Continuous expansion to new environments is the most general overriding feature of all adaptive radiation. The pattern of this divergence between populations and species is partly predictable from principles of quantitative genetic covariation. The initials stages of divergence between populations and species tend to occur along directions of maximum genetic variance within populations, a tendency that seems to decay with time.
-D. Schluter, The Ecology of Adaptive Radiation (2000. p. 237)
Entry into a new adaptive zone is often a unique event. There is general agreement, for instance, about which “preadaptations” allowed freshwater organisms to invade land (sturdy limbs and lungs). But in most cases of adaptive radiation, the key traits–if any–are impossible to determine without the replication necessary for sister-group analysis. Because some adaptive radiations have occurred repeatedly, such as the invasion of angiosperms by insects, one can test whether any traits are consistently associated with the diversification.
-J. Coyne & H.A. Orr, Speciation (2004, p. 431)
The null model for early Homo should be the kind of evolutionary pattern that we now know to be true for Late Pleistocene humans. Multiple populations, much more highly differentiated than today’s human populations, existed during the Late Pleistocene and exhibited nonuniform patterns of expansion and mixture. The expansions of some groups within and outside Africa were likely driven by gene-culture coevolution, as both technological changes and physiological changes affected population growth. We are beginning to appreciate that similar episodes of expansions and mixture happened throughout the Pleistocene. The origin of our genus, initiating the first expansions of hominins into Eurasia, was surely driven by a similar process.
-John Hawks, Polytypism and complexity (http://johnhawks.net/weblog/topics/dynamics/complexity-early-homo-van-arsdale-2012.html)
