Monday, March 25, 2013

The Physiology/Life-history Nexus: life history a la Ricklefs and Wikelski (2002)

Ricklefs and Wikelski (2002) present a conceptual model linking genotypes, phenotypes, performance, and demography to evolutionary responses in the context of the environment.

It is a little mushy because they define life-history in as a set of ... physiological adaptations, and then argue that "physiology mediates the relationship between life-history and the environment" (R&W p. 463).

Demography refers to traits of populations, where the state variable is typically population size, N, and we may characterized rates of change of N due to birth rates, death rates and migration. In contrast, life history refers to traits of individuals, especially individual probabilities of survival and death, lifespan, and the sizes and number of offspring in one bout of reproduction and over an organism's lifetime.

I think they take it for granted that their readers know that life history refers literally to the history of "significant" events in the life of an average individual of a population, focused exclusively on those events, such as clutch size or lifespan, that govern population demographic rates. For instance, different life history stages refer to elements of a life cycle are relatively recognizably distinct, and which might be characterized by different probabilities of death or survival, birth, or different average fecundities. Thus distinct life history stages are characterized by individuals having different properties. The study of life histories includes the study of traits of individuals related directly to survival and reproduction. The traits of interest most commonly include:
  • lifespan and senescence;
  • age at maturity;
  • metamorphosis between stages;
  • age-specific or stage-specific probabilities of survival or death;
  • number of seeds, eggs, or offspring per bout of reproduction (e.g., mast event, clutch, or litter);
  • semelparity vs. iteroparity
  • average size of individual seeds, eggs, or offspring;
  • lifetime reproductive success. 
  • body size.
Life history strategies are set of these traits that seem to us to optimize fitness in a particular context. For instance, r-selection is a life history strategy characterized by early onset of reproduction and large numbers of offspring and which often maximizes fitness in highly unpredictable environments. This strategy can maximize fitness when adult survival (and therefore future reproduction) is unpredictable. At the other end of the r vs. K-selection continuum, K-selected species are characterized by delayed onset of reproduction, and multiple bouts of reproduction (iteroparity). The K-selected strategy tends to maximize fitness in predictable environments. These two life history strategies seem to represent to ends of a continuum in which many of the above life history traits seem to covary.

The study of life histories focuses on the proximate (e.g., phsyiological) and ultimate (evolutionary) causes of variation and covariation in the above traits.

Non-sequitor: Why do we have the impression that aggregate properties (ecosystem variables, diversity, N) exhibit patterns and are suitable objects of study? (I ask this, I think, because of Ricklefs' focus on individuals and species).

The five principles of Ricklefs and Wikelski (2002):
  1. individuals respond to variation in their environments.
  2. responses are constrained by the allocation of limited resources among competing functions, 
  3. individual organisms assume alternative physiological states at different stages in their life cycles because these states are incompatible.
  4. individuals might also assume different states as phenotypic responses to the environment,
  5. the assumption of one or another state can be modulated by demography, especially reproductive value (future reproductive potential).
Specific points
  •  It seems to me that their primary point is that we need to study physiology in order to understand life history.  
  • I could not determine whether they were implying that the environment caused covariation in life history traits, or the covariation was due primarily to physical constraints on different components of organisms' physiologies.
  • Figures I and II in Box 1 seems orthogonal or perpendicular to life history traits. That is, we might imagine that a particular life history strategy such as r-selected traits occupies the phenotype box but different points on the r-K continuum lie perpendicular to the figure, extending out of and into the page.
  • I thought it was odd that they chose to not mention tradeoffs that might arise through "simple" laws of conservation of matter and energy.

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