In evolution the only measure of fitness is how many of your genes spread in the population. You can die of old age, but if you don't reproduce you are not truly "fit" from an evolutionary perspective. (unless one counts kin selection)
I asked mainly because evolutionary algorithms talk about fitness and optimization in ways that I suspected differed from biologists. But it was also papers like the following:
"These definitions of fitness in terms of actual survival and reproductive success are straightforward and initially intuitively satisfying. However, such definitions lead to justifiable charges that certain explanations invoking fitness differences are circular." [and later on in the paper]
"The identical twins [mentioned earlier in the paper, in a hypothetical situation when one is crushed by lightning] are equally capable of leaving offspring. And the camouflaged butterfly is more capable of leaving offspring than is the noncamouflaged butterfly.
Thus, we suggest that fitness be regarded as a complex dispositional property of organisms.
Roughly speaking, the fitness of an organism is its propensity to survive and reproduce in a particularly specified environment and population."
from "The Propensity Interpretation of Fitness" by Mills & Beatty (a paper from the volume
Conceptual Issues in Evolutionary Biology 3rd ed.; MIT Press 2006). The next paper in that volume is by Sober, and although he has more detailed stuff going back decades, his comment (or rhetorical question) in his contribution is short and to the point: "The definition of fitness as expected number of offspring has a one-generation time scale. Why think of fitness in this way rather than as having a longer time horizon?" (p. 28)
Second, it isn't clear to me how the use of the term "fitness" computational biology and computational intelligence techniques like evolutionary algorithms (things I am more familiar with) compared to evolutionary biology. Sometimes it seems as if biologists use the term to refer to an organism's "fitness" or "fitness traits" in ways more or less identical to the use in texts which talk about fitness functions (even those which have nothing to do with actual evolution or organisms). For example, "Any phenotypic change, before being fixed as an evolutionary change, goes through two stages. First, it is generated, and then it must undergo a process of selection, in which useful changes, those that improve the fitness of the organism to the environment, or at least do not reduce it, will be conserved and propagated to the progeny." (from the intro to Cabej's
Epigenetic Principles of Evolution (Elseviar, 2012).
And in 10.3 of
Systems Biology in Practice: Concepts, Implementation and Application (Wiley, 2005) entitled "Prediction of Biological Systems from Optimality Principles" the authors state "Evolution is considered to be without aim or direction, but it forces development of species towards maximizing fitness. The formulation of a function that measures fitness is not straightforward. Several optimality criteria have been proposed."
Which (like the book itself) is an approach to fitness as a function of optimal traits, whether in an organisms or species, but not a measure of progeny nor the number of progeny.
However, this is in contrast to descriptions like yours, also found in numerous other places such as biology textbooks: "Unfortunately, fitness is also commonly used to denote the survival-enhancing qualities of individuals, such as size, speed or strength (survival of the fittest), or individual reproductive success (reproductive fitness)...we shall use fitness to refer to the spread of alleles rather than any quality of individuals." (from p. 64 of Barnard's
Animal Behaviour: Mechanism, Development, Function and Evolution).
Here Barnard talks about the use of "fitness" as I am more used to the term, but calls it unfortunate compared using the spread of alleles as a definition.
The only way to measure total fitness is long term over single or several generations.
There are two measures used to study fitness over generations... absolute and relative.
I understand (I think) the idea behind frequency approaches (absolute or relative), but it seems as if (and again, prior to this thread my knowledge of biological fitness was quite limited, and although I've done a fair amount of reading since, I'm clearly not in any way capable of knowing what the state of research is here) there are a number of studies which show that frequency of direct or indirect offspring are inadequate. The most scathing evaluation of this approach comes from a fairly recent book in
The Vienna Series in Theoretical Biology, Robert Reid's
Biological Emergences: Evolution by Natural Experiment (MIT Press, 2007):
"neo-Darwinists had no qualms about establishing a precedent when they redefined it as
differential survival and reproduction. While this addressed the effects of the process, it left putative causal agents such as competition, predation, and the literal choices that are made in reproductive pairing, and co-evolution, to be tacitly implied. Natural selection is not simply the effect of evolutionary change, but a syndrome of secondary causes and effects. As such, it is a real phenomenon, based in some part on the participation of genes, and not to be abandoned for its creaky logic.
To make matters worse, without a murmur of dissent from the orthodox, evolution itself was re-invented as
changes in the distribution of alleles in populations, for the sole purpose of making it match the new definition of natural selection. What cloud of unknowing allowed, and still allows, this to pass without protest?
You can track both of these from one generation to the next, to get an idea of long term fitness. Your long term fitness is not a single steady measure. " (p. 9-10; italics in original)
Only if they do not spread their genes into the population.
In computational appoaches to evolution and fitness "fitness" is based on solutions given the environment in question (which, for my work anyway, rarely has anything to do with an actual physical environment). But wouldn't at least some computational/algorithmic measures of fitness be useable in biological evolution, especially given their use in mathematatical/computational biology and systems biology. See, for example, the edited volume
Information Processing and Biological Systems (
Intelligent Systems Reference Library,Volume 11; Springer, 2011). Also, as far back as 1998, Auyang's
Foundations of Complex-system Theories: In Economics, Evolutionary Biology, and Statistical Physics, Auyang talks about the implicit yet idealized individualism within frequency approaches to fitness in biology, and notes the problems here:
"Does the statistical averaging preserve most important causal mechanisms? Is the correlation among various character types of organisms really negligible? To these controversial questions, genie selectionism not only answers with a blanket yes but asserts that the allele distribution is the correct way to represent evolution. Its opponents counter that although some factorization is inevitable, usually we must retain the correlation among some character types because their functions are biologically inseparable.
The indiscriminate use of allele distributions in evolution theory obscures and distorts the causal mechanisms of evolution." (p. 142; emphasis added).
Additionally, using evolutionary algorithms, graph theory, gene expression, and similar adapative algorithms has produced at least
some evidence (I say "some" because I can only speak to the fact that I've read some work on this, but have no idea how accepted it is within evolutionary biology or biology as a whole) that emergent genetic networks tend to diverge from the original "parent" patterns, and thus (among other things) frequency approaches to fitness are missing a lot of the actual dynamics in genetic expression by defining fitness based on mutation and its role in the number of heritable genetic traits an organism passes on. The entirety of epigenetics and the ways in which a single organism can produce characteristics which are not the result of inherited genes or of mutations, but rather selectional expression of the organism itself (particularly in development).