The Biology and Culture of Tilapias

earlier age of first reproduction will often dramatically increase "r" (Lewontin
1965; Green 1980). Thus earlier age of first reproduction will likely be more
significant than changes in fecundity. Ecologists have provided extensive
discussions of these considerations, particularly the concept of reproductive
value, i.e., the allocation of resources to growth or reproduction at a particular
time (Southwood 1976; Horn 1978; Krebs 1978; Hirschfield 1980). More
altricial forms would either occupy habitats with persistent, unpredictable
fluctuations, or invade "new" habitats before they stabilize (e.g., Courtenay
and Hensley 1979). More precocial forms would be better suited for stable
conditions and resultant competition, or specialization, or both (Baker and
Stebbins 1965; Southwood 1976; Horn 1978; Balon 1981a). In this regard,
it is worth noting the generalizations made by Philippart and Ruwet (this
volume). They conclude that the macrophagous, substrate-spawning(guarders)
Tilapia species of the Coptodon group all have wide distributions, exclude
each other geographically, have been slow to speciate and are closer to the
original stock. The microphagous, mouthbrooding (bearers) Sarotherodon,
on the other hand, are diversified and specialized into small local populations
with restricted distributions in the Rift Lakes (with a few exceptions). The
distinctions they draw correspond well to the separation we have suggested
between the altricial life style of the guarders and precocial life style of the
bearers, with attendant adaptations as "generalist" and "competitive"
species, respectively.
Also, the report by Lowe-McConnell (this volume) of the results of
simultaneous stocking of T. zillii (a guarder) and S. leucostictus (a bearer) in
the Teso dams of Uganda fits the same pattern. T. zillii predominated at
first, but was later overtaken in numbers by S. leucostictus, even though the
fecundity of the latter was lower. As we would predict, the more altricial
guarder would be better suited (ecologically) for the initial invasion, but
with time (and presumably increasing ecological maturity of the system) the
more precocial life style of the bearer would be increasingly favored. How-
ever, not all such cases of apparent competition fit this pattern: see discussion
of the advantage of S. macrochir over T. rendalli in initial colonization
of the man-made Lake Mwadingushu, Shaba (Philippart and Ruwet, this
volume) and ousting of S. variabilis by T. zillii in L. Victoria (Lowe-McCon-
nell, this volume).
But the intrinsic appeal of our formulation is that it does not require
dramatic or drastic alteration in a species to produce substantial evolutionary
changes, i.e., a species would not necessarily require major new genetic
"mutations" to shift its life history style. Such shifts could be the result of
relatively minor, geneticallydetermined changes in the timing of ontogenetic
processes. In fact, a more precise formulation provides a mechanism for
evolutionary change in either direction between altricial and precocial forms.
The sympatric occurrence of two forms of a species, altricial and precocial,
has been discussed at length elsewhere (Balon 1980,1981a, 1981b).
Such a heterochronous shift could make possible a kind of sympatric
speciation (through allochronic speciation, perhaps), or only lead to adaptive
changes in life histories. As an aside, however, as we mentioned previously, it
is worth noting the possible significance of this as a mechanism for the
proposed dichotomous splitting of taxa (Lq5vtmp 1974; Balon 1981b). The