breeding lying on one end, and pure
income breeding on the other.
Capital breeding is more often seen in organisms that deal with
strong seasonality. 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.
In less seasonal environments, income breeding is likely to be favoured
because waiting to breed would not have fitness benefits.
Phenotypic plasticity
Phenotypic
plasticity focuses on the concept that the same genotype can produce
different phenotypes in response to different environments. It affects
the levels of genetic variability by serving as a source of variation
and integration of fitness traits.
Determinants
Many
factors can determine the evolution of an organism's life history,
especially the unpredictability of the environment. 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 rates—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.
Human life history
In studying humans, life history theory is used in many ways, including in biology, psychology, economics, anthropology, and other fields. For humans, life history strategies include all the usual factors—trade-offs, constraints, reproductive effort,
etc.—but also includes a culture factor that allows them to solve
problems through cultural means in addition to through adaptation.
Humans also have unique traits that make them stand out from other
organisms, such as a large brain, later maturity and age of first
reproduction, and a relatively long lifespan, often supported by fathers and older (post-menopausal) relatives.
There are a variety of possible explanations for these unique traits.
For example, a long juvenile period may have been adapted to support a
period of learning the skills needed for successful hunting and
foraging. This period of learning may also explain the longer lifespan, as a
longer amount of time over which to use those skills makes the period
needed to acquire them worth it. 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. The large brain allows for a greater learning capacity, and the ability to engage in new behaviors and create new things.
The change in brain size may have been the result of a dietary
shift—towards higher quality and difficult to obtain food sources—or may have been driven by the social requirements of group living, which promoted sharing and provisioning. Recent authors, such as Kaplan, argue that both aspects are probably important. Research has also indicated that humans may pursue different reproductive strategies.
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.
Tools used
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.
Criticism
The
claim that long periods of helplessness in young would select for more
parenting effort in protecting the young at the same time as high levels
of predation would select for less parenting effort is criticized for
assuming that absolute chronology would determine direction of
selection. 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. One cited example is
that small animals that have more natural enemies would face
approximately the same number of threats and need approximately the same
amount of protection (at the relative timescale of the animals) as
large animals with fewer natural enemies that grow more slowly (e.g.
that many small carnivores that could not eat even a very young human
child could easily eat multiple very young blind meerkats).
This criticism also argues that when a carnivore eats a batch stored
together, there is no significant difference in the chance of one
surviving depending on the number of young stored together, concluding
that humans do not stand out from many small animals such as mice in selection for protecting helpless young.
There is criticism of the claim that menopause
and somewhat earlier age-related declines in female fertility could
co-evolve with a long term dependency on monogamous male providers who
preferred fertile females. 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. These critics argue that unless male
preference for fertile females and ability to switch to a new female was
annulled, any need for a male provider would have selected against
menopause to use her fertility to keep the provider male attracted to
her, and that the theory of monogamous fathers providing for their
families therefore cannot explain why menopause evolved in humans.
One criticism of the notion of a trade-off between mating effort
and parenting effort is that in a species in which it is common to spend
much effort on something other than mating, including but not exclusive
to parenting, there is less energy and time available for such for the
competitors as well, meaning that species-wide reductions in the effort
spent at mating does not reduce the ability of an individual to attract
other mates. These critics also criticize the dichotomy between
parenting effort and mating effort for missing the existence of other
efforts that take time from mating, such as survival effort which would
have the same species-wide effects.
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.
