more accurately, by any one of a variety of processes) that has no 
necessary relationship to genetic/morphological change under the 
guiding hand of natural selection. Among mammals, speciation ap- 
pears to require complete—or at least effective—physical separation 
of populations (see, for example, the discussion by Mayr, 1966); 
but, once such separation has occurred and speciation has inter- 
vened, the resulting sister species will compete rather than interbreed 
should environmental circumstances change and place them in con- 
tact once more. Habitat change of this kind will normally involve 
upheavals in whole faunas rather than in the lives of individual 
species, placing each population in a complex new competitive sit- 
uation. In such circumstances winnowing effects will take place 
among species that are analogous to the winnowing of individuals 
under natural selection that occurs within local populations. 
I have, I hope, said enough to persuade you that evolution is a 
complex process into which many influences enter that are entirely 
random with respect to excellence of adaptation (see discussion in 
Tattersall, 1998). Environments fluctuate, adaptations become irrel- 
evant, disasters befall. Yet discourse on the subject of human cog- 
nitive evolution has, almost without exception, proceeded as if in- 
creasing brain sizes and “‘‘intelligence’’ were simply matters of in- 
exorable improvement under the benign influence of natural selec- 
tion. We have big brains; we take their advantageousness for 
granted; and we thus tend to see a certain inevitability in our having 
become the way we are. Yet a moment’s thought is sufficient to 
indicate how implausible it is that bigger and better brains or im- 
proved brain structures evolved in this tranquil and linear fashion, 
most especially in the highly unsettled ecogeographic conditions of 
the past few million years. The fact that brain enlargement, at least, 
has-been a fairly consistent if irregular theme throughout the last 
couple of million years or more of hominid evolution does, of 
course, suggest that on balance, and on average, larger brains have 
conferred advantage; but we should bear in mind that the trend we 
perceive is almost certainly not a simple within-species phenome- 
non, smarter individuals surviving and reproducing more success- 
fully, generation by generation, in an inexorable process of improve- 
ment over the eons. Instead, as Gould and Eldredge (1993) empha- 
