Some creatures (and some plants) enjoy more reproductive success than others. This variability in reproductive success determines the allotments of genes that the next generation inherits. Specifically, the next generation inherits more of the genes of the more successful reproducers, and it inherits fewer of the genes of the less successful reproducers. These varying inheritances express themselves as various distributions of phenotypic traits, and that explains how phenotypes get to be how they get to be. (Read for "genes" shorthand for DNA, epigenetic markers and whatever else constitutes the machinery of inheritance.)
So far so good for the theory of natural selection. But the truisms raise a question: Why do some creatures (and some plants) enjoy more reproductive success than others?
The theory of natural selection assumes and asserts that reproductive success is a function of heritable phenotypic traits. According to the theory, variability among the heritable phenotypic traits in a local population causes the members of the population to exhibit variable reproductive success. Heritable phenotypic traits affect reproductive success by interacting with the environments in which their bearers live. A longer neck reaches higher fruits; a sharper eye detects more hidden prey, and so on. This is how traits lead to reproductive success, according to the theory.
But this explanation begs more questions. Which attributes of a creature constitute phenotypic "traits"? And why should the heritable ones be credited with determining reproductive success?
We coin a term, "trait", and identify, say, nose length, as one. We coin a term, "adaptation," and declare that a long (or short) nose is one. Its degree of adaptation relates the trait to "fitness," which determines reproductive success, which is a measure of the given creature's progeny: their number and viability and fertility.
If we measure the noses of a generation of offspring, we might find an over-representation of nose-types associated with certain members of the parental generation. Natural selection theorists would regard this outcome as testifying to the reproductive success of those members of the parental generation, which would testify to their fitness, which would testify to their being adapted, which would testify to their possessing nose lengths within some range. To repeat: Reproductive success is a function of fitness, which is a function of adaptation, which is a function of heritable traits, goes the theory.
The elaboration begs more questions: Which heritable traits are the adaptive ones? We can't say, offhand, because we can't distinguish between adaptive traits and other traits until we measure reproductive success. Once we do that, then we can credit whatever traits are overrepresented in the offspring generation with being adaptive and thereby conferring the fitness that led to the reproductive success of their earlier bearers. But we can credit traits with being adaptive only after reproductive success is measured, if we are to evaluate the theory of natural selection by its own terms.
One can argue that traits contribute to reproductive success without being determinative, but then what becomes of the formula that says reproductive success is a function of fitness, which is a function of adaptation, which is a function heritable phenotypic traits? If that formula fails--if reproductive success is due to something else--then the theory of natural selection goes out the window, and we are left without a theory of how phenotypes get to be how they get to be.
Variability of reproductive success necessarily reflects the interplay of countless variables, heritable phenotypic traits (nature) among them. Nonheritable phenotypic traits (nurture), along with countless environmental contingencies, also will affect reproductive success, positively or negatively. Dumb luck, being in the right place at the right time (or bad luck, being wrongly situated), might have more to do with reproductive success in many cases than having certain of one's phenotypic traits be more or less pronounced that the corresponding traits of stronger, faster, smarter--seemingly more fit--rivals.
For example, if the rivals are north of the river and you're south of the river when the blaze consumes half the forest, or the pack of predators converges upwind of you but downwind of your rivals, or the rivals are sterile for having been caught in the plague as infants, then you might emerge as the more fit, based on your reproductive success, but not due to heritable phenotypic traits. Factors responsible for your reproductive success need have no connection to heritability.
Nonetheless, in these scenarios your genes will be overrepresented in the next generation--all of your genes, whether we can concoct stories about any particular ones contributing to fitness, by playing adaptive roles, or not.
The same thing applies to traits acquired through diet and/or exercise. In the case of acquired traits, no matter how important the traits are in determining reproductive success, they are not heritable. Only their genetic potential is heritable. Identical twins, even in the same environment, will not necessarily enjoy identical reproductive success. Nonheritable effects might rule the day.
The natural selection theorist can counter that the local habitat's variability, typically small and random, will tend to cancel itself out over time, leaving genetic effects to determine reproductive outcomes. But the formula can be read either way: The local population's phenotypic variability, typically small and random, will tend to cancel itself out over time, leaving environmental (nonheritable) effects to determine reproductive outcomes.
The foregoing suggests that natural selection theory be formulated as a problem of signal-to-noise ratio. That is, the burden on the theory is to show that the variability of heritable phenotypic traits within a species in a local population, limited as it is by developmental constraints, nonetheless is significant enough to account for the variability of reproductive success among the members of a generation. Can the variability of the heritable traits in a given generation, the signal, rise above the day-in day-out contingencies of the environment and the intrinsic developmental constraints that limit the variability of phenotypes in a given generation, the noise, to override these factors and determine reproductive outcomes generation after generation?
What magnitude of variability is necessary to tip the scales? A nose-length variability of one cell? A hundred cells? A nanometer? An inch?
In a given habitat, selection pressures will operate above a given magnitude of variability for any given trait. But for any given trait, how does nature determine that threshold of significance? If the determination depends on someone first measuring reproductive success, then we've made no progress beyond the truisms of the first paragraph of this post.
Even computers programmed to simulate evolution use criteria to decide which simulated creatures enjoy which levels of reproductive success. This must be so, if evolution is to occur among the simulated creatures. But nature has no criteria by which to select winners and losers. Fitness? By which criteria is that to be assigned? Adaptation? Ditto. The buck stops at varying reproductive success itself.
As a guiding narrative, natural selection is becoming shopworn. We notice the scent of biological phlogiston. Phenotypes get to be how they get to be by some other means.