‘Fitness’ is a very overloaded term, as you’ve delved into above. I’d like to attempt to describe a few carvings which help me to firm things up and avoid equivocation in my own thinking.
The original pretheoretic term ‘fitness’ meant ‘being fitted/suitable/capable (relative to a context)’, and this is what Darwin and co were originally pointing to. (Remember they didn’t have genes or Mendel until decades later!)
The modern technical usage of ‘fitness’ very often operationalises this, for organisms, to be something like number of offspring, and for alleles/traits to be something like change in prevalence (perhaps averaged and/or normalised relative to some reference).
So natural selection is the ex post tautology ‘that which propagates in fact propagates’.
If we allow for ex ante uncertainty, we can talk about probabilities of selection/fixation and expected time to equilibrium and such. Here, ‘fitness’ is some latent property, understood as a distribution over outcomes.
If we look at longer timescales, ‘fitness’ is heavily bimodal: in many cases a particular allele/trait either fixes or goes extinct[1]. If we squint, we can think of this unknown future outcome as the hidden ground truth of latent fitness, about which some bits are revealed over time and over generations.
How can we reconcile this claim with the fact that the operationalised ‘relative fitness’ often walks approximately randomly, at least not often sustainedly upward[2]? Well, it’s precisely because it’s relative—relative to a changing series of fitness landscapes over time. Those landscapes change in part as a consequence of abiotic processes, partly as a consequence of other species’ changes, and often as a consequence of the very trait changes which natural selection is itself imposing within a population/species!
So, I think, we can say with a straight face that natural selection is optimising (weakly) for increased fitness, even while a changing fitness landscape means that almost by definition relative fitness hovers around a constant for most extant lineages. I don’t think it’s optimising on species, but on lineages (which sometimes correspond).[3]
In cases where the relative fitness of a trait corresponds with its prevalence, there can be a dynamic equilibrium at neither of these modes. Consider evolutionary stable strategies. But the vast majority of mutations ever have hit the ‘extinct’ attractor, and a lot of extant material is of the form ‘ancestor of a large proportion of living organisms’.
Though note we do see (briefly?) sustained upward fitness in times of abundance, as notably in human population and in adaptive radiation in response to new resources, habitats, and niches becoming available.
Now, if the earlier instances of now-extinct lineages were somehow evolutionarily ‘frozen’ and periodically revived back into existence, we really would see that natural selection pushes for increased fitness. But because those lineages aren’t (by definition) around any more, the fitness landscape’s changes over time are under no obligation to be transitive, so in fact a faceoff between a chicken and a velociraptor might tell a different story.
‘Fitness’ is a very overloaded term, as you’ve delved into above. I’d like to attempt to describe a few carvings which help me to firm things up and avoid equivocation in my own thinking.
The original pretheoretic term ‘fitness’ meant ‘being fitted/suitable/capable (relative to a context)’, and this is what Darwin and co were originally pointing to. (Remember they didn’t have genes or Mendel until decades later!)
The modern technical usage of ‘fitness’ very often operationalises this, for organisms, to be something like number of offspring, and for alleles/traits to be something like change in prevalence (perhaps averaged and/or normalised relative to some reference).
So natural selection is the ex post tautology ‘that which propagates in fact propagates’.
If we allow for ex ante uncertainty, we can talk about probabilities of selection/fixation and expected time to equilibrium and such. Here, ‘fitness’ is some latent property, understood as a distribution over outcomes.
If we look at longer timescales, ‘fitness’ is heavily bimodal: in many cases a particular allele/trait either fixes or goes extinct[1]. If we squint, we can think of this unknown future outcome as the hidden ground truth of latent fitness, about which some bits are revealed over time and over generations.
A ‘single step’ of natural selection tries out some variations and promotes the ones which in fact work (based on a realisation of the ‘ex ante’ uncertain fitness). This indeed follows the latent fitness gradient in expectation.
In this ex ante framing it becomes much more reasonable to treat natural selection as an optimisation/control process similar to gradient descent. It’s shooting for maximising the hidden ground truth of latent fitness over many iterations, but it’s doing so based on a similar foresight-free local heuristic like gradient descent, applied many times.
How can we reconcile this claim with the fact that the operationalised ‘relative fitness’ often walks approximately randomly, at least not often sustainedly upward[2]? Well, it’s precisely because it’s relative—relative to a changing series of fitness landscapes over time. Those landscapes change in part as a consequence of abiotic processes, partly as a consequence of other species’ changes, and often as a consequence of the very trait changes which natural selection is itself imposing within a population/species!
So, I think, we can say with a straight face that natural selection is optimising (weakly) for increased fitness, even while a changing fitness landscape means that almost by definition relative fitness hovers around a constant for most extant lineages. I don’t think it’s optimising on species, but on lineages (which sometimes correspond).[3]
In cases where the relative fitness of a trait corresponds with its prevalence, there can be a dynamic equilibrium at neither of these modes. Consider evolutionary stable strategies. But the vast majority of mutations ever have hit the ‘extinct’ attractor, and a lot of extant material is of the form ‘ancestor of a large proportion of living organisms’.
Though note we do see (briefly?) sustained upward fitness in times of abundance, as notably in human population and in adaptive radiation in response to new resources, habitats, and niches becoming available.
Now, if the earlier instances of now-extinct lineages were somehow evolutionarily ‘frozen’ and periodically revived back into existence, we really would see that natural selection pushes for increased fitness. But because those lineages aren’t (by definition) around any more, the fitness landscape’s changes over time are under no obligation to be transitive, so in fact a faceoff between a chicken and a velociraptor might tell a different story.