This explanation of Darwinian fitness from 10/29/2002 is being reformatted for the current website. It’s still worth considering.
Fitness for Dummies: Is It Running in Circles? 10/29/2002
The Oct. 29 issue of Current Biology has a primer on “Fitness” by John Brookfield, in question-and-answer form. First, since fitness is such a key word in Darwinian evolution theory, he defines the term:
Fitness in evolution — what is it? Not fitness in the sense of health, but rather an ability to survive and reproduce, as when the Darwinian theory of evolution was characterized as “survival of the fittest”.
He hastens to explain that while straightforward, there are subtle shades of meaning that differ between geneticists and ecologists. Brookfield hastens to answer the tautology criticism:
What about the old chestnut – if fitness is defined by survival, and then we say the fittest survive, isn’t this all circular? Not really. Suppose we have a population where half the offspring survive to adulthood. An individual with a new advantageous mutation might have a 51% chance of surviving to adulthood. Its genotype’s relative fitness is 2% higher than that of the other individuals. But the individual with this new unique genotype will either survive or it will not: an individual cannot have a survivorship of 51%. Fitness is an average, or an expected, outcome.
He adds the point that fitness can decrease in a population by chance, if, for example, a bad mutation spreads through a population through a series of lucky chance events. “One thing for sure,” he explains: “if a population’s fitness can go down, it cannot be a circular argument to say it will usually go up.”
Brookfield then addresses the relation of adaptation to fitness:
Is an organism’s fitness the same as its adaptation to its environment? Evolutionary biologists look for adaptations in the phenotypes (morphologies and behaviors) of organisms. In population genetic studies of fitness, relative fitnesses of organisms’ genotypes can be compared empirically.
Isn’t it the same with all studies of adaptation? Not exactly. When studying a phenotype universally seen in a population of organisms, what does it mean to describe the phenotype as causing high fitness or as being an adaptation? High fitness relative to what? In these cases, phenotypes typically have to be compared with a range of imagined possible phenotypes, as in optimization theory.
He dodges getting embroiled in the controversy over whether memes (ideas that evolve) can have fitness, or whether that is a circular argument.
Look carefully at these answers, and read our Baloney Detector entries on equivocation and circular reasoning. We are assuming that Brookfield’s answers are current and valid representations of what Darwinians believe, being published in Current Biology. We assume they would receive nods of agreement from Eugenie Scott and Ken Miller. (If you are an evolutionist reading this, and disagree with Brookfield’s assessment, feel free to write us and clarify what fitness really means.)
Now consider whether he has successfully dispensed with the “old chestnut” that fitness is a circular argument. Notice how right off the bat, he defines fitness in terms of survival. How do you know this moose is fit? Because it survived. Why did it survive? Because it is fit. We seem to be off on the wrong hoof. Now read and re-read his explanation of why this is not circular, and see if it is clearer than mud. The percentages are all a smokescreen if fitness has already been defined in terms of survival. How does the biologist know that a mutation is advantageous unless it causes the bull moose to have a 51% chance (2% higher than his moosemates) of surviving? But then he says mutations can lower the fitness, and this proves the argument is not circular. But lower fitness relative to what? To survival! Yet if all the individuals in the population have the same survival, they have equal fitness. Confused? This is a good sign; it means you are not being swayed by this shell game.
Fitness is such a nebulous term, it can mean anything. If you’re picturing a bodybuilder in a fitness center when you hear the word fitness, you’ve got it all wrong. Fitness can be a beer belly, if that gets you the girl. Fitness can be anything the biologist wants it to be, good, bad or ugly, and if all the population is ugly but cute (like fat elephant seals on the beach), then they’re fit. To top it off and show how meaningless the word fitness is in evolutionary biology, look at what Brookfield says about adaptation. How do you know it’s an adaptation if the whole population has the trait, and has a survival ratio? Adapted relative to what? High fitness relative to what? How do the geneticists measure fitness in the genes if they don’t relate to survival, putting them back into the tautology hammerlock? All he can think of are comparing the observed phenotype to some imagined possible phenotypes. What are these? Superman traits, like X-ray vision? Come on. If Brookfield thinks he tossed away an old chestnut, the chestnut tree just dropped more on his head.