804 THE STRUCTURE OF EVOLUTIONARY THEORY
- As emphasized in my previous discussion of publication bias (see p. 763), I
remain unconvinced that a predominant relative frequency for gradualism—as
opposed to genuine documentation of several convincing cases—has been
established, even for this maximally promising taxon. No one has ever compiled an
adequately random, or even an adequately numerous, sample of planktonic species
drawn from the entire clade. Gradualistic lineages have been highlighted for study
as a consequence of their greater "interest" under conventional views, while
putatively stable lineages have tended to remain in unexamined limbo as
supposedly uninformative, or even dull. Thus, the fact that gradualism prevails in a
high percentage of published studies tells us little about the relative frequency of
gradualism in the clade as a whole.
A telling analogy may be drawn with a crucial episode in the history of
genetics. With classical techniques based on the Mendelian analysis of pedigrees,
only variable genes could be identified. (If every Drosophila individual had red
eyes, earlier researchers could legitimately assume some genetic bases for the
invariance, but no genes could be specified because traits could not be traced
through pedigrees. But once a white-eyed mutant fly appeared in the population,
geneticists gained a necessary tool for identifying relevant genes by crossbreeding
the two forms and tracing the alternate phenotypes through successive generations.
In other words, genes had to vary before they could be specified at all.)
Therefore, under these methodological constraints (which prevailed during
most of the 20th century history of genetics), a dominant measured frequency for
variable genes taught us nothing about the actual frequency of variable genes
across an entire genome—for we knew no way to generate a random or unbiased
sample by selecting genes for study prior to any knowledge about whether or not
they varied. The fact of variation in all known genes only recorded a
methodological limitation that precluded the identification of nonvariable genes.
I don't, of course, claim that methodological strictures on paleontological
lineages have ever been so strong—that is, we could always have selected stable
lineages for study, had we chosen to do so. But, in practice, I'm not sure that the
actual procedural bias has operated with much less force in paleontology than in
genetics, so long as researchers confined their attention to lineages that appeared
(by initial qualitative impression) to evolve by gradual anagenesis. Just as all
known genes might be variable (while variable genes actually represent only a few
percent of the total complement, because the remaining 95 percent of invariant
genes could not be recognized at all), most studied species might illustrate gradual
trends (while gradualistic species represent a small minority of all lineages because
no one chooses to study stable species).
Genetics resolved this problem by inventing techniques—with electrophoresis
as the first and historically most important—for identifying genes prior to any
knowledge about whether or not they varied. This methodological advance
permitted the resolution of several old and troubling questions, most notably the
calculation of average genetic differences among human races.