This correspondence in life history features of the two groups of tilapias is
not surprising, and at least some of these features have been noted by a!most
all previous investigators who have studied more than a few species (e.g.,
Baerends and Baerends-van Roon 1950; Fryer and Iles 1972; Lowe-McConnell
1975, this volume). We emphasize it since it agrees with our basic hypothesis,
and leads to some interesting predictions. These are particularly relevant as
they may apply to management of these fishes, and their responses to
environmental perturbations over an ecological time frame (i.e., during the
ontogeny of individual fish).Saltation of Ontogeny, and Altricial to Precocial TilapiasThe hypothesis of saltatory development predicts that species should
respond to environmental situations, both on the time scale of ontogeny and
of phylogeny, by heterochronous rates in development. These rates will
provide a more appropriate "match" to existing environmental conditions by
moving the individual more towards the altricial or towards the precocial
trend of the life history style. Since these conditions correspond, in the
general way we have already described, to "r- and K-selection", respectively,
they are of some considerable significance.
Heterochrony is the "shifting of gears" in development (Jones 1972;
Gould 1977,1979; Balon 1980), or change in ontogeny as a result of relative
timing of developmental events and intervals. Since we have a uniform set of
terminology to apply to ontogeny, we can ask whether heterochrony has
occurred, and if so, at what point(s). The answers appear to be directly
related to considerations of selective pressures acting on life history styles.
Heterochrony provides the ability to adapt to a fluctuating environment
by shifting the timing of appearance of structures and functions. It likely
varies among species, or even forms within a species, depending on previous
evolutionary history. The trend in evolution will be from altricial to precocial
(Balon 1980 ; Noakes 1981). This is based on the assumption that, other
things being equal, communities will tend towards a more mature, com-
petitive ("K-selected") condition through ecological succession, favoring a
more precocial life style in the fishes. But juvenilization (paedomorphosis)
remains as the process available for "retracing" phylogeny, and prolonging
early ontogeny, with its concomitant flexibility and adaptability. By this
mechanism, fish will retain some flexibility in ontogeny, and be able to shift
back more towards an altricial life style.
In theory, at least, we can derive an altricial form (more or less equivalent
to "r-selected"), or a precocial (more or less equivalent to "K-selected")
form for a particular tilapiine species by altering the timing of ontogeny
(heterochrony). A shortened (or absent) larval period would be related to a
longer growth interval, to a later reproductive maturation, and consequent
shifts in life history (and vice versa). Since size (a consequence of growth)
and fecundity are typically positively correlated in fishes (bigger fish have
bigger gonads with more gametes; Lowe-McConnell, this volume), fecundity
would also be altered, as would longevity (Balon 1981b). But the net effect
of such shifts in ontogeny would depend on the details of each case, as an