Agriculture Reference
In-Depth Information
evolutionary rates and, ultimately, diversification (Wells
1969
; Wisheu
et al.
2000
).
Furthermore, the resulting non-overlapping generations of seeders increase the
probability of manifestations of genetic novelties associated with different condi-
tions present in each generation (Cowling & Pressey
2001
), and thus it has been
theorized that seeders should speciate more than resprouters, and consequently
diversify more as well.
Although there have been several attempts to test the differential diversification
of seeders vs. resprouters (Bond and Midgley
2003
; Lamont & Wiens
2003
; Verdu´
et al.
2007
; Boatwright
et al.
2008
), all have found limited support for a significant
difference between these life history modes. These results suggest that seeders have
not diversified more than resprouters, although it is to be expected that differences
would be rather subtle since many of the resprouters studied were not obligate
resprouters, and thus the comparisons are between taxa that replace the entire
population each fire cycle with taxa in which a variable proportion of the popula-
tion is replaced each fire cycle.
Complicating this comparison is the fact that there are also potential genetic
advantages associated with resprouters as repositories of genetic innovations.
Obligate seeding species subject their gene pool to intense selection because each
fire results in a new generation. Thus, the characteristics of individuals that are
selected by circumstances associated with one fire may not be subsequently
selected if the environment changes. This is evident in that obligate seeder popu-
lations are generally much more phenotypically homogeneous than resprouter
populations, which often exhibit substantial clonal variation (e.g. Keeley
et al.
2007
; Premoli & Steinke
2008
). Considering that environmental change is not
monotonic but cyclical at various scales, species capable of retaining suboptimal
variants under certain conditions may ultimately be selected in the long run. This
could have been of particular selective value during the Quaternary alternation
between glacial and interglacial episodes.
As discussed more fully in
Chapter 10
, different fire histories are associated with
different patterns of diversification in lineages and these appear to distinguish
themselves between the northern and southern hemispheres. In the southern
hemisphere the origins of genera such as
Banksia
are tied to nutrient-poor sites,
which have selected for sclerophyllous fire-prone vegetation, and these occurred
very early in the Tertiary or even earlier. As a consequence vast expanses of fire-
prone landscape potentially would have provided the stage for fire-driven diversi-
fication in
Banksia
over much of the Tertiary, accounting for its spectacular
speciation by the Eocene (Hill & Christophel
1988
). In contrast, in the northern
hemisphere early Tertiary fire-prone landscapes were likely marginal habitats on
widely disjunct sites. Although these habitats selected for sclerophyllous fire-prone
vegetation, the discontinuity of fuels would have led to a less predictable fire
regime, one that selected for resprouting, but a regime lacking sufficient predict-
ability and intensity to select for postfire seeding until climatic changes later in the
Tertiary. Not surprisingly most of the northern hemisphere fire-prone genera with
a clear linkage to early Tertiary origins are depauperate genera of obligate
