Academic journal article Mankind Quarterly

Population-Level Life History in Italy, Spain and Mexico: The Impact of Regional Climate, Parasite Burden, and Population Density

Academic journal article Mankind Quarterly

Population-Level Life History in Italy, Spain and Mexico: The Impact of Regional Climate, Parasite Burden, and Population Density

Article excerpt

1. Introduction

Life history theory is a mid-level evolutionary theory derived principally from natural selection (Darwin, 1859) and parental investment theories (Trivers, 1972). This theory posits that organisms operate with a limited energy budget and must strategically allocate both bioenergetic and material resources to survive and reproduce. Much of life history research focuses on the trade-offs between Somatic Effort (investment in one's own growth, development, and maintenance) and Reproductive Effort, which itself may be disaggregated into Mating Effort (investment in finding and securing mates) and Parental Effort (investment in offspring). An organism's "life history strategy" refers to a co-adapted constellation of traits described on a continuum of fast to slow. Fast (r-selected) life history strategies are those characterized by rapid growth, early maturation, and having more offspring, reflecting a decrease in Somatic Effort and a corresponding increase in Mating Effort. Slow (K-selected) life history strategies, on the other hand, are characterized by slow growth, later maturation, and having fewer offspring; these individuals therefore invest more in Somatic Effort and Parental Effort.

Early models of life history evolution focused on ecological parameters, addressing the importance of ecological factors on modifying the development and evolution of life history strategies of organisms residing within the immediate environment. One theory proposed that population density would be an important antecedent of life history strategies (Pianka, 1970). Environments abutted with large groups of species and limited resources would show niche diversification and increased specialization and competency to extract resources from the environment (Woodley, 2011). Environments with abundant resources, on the other hand would be oriented toward niche generalization, capitalizing on the resources that are temporally available at any given moment. Put in other words, increased population density would increase resource competition, creating an environment that would favor slower life history strategies; conversely, lower population density would produce a faster life history strategy.

Other models emphasize the importance of adult morbidity-mortality cues on life history evolution and development (Charnov, 1991; Ellis et al., 2009). According to this model, life history strategies are expected to be especially sensitive to environmental cues to harshness and unpredictability (Ellis et al., 2009), which directly impact an organism's ability to allocate its available bioenergetic and material resources. Environmental harshness is characterized by high extrinsic mortality, where extrinsic mortality refers to "external sources of disability and death that are relatively insensitive to the adaptive decisions or strategies of the organism" (p. 206).

Environmental unpredictability refers to temporal or spatial variance in environmental harshness. If sources of threat are unavoidable (i.e. extrinsic) and thus morbidity-mortality rates are high, it would be maladaptive to allocate resources towards future rather than immediate reproduction, in slow development, and in long-term investments. Under those circumstances, faster life history strategies must evolve. Adults facing extrinsic morbidity or mortality are expected to pursue a fast life history strategy characterized by high mating effort, particularly when population density is low. When threats to morbidity and mortality are low, and population density is high, the optimal strategy is to invest highly in Somatic Effort and Parental Effort to produce highly competitive offspring that can acquire the resources needed to survive and reproduce. Support for this theory comes from correlations showing that decreases in adult mortality rate are associated with a later age of maturity, while an increase in adult mortality is associated with increased childhood mortality and increased fecundity (Charnov, 1991). …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.