As a form of historical explanation, natural selection is very restrictive; the world and its history must have a particular and atypical shape to fit it (Taylor 1987).

To develop this claim, let me start with Darwin himself, who, in the first four chapters of On the Origin of Species set out an argument for natural selection as a means of evolution. Part of this argument involves straightforward deductions. If there exist (some modern terms are inserted here):

• Variation among organisms in characters;
• Inheritance (at least partially) of characters; and
• Hyperfecundity, which ensures that not all can survive to reproduce and that there will be a struggle for existence;

then there will be differential representation in lineages of organisms of variant characters over time, i.e., modification by descent or evolution.
Now, if there is also

• Natural selection, that is, greater survival and reproduction of those organisms with characters that fit them better to their environment (including potential mates),

then evolution will result in improvement of adaptation to conditions of existence.

(Many variants of this schema exist in the published literature, referring to heritable variation, genes, and so on, but these post-Darwin elaborations do not, in my opinion, help us to understand the logic of Darwin’s explanatory schema.)

This schema is put to use in four ways, but only the fourth approach can lead to a well-confirmed explanation (Lloyd 1988) of actual observed evolutionary changes:

• i) Forward speculation. If there existed heritable characters (or genes, genotypes…) that resulted in these specific differential survival and reproductive success rates (the inaptly named “Darwinian fitness” values), then… (Here the philosopher or mathematician fills in the result.)
• ii) Backward fitting. If we assume there exists heritable characters with certain differential fitness values, then, given this specified model of their genetics (linkage, number of loci, number of alleles, etc.), the fitness values and other parameters would have to be… in order to match the observed data. (Here statisticians or empirical population geneticists fill in their estimates, at least for the cases in which they can overcome the substantial problems of estimation; Lewontin 1974.)
• iii) Hybrid speculation-fitting. For a given speculative scenario, the biologist finds examples that appear to fit the predicted result. Conversely, given a certain observation, a speculative scenario is fashioned that produces that outcome. A third hybrid mode is to use the estimates from backward fitting for a given model and, in an empirically grounded form of forward speculation, predict the most likely future.
• iv) Integrated interpretation, i.e, empirical observation and rational interpretation simultaneously. For this, we need not to assume natural selection when we speculate or interpret observations, but to demonstrate that some actual observed character change was produced by a process of natural selection.

Unfortunately, the form of the theory of natural selection makes this Integrated interpretation very difficult to do properly. Let me explain. Natural selection holds that organisms enjoy differential survival and reproductive success because of the effect of some character they possess. When seeking to demonstrate natural selection it is not sufficient to point to differential representation of a character. This is, at best, a promissory note for a natural selective account. (There are a multitude of uncashed notes in the literature, most notably, in Endler’s 1986 impressive compilation of purported cases of natural selection in the wild.) Now the existence of natural selection in the short term, say, for a generation, is not my issue; I am not positing chance and genetic drift instead of natural selection. The question is whether the generation-by-generation natural selection adds up over time to an outcome that we can, retrospectively, assign to natural selection associated with some specific character that increased in frequency in the population.

In the next installment, I explain why Darwin’s schema is a very restrictive form of historical explanation and explore the implications of that restrictiveness.

References

Endler, J. A. (1986). Natural Selection in the Wild. Princeton, NJ: Princeton University Press

Lewontin, R. C. (1974). The Genetic Basis of Evolutionary Change. New York: Columbia University Press

Lloyd, E. A. (1988). The structure and confirmation of evolutionary theory. New York: Greenwood Press.

Taylor, P. J. (1987). Historical versus selectionist explanations in evolutionary biology. Cladistics 3: 1-13.

Another extract from “From natural selection to natural construction to disciplining unruly complexity: The challenge of integrating ecology into evolutionary theory,” in R. Singh, K. Krimbas, D. Paul & J. Beatty (eds.), Thinking About Evolution: Historical, Philosophical and Political Perspectives, Cambridge: Cambridge University Press, 377-393, 2000.