Brief history of field[edit]
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]
Life cycle[edit]
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.[3]
The major events in this life cycle are usually shaped by the demographic qualities of the organism.[2] Some are more obvious shifts than others, and may be marked by physical changes—for example, teeth erupting in young children.[8] 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,[3] 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.[9] 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.[8]
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.[12] Evolution then works upon these stages to ensure that an organism adapts to its environment.[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.[3][12] All of these are defined in a specific biological way, which is not necessarily the same as the way that they are commonly used.[12]
Darwinian fitness[edit]
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.[5] 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,[2] which in turn establishes an organism's level of Darwinian fitness.[5] 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.[5]
Strategies[edit]
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. 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.[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.[14]
Determinants[edit]
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.[32] 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.[33]
Human life history[edit]
In studying humans, life history theory is used in many ways, including in biology, psychology, economics, anthropology, and other fields.[9][34][35] 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.[5] 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,[7] and a relatively long lifespan,[7][36] often supported by fathers and older (post-menopausal) relatives.[36][37][38] 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.[7][36] 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.[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][38] The large brain allows for a greater learning capacity, and the ability to engage in new behaviors and create new things.[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.[8] Recent authors, such as Kaplan, argue that both aspects are probably important.[36] Research has also indicated that humans may pursue different reproductive strategies.[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]
Perspectives[edit]
Life history theory has provided new perspectives in understanding many aspects of human reproductive behavior, such as the relationship between poverty and fertility.[43] 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.[44]
Criticism[edit]
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.[45][46]
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.[47][48]
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.[49][50]
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][52]