The Modern Synthesis as a Limited Consensus 535
- Most geographic variation in clines:
It is difficult to see why the gradual decrease from the north to the south in
the number of the bridled individuals (ringvia) in populations of the
Atlantic murre (Uria aalge) should have an adaptational significance ... The
convergent development in several species of Draco also seems to belong
to the category of non-adaptive clines (p. 96). - Much geographic variation in general:^
It should not be assumed that all the differences between populations and
species are purely adaptational and that they owe their existence to their
superior selective qualities... Many combinations of color patterns, spots,
and bands, as well as extra bristles and wing veins, are probably largely
accidental. This is particularly true in regions with many stationary, small,
and well-isolated populations, such as we find commonly in tropical and
insular species... We must stress the point that not all-geographic variation
is adaptive (p. 86).^
Mayr's later book (1963) expanded to more than twice the number of pages,
and became even more weighty in its assurances. This work shaped my own
evolutionary thinking more than any other book—and I am confident that most
naturalists of my generation would offer the same testimony. As I reread Animal
Species and Evolution in preparing to write this chapter—and examined my old
marginalia, pencilled in preparation for the deciding oral exam of my Ph.D.
program—I came to appreciate even more (now that I know the genre's difficulty
through personal experience) the enormous labor and creative thought involved in
bringing so much material together. And I finally understood the defining word
that once puzzled me in Julian Huxley's review of Mayr's book—"magisterial."
(The etymological source does not reside in "magnificent" or "majestic," though
Mayr's book surely merits either of these accolades, but in magister, the Latin
word for teacher. A great magister is not a schoolroom pedant, but a wise preceptor
who holds mastery within his teaching authority, or magisterium. Magisterial,
above all else, means authoritative. And to what greater virtue, after all, may an
author aspire?)
Although Mayr's 1963 book covers the same general material, and in similar
order, as the 1942 version, the works differ profoundly, and Mayr chose a new title
(just as Simpson had done in noting the changes between his 1944 and 1953
volumes). I would specify two thematic changes as most important.
- The primary role of geographic isolation as a sine qua non, and the
consequent near universality of allopatric speciation, has consistently formed the
centerpiece of Mayr's worldview. But, in 1942, pure continuationism reigned.
Populations split into roughly equal divisions and each subgroup then functioned
as a microcosm of the ancestral mass—as in the model now called "dumb-bell
allopatry" and considered (by Mayr at least) both rare and relatively ineffective in
producing new species. In other words, Mayr (1942) originally identified no
distinctive properties promoting speciation in certain kinds of isolated populations
vs. others. Isolation itself, and the severing of