4 | DISCUSSION
The convergent evolution of phenotype in correlation with behavioral
niche is clearly a pervasive trend in the evolution of mygalomorph
spiders. Their adaptive landscape is simple and constrained at two
extremes: at one end are opportunistic taxa that inhabit existing spaces
and construct capture webs, and at the other are taxa that construct
their own burrow or nest, and structurally modify the entrance, for
example with a trapdoor (Fig. 2). A spectrum exists between these
extremes, but most intermediate taxa still burrow, or show facultative
burrowing habits, but do not structurally modify the entrance. Within
these constraints, changes in the niche occupied have been common in the
evolution of the infraorder, and have occurred in both directions (Fig.
1). For example, the general trend in both the Atypoidea and the
Avicularioidea is that burrowing, trapdoor-building taxa have evolved
from opportunistic, web-building ancestors, yet in (at least) the Venom
Clade and the Nemesioidea, the opportunistic, web-building niche has
evolved again, independently (Fig. 1). Adaptation to different optima in
this narrow adaptive landscape is one of the primary forces shaping
somatic morphology in the Mygalomorphae, and this trend is clear in both
overall morphology (Fig. 2) and in those morphological features that are
intuitively adaptive (Tables 1,2; Fig. 3). The historical use of these
characters to infer phylogenetic relationships explains, at least in
part, the conflict between traditional morphological hypotheses and new
molecular ones. Indeed, it is now clear that the “Dipluridae”sensu lato and the previous higher classification
“Rastelloidina” are both artificial groups lumping together taxa from
either end of the mygalomorph adaptive landscape (Raven, 1985).