Evolutionary Biology
Middle-level evolutionary theories are consistent with general evolutionary theory, but focus on certain domains of functioning (Buss, 2011) Specific evolutionary psychology hypotheses may be derivative from a mid-level theory (Buss, 2011). Three very important middle-level evolutionary theories were contributed by Robert Trivers as well as Robert MacArthur and E. O. Wilson
The theory of parent-offspring conflict rests on the fact that even though a parent and his/her offspring are 50% genetically related, they are also 50% genetically different. All things being equal, a parent would want to allocate their resources equally amongst their offspring, while each offspring may want a little more for themselves. Furthermore, an offspring may want a little more resources from the parent than the parent is willing to give. In essence, parent-offspring conflict refers to a conflict of adaptive interests between parent and offspring. However, if all things are not equal, a parent may engage in discriminative investment towards one sex or the other, depending on the parent’s condition.
The Trivers–Willard hypothesis, which proposes that parents will invest more in the sex that gives them the greatest reproductive payoff (grandchildren) with increasing or marginal investment. Females are the heavier parental investors in our species. Because of that, females have a better chance of reproducing at least once in comparison to males, but males in good condition have a better chance of producing high numbers of offspring than do females in good condition. Thus, according to the Trivers–Willard hypothesis, parents in good condition are predicted to favor investment in sons, and parents in poor condition are predicted to favor investment in daughters.
r/K selection theory, which, in ecology, relates to the selection of traits in organisms that allow success in particular environments. r-selected species, i.e., species in unstable or unpredictable environments, produce many offspring, each of which is unlikely to survive to adulthood. By contrast, K-selected species, i.e., species in stable or predictable environments, invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood.
Life history theory posits that the schedule and duration of key events in an organism’s lifetime are shaped by natural selection to produce the largest possible number of surviving offspring. 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. Examples of some major life history characteristics include: age at first reproductive event, reproductive lifespan and aging, and 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 example, attachment theory proposes that caregiver attentiveness in early childhood can determine later adult attachment style. Also, Jay Belsky and others have found evidence that if the father is absent from the home, girls reach first menstruation earlier and also have more short term sexual relationships as women.