Historical Constraints and the Evolution of Development 1031
Darwin wrote to Lyell in 1862 (quoted in F. Darwin, 1903, vol. 2, p. 338— see pp.
330 - 341 for further discussion, and especially p. 341 for Darwin's comprehensive
architectural metaphor for dismissing constraint as a theoretical challenge to natural
selection):
Mere variability, though the necessary foundation of all modifications, I
believe to be almost always present, enough to allow of any amount of
selected change; so that it does not seem to me at all incompatible that a group
which at any one period (or during all successive periods) varies less, should
in the long course of time have undergone more modification than a group
which is generally more variable.
Placental animals, e.g. might be at each period less variable than
Marsupials, and nevertheless have undergone more differentiation and
development than marsupials, owing to some advantage, probably brain
development.I label this negative category of constraint based on lack of sufficient variability
as "less benign" because its operation does place a genuine damper, both actual and
theoretical, upon the exclusivity of natural selection as the cause of evolutionary
change. Darwin himself certainly read the issue in this light, as the above quotation
indicates—for if his argument fails, and constraint often trumps selection as a
regulator of evolutionary rate, then his resulting disappointment, in needing to
recalibrate and downgrade the relative importance of natural selection, will evidently
be severe.
But this far even the most devoted selectionist must proceed—for the logic of
this "less benign" category cannot be gainsaid. The basic formulation of the theory of
natural selection does require structural input of raw material by variation to fuel the
functional outcome of evolutionary change by selection. (And since natural selection
cannot, in principle, manufacture this necessary fuel for its own operation, lack of
input can stymie output—just as the niftiest motor car can't move if you run out of
gas in the middle of the Sahara, hopelessly far from the nearest petrol station in
Timbuctu.) Therefore, one cannot brand limitation in raw material as an incoherent
concept, or even an empirical rarity, a priori.
And yet, if selectionists can hold the line—as they generally attempt to do—at
this negative definition of constraint, their theory, while deniably impacted, suffers
no serious setback to its truly essential postulate that natural selection controls the
direction of evolutionary change. At most, the negative forces of constraint may slow
down, or even prevent, modifications. But so long as these structural factors do not
operate in a positive sense—either to determine important variation in rates and
extents of change or, more threateningly, to impact or set the actual direction of
change—then the fundamental Darwinian rule still prevails: variation proposes, but
only natural
change—even though I remained quite willing to believe that natural selection always set
the direction of change.