"Fitness" is one of those words that pops up in every discussion of evolution, but which turns out to have almost as many meanings as people using it. The varied meanings have led to confusion, uncertainty, and doubt in public discussions of evolution, and one deep misunderstanding led to the political movement for "eugenics", which cast a shadow over the public discourse on evolution for a generation or more. This same misunderstanding is at the heart of "Social Darwinism", a pernicious political philosophy that is still popular in some circles today.
Darwin saw fitness as the measure of how well an organism can survive to pass on its heritage (genetics and the biochemistry of reproduction was as yet unknown when Darwin wrote Origin of the Species). It consisted, he said, of 3 attributes:
- Survival or mortality selection - Organisms that survive at least to the end of their reproductive phase are fitter than those who don't, because they're likely to have more offspring.
- Mating success or sexual selection - Many species have some form of mate selection process which makes some organisms more likely to mate or likely to mate more often and thus produce more offspring.
- Family size or fecundity selection - The more mature offspring (that is, those that live to reproduce themselves) produced by an organism, the fitter it is. This takes into account the two major reproduction strategies:
- produce as many offspring as possible, putting as little resource into each as possible (squids, for instance, do this),
- produce fewer offspring, putting significant resources into each, to increase each one's chances of making it to reproductive age (humans do this).
Even many scientists who were not experts in the theory of evolution thought that fitness was measurable: computer simulations of evolution (see Tierra, for example) that used a simple measure of fitness that seemed successful in terms of demonstrating the evolution of new forms of life were offered as evidence for this position. Unfortunately, such programs are fundamentally limited in what can evolve within them (see below), and so the statistics of the changes in them don't match those we find in biological evolution.
Evolutionary theory has itself evolved in the last half century or so to include concepts from mathematics and physics like the thermodynamics of information, and algorithmic complexity theory. An algorithm is a precise set of instructions for carrying out some computation; all computer programs are implementations of one or more algorithms. But it turns out that fitness cannot be measured algorithmically; some aspect of an organism's fitness that isn't part of a given algorithm for measuring fitness may have a major effect on the organism's ability to survive or reproduce if the environment of the organism changes. And as new traits evolve, any one of them may have an effect on the organisms fitness that isn't measured by a given fitness algorithm.
To see why this is so, consider the K-T transition, the short period between the Cretaceous and Tertiary eras when the dinosaurs went extinct. The current (still somewhat controversial) consensus theory is that the long-term effects of the impact of an asteroid in Central America changed the Earth's climate so that the entire food chain many animals were dependent on was disrupted. What algorithmic measure of fitness for a given individual of one dinosaur species could take all such possible catastrophic changes into account? And what measure of fitness for the mammal species that survived would include all the characteristics that let them survive? Yes, it's always possible to create a fitness measure that includes any given set of characteristics, but it's not possible to create a fitness measure that includes all characteristics that might, under any circumstances, have an affect on selection of an organism. There's no way to predict what characteristics any organism may have in the future, let alone what characteristics will be selected for or against.
In that light, it seems that fitness depends on all the attributes of an organism in connection with all the aspects of its environment, where the environment includes all the physical aspects of the world that it interacts with, as well as the attributes and actions of all other organisms that it interacts with. And that those attributes and actions are the result of the process of evolution, which is not itself algorithmic or predictable in any useful way.
In a sense, fitness is a tautological concept: an organism is fit if it survives to have mature offspring; one organism is more fit than another if it produces more mature offspring than the other. There's no way to predict which organisms are fitter; it's possible to make reasonable bets based on the probability of survival and reproduction of an organism with a given genome in a given environment, but impossible to predict how that environment may change in detail at the level of each organism.
So if the Eugenics movement was wrong in trying to measure fitness and use the measure to evaluate people's worth in evolutionary terms, what strategy might work to maximize the potential fitness of the human race? Remember, we're talking about humans here, so this strategy can be consciously-planned. One strategy is to retain a level of diversity within a population of organisms so that there's a repository of attributes that might be useful in the event of changes in the population's environment. In other words, don't try to optimize the population by making them all perfectly-fitted for the current environment, but leave some room for fitness in changed circumstances. And, of course, don't try to measure fitness, because you really can't. This is clearly not a perfect strategy, but then nothing about evolution is perfect. Evolution is concerned with what works using what's available.
If you want to get deeper into the nature of fitness, and how it fits into the overall process of evolution, here is a brief, eclectic, and personal list of books and websites related to that subject. It's a list of writings I've found interesting, not necessarily all the influential writings, or even a good survey of the important positions on the subject. The authors of these books are not necessarily in agreement, in fact, there's some rather acrimonious debate among them.
The Extended Phenotype - Richard Dawkin's take on natural selection, fitness, and what it is that is actually selected.
The Spandrels of San Marco and the Panglossian Paradigm - Stephen Jay Gould and Richard C. Lewontin's famous critique of adaptationism. "Spandrel" has entered the technical vocabulary of evolutionary theory because of this paper.
Darwin's Dangerous Idea - Daniel Dennet's exposition of a sort of Darwinian fundamentalism. Fascinating, well-written, and not quite right (IMHO) on the question of whether evolution can be an algorithmic process.
Stephen Jay Gould's Reply to Dennet - It gets hot and heavy in here; neither Gould nor Dennet is shy about attacking other's positions and defending their own, no holds barred.