Biology textbooks usually define evolution as a change of gene frequencies in populations over time. A change in the frequency of some observed trait over time might be related to changes in environmental conditions and reversed if those conditions revert to earlier levels.
Evolution could be defined as a change of trait frequencies in populations over time, leaving for investigation whether the change is reversible, accompanied by a change of gene frequencies, and so on.
Even with a focus on gene frequencies, the cause of the change in frequencies may not be readily associated with genes. Contrary to many popular accounts, the origin of the heritable variation that is required for natural selection to lead to a new adaptation does not need to be a random mutation. Reassortment of chromosomes through sexual reproduction, which sometimes involves crossing over and recombination, can provide new variants even in the absence of a new gene mutation.
Moreover, existing uniformity in some traits can turn into variation given a change in the environment. Existing variation may be associated with variation in behaviors that open up new environments, in which subsequent evolution takes place.
Models of evolution by natural selection that begin from the definition of evolution as a change of gene frequencies in populations over time include parameters, such as the “fitness” of genes or genotypes (pairs of genes at a given place on the paired chromosomes), that are difficult or impossible to estimate. This situation led population geneticist, Richard Lewontin, to remark in his 1974 book, The Genetic Basis of Evolutionary Change, that:
“What we can measure is by definition uninteresting and what we are interested in is by definition unmeasurable” (23). The problems of relating models of selection to observations become astronomically worse when there are multiple, linked loci (317). He concluded that we could shift our attention to the fitness effects of variation at the level of chromosomes, and such effects could be measured. (Taylor 2003)
Lewontin, R. C. (1974). The Genetic Basis of Evolutionary Change. New York: Columbia University Press.
Taylor, P. J. (2003) “Gene-environment complexities: What is interesting to measure and to model?” pp. 233-253 in The Evolution of Population Biology: Modern Synthesis, ed. R. K. Singh and M. Uyenoyama, Cambridge: Cambridge University Press, 2003.
(Introduction to this series of posts)