Die Life History Theorie besagt, dass die zeitliche Ausgestaltung und Dauer des Lebens jedes Lebewesens durch natürliche Selektion so ausgestaltet wurde, dass die größtmögliche Zahl von Nachkommen gezeugt werden kann.
Insbesondere sind Ereignisse wie:
- kindliche Entwicklung
- Alter bei dem die Geschlechtsreife Eintritt
- das Alter für die Reproduktion
- die Anzahl der Nachkommen
- der Umfang der elterlichen Investition
- der Alterungsprozess
- und der Tod
abhängig von der tatsächlichen und ökonomischen Umgebung des Organismus.
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 favored 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 aging 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 selected for flies with both shorter lifespans and higher fecundity.[
Es ist also in vielen Fällen ein Ressourcenproblem. Die Lebewesen müssen Kompromisse machen, die eben dann zu gewissen Gruppen und Zusammenhängen führen.
Die Berechnungen werden in der Wikipedia auch noch einmal weiter ausgeführt:
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 gives birth 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
Es ist unschwer zu erkennen, dass wir Menschen zu der zweiten Gruppe gehören Wir werden relativ spät erwachsen und pflanzen uns noch viel später fort. Wir haben eine lange Lebensspanne, haben wenige Nachkommen auf einmal und haben eine vergleichsweise geringe Kindersterblichkeitsrate. Die elterliche Investition ist ebenfalls sehr hoch.
Ich finde diese Zusammenhänge sehr interessant. Sie zeigen auch, dass Evolution zwar ein zufälliger Prozess ist, bei dem zufällige Mutationen auftreten und der keiner Planung unterliegt, aber dennoch bestimmte Regeln herausgebildet werden können, die einfach aus den Kosten-Nutzen-Betrachtungen entstehen, die durch Selektion entstehen.
Anhand dieser Theorie lassen sich interessante weitere Forschungen zum Menschen durchführen. Beispielsweise die Folgenden:
Human life histories, as compared to those of other primates and mammals, have at least four distinctive characteristics: an exceptionally long lifespan, an extended period of juvenile dependence, support of reproduction by older postreproductive individuals, and male support of reproduction through the provisioning of females and their offspring. Another distinctive feature of our species is a large brain, with its associated psychological attributes: increased capacities for learning, cognition, and insight. In this paper, we propose a theory that unites and organizes these observations and generates many theoretical and empirical predictions. We present some tests of those predictions and outline new predictions that can be tested in future research by comparative biologists, archeologists, paleontologists, biological anthropologists, demographers, geneticists, and cultural anthropologists
Our theory is that those four life history characteristics and extreme intelligence are co-evolved responses to a dietary shift toward high-quality, nutrient-dense, and difﬁcult-to-acquire food resources.
Und hier ein Artikel, in dem die verschiedenen Kostenfaktoren beim Menschen durchgegangen werden. Und hier ein weiterer.