1969] 
Burns and Kendall — Pyrgus 
51 
Temporal distribution in Texas 
Both species are multivoltine and occur almost throughout the year 
(see fig. 5, bottom) wherever a suitably mild climate prevails. Even 
where they are sympatric, there is no temporal displacement of these 
two related populations. 
Discussion 
Reproductively isolated populations recently evolved from a. com- 
mon ancestor may remain allopatric, not because of competitive ex- 
clusion or the persistence of an old extrinsic barrier to distribution, 
but because, in the course of inevitable ecologic differentiation, they 
have specialized in ways that put new restrictions on their distribution. 
If a potential distribution-limiting factor varies abruptly in the space 
between them, allopatric sister populations may become finely ad- 
justed to distinct modes of that factor and thereafter be geographic- 
ally segregated by this very adaptation. Should such a condition 
persist, even widely diverging populations would continue to replace 
each other geographically like subspecies of a polytypic species or 
species of a superspecies, without long qualifying as either. It would 
be most unreasonable to insist that ecologic divergence of sister pop- 
ulations to a patently non-competitive, coexisting state be the sole 
admissible evidence of complete speciation. 
The mutual geographic replacement of P. oileus and P. philetas 
in the United States suggests competitive exclusion and suggests that 
they may be more closely related than their sympatry in Mexico and 
their level of morphologic divergence indicate. But competitive ex- 
clusion is probably not involved. Along its eastern margin through 
central Texas, P. philetas , as noted above, stops almost on a line; 
yet it regularly encounters P. oileus only along the southern part of 
this line, and there there is some overlap. Rather than limiting each 
other, P. oileus and P . philetas are more likely limited — directly or 
indirectly — by humidity, an abiotic factor that decreases rapidly from 
east to west in central Texas. Direct limitation could stem, of course, 
from physiological adaptation of P. oileus to moister and P. philetas 
to drier conditions; indirect limitation could take the form of dif- 
ferent larval foodplants that are, in turn, directly limited by humid- 
ity. P. oileus and P. philetas are limited northward at similar latitudes. 
Altogether it appears that these species, which are substantially sym- 
patric southward, gradually sort themselves out geographically in 
response to humidity as they approach their northern temperature 
limits. 
