The concept of life history refers to the time spanning the initial origination of species that are now extinct and takes into consideration the process of change in species that are still extant, either through evolutionary mechanisms or ecological influences or through the indispensable interaction between the two forces. For the most part however, life history pertains to the story of how life first came into existence on this planet and how life went through the ramifications of evolution to bring about the level of complexity that characterizes living systems at present. Of course, the assertions inherent in the history of life based on evolutionary claims, take validity on account of the premises of evolution.
Peter J. Bryant of the University of California at Berkeley, in his book titled, “Biodiversity and Conservation”, described the concept of life history as “a complicated series of geological, climatic and biological events that have led up to the present day situation.” He speculated that the history of life and for that matter, the history of global biological diversity is best seen in marine habitats on account of the premise that the first life forms emerged from the bodies of water.
In support of Bryant’s presupposition, the geologic time scale describes the dawn of life as having started from the pre-cambrian phagocytic unicells. Henceforth, it is believed that this primitive life form went through the different evolutionary mechanisms to bring about the level of complexity characteristic of the many species that survived the tests of time.
The underlying premise here is that different species have evolved different structural mechanisms to enable them to shift their reproductive strategies in such a way that they are better able to cope with ecological pressures arising from scarcity of available resources or uncertainty of existing environmental conditions. The species’ continuous interaction with their environment created in them, the needed changes in their gene-pool that either will prepare their generation for changes in their ecospaces or will evolve new generations with new levels of fitness.
Begon in his book titled, “Ecology: Individuals, Populations, and Communities”, enumerated seven of the more apparent components of life history patterns: size, growth rate, pattern of development, reproduction, resource capture and allocation, and dispersal. The underlying premise here is that, variations to this components will have corresponding trade-offs if only to make the individual members of a species better able to utilize available resources or to compete for such resources.
In the case of the trade off between size and fitness for example, the energy used to compensate for an increase in the size of organisms is traded off with the energy that is thought to compensate for reproductive functions. This is consistent with the r/K Selection Theory. According to this theory, organisms that have capitalized on the advantages of reproduction (r-selection) will have to bear a multitude of offprings even if in the end, only quite a few of them survive. Organisms that have adapted to this evolutionary predisposition will have to reproduce at a fast-tracked rate and at the soonest time possible in their life cycle. This is an apparent trade off in terms of fitness relative to organisms that have capitalized on body size. This evolutionary predisposition takes on the advantage of bearing “fitter” offsprings (K-selection) even if to trade off the advantage that comes with reproducing in multitude.
In the case of developmental patterns, it is presupposed that the phenotypic attributes of individuals in a population must establish an “appropriateness of fit” with the environment that they will grow into later on in life to make them cope better by adapting a function that corresponds to the habitat’s intrinsic stresses and pressures. Phenotypic plasticity figures significantly in this component of life history. Through phenotypic plasticity, a genotype common among individual organisms of the same species will have variable phenotypic expressions. This is achieved in part through the physiologic response of the organism to fluctuations in environmental conditions and in part through the behavioral match that an organism will have to adapt itself to in order to accommodate a particular environmental setting.
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