whereas S. galilaeus has small green eggs brooded by both parents. What
further differences may exist among the species apparently related to
these we do not know. Perhaps they even evolved from different species
of Tilapia or from the same species at different times. Therefore we may
eventually recognize different lines of evolutior? in these species and wish
to call them by different generic names. Meanwhile let us be conservative
and call them all Tilapia.
This is the position of Thys and Peters. My view (Trewavas 1973a, 1973b,
1980 and in press) is, however, that the mouthbrooding species showing
affinity to Tilapia are sufficiently alike in their feeding adaptations and even
in their breeding habits to show close interrelationship and that this justifies
separating them from the substrate-spawning Tilapia under the one generic
name. The substrate-spawners have themselves branched out into a number
(about 29) of species and I find it difficult to lump all the mouthbrooders
together with them as Tilapia. In the last few years I notice that fish culturists
and others have followed my proposal to call the mouthbrooders Saro-
therodon.
Speciation
The pattern of speciation in Sarotherodon and Tilapia leads to the con-
clusion that it has occurred in two ways: 1) by geographical isolation;
allopatric and 2) by ecological diversification in one area; sympatric (see
Figures 1 and 2).- Examples of allopatric speciation are:
a. Tilapia zillii and T. rendalli, described above. Their coming together at
Yangambi is probably secondary. A number of allopatric and geographically
more restricted species related to these are present in west Africa.
b. Sarotherodon melanotheron and its subspecies in areas along the
coast, separated by steeper shores. (These have been ranked as species by
Thys (1971b), a matter of policy).
c. The tasselled tilapias surrounding the Rift Valley: S. variabilis, S.
upembae and its related form in the Malagarasi, S. rukwaensis, S. macrochir
and the Malawi flock considered as a unit.
d. S. mossambicus and related forms in the eastern rivers. - Examples of sympatric speciation are:
a. The species of the Malawi flock, S. squamipinnis, S. saka, S. lidole and
S. karongae, whose ecological differences are described by Lowe (1952,
1953).
b. The two species of the Malagarasi swamps, S. karomo and the other
species related to S. upembae. The highly specialized jaws and teeth of S.
karomo must indicate a specialization in feeding on the epiphytic growths.
c. The species of the small lake Barombi Mbo in Cameroon, especially
evident in S. linnellii and S. caroli (Trewavas et al. 1972).
When two species occur together but each has its nearest relatives else-
where, this is regarded as allopatric speciation in two lines that have come
together geographically, and not, of course, as sympatric speciation. Such
are S. esculentus and S. variabilis in Lake Victoria related respectively
to S. niloticus and the tasselled tilapias. Other examples are S. shiranus
(related to S. mossambicus) and the S. squamipinnis group considered as a