My comment on heritability is in reference to your example.

Yes, a trait may have a negative and positive effect. That is actually quite common .

Please look up “sickle cell trait.” It is not the only example, but it’s the canonical one used in lay discussions.

In my mind, that seems like it would alter the environmental context. The trait would be good enough *until* the context is changed. And wouldn’t the whole of the environment, including competing species, be the context?

> Also, the trait has to be heritable/genetic.

I did say “heritable”. 🙂

> Something we see as a disease may have a benefit somewhere else in someone’s life history.

Within the same species? If so, that’s an interesting concept I hadn’t heard of. If not, then it seems that it would be a different context.

Also, the trait has to be heritable/genetic.

And then there are polymorphisms. Something we see as a disease may have a benefit somewhere else in someone’s life history.

See: http://scienceblogs.com/gregladen/2008/02/05/the-three-necessary-and-suffic-1/

By “not leading to a disadvantage” I mean we don’t pass on the Best Adaptations, just the Good Enough Adaptations. After all, arthritis doesn’t appear to provide any advantage. But since it hasn’t stood in the way of species survival (or individual survival), then it persists. Survival: persistence of the adaptation. Fittest: Appropriate enough for the context.

Is this not accurate?

Ashwin,

Fitness is the relative effect of an allele (though sometimes measured as a trait, which is somewhat indirect) on representation of itself in the next generation. That is very precise concept.

However, as you imply, the actual operationalization of this is not as clear. Defining what the genetic underpinning of a trait is is difficult. (Fitness can be used in non-genetic systems as long as it is something that replicates fairly reliably). The actual fitness of a variant often depends on the context, so the fitness can change with the context. Because of this, the actual fitness is probaby best determined after the fact, and that makes fitness look to many like a tautology. But the concept of fitness, what it actually is, and how one would measure it given enough control, is not a tautology and not ambiguous.