A JOURNAL OF ACADEMIC WRITING VOLUME 5

morphology have changed along with morphological
diversification in cichlids, then the amino acid
substitutions should have occurred more frequently in
Great Lake cichlids than in other fishes. Conversely,
if these genes are not responsible for producing the
cichlid diversification in jaws, then the rate of amino
acid substitutions should be similar in these cichlids
and other fishes. To test this, they amplified parts of
the cichlid genome known to be homologous with
genes that control jaw and dentition features in mice.
They also amplified the genome of an outgroup
fish. From the DNA sequences, they found that the
highest evolutionary rate of change in this part of the
genome corresponded to a single gene previously
known from mice called Bmp4. Furthermore, they
found that the rate of amino acid substitutions in
Bmp4 was five times higher among the Great Lake
cichlids than among the outgroup fish. Thus, results
suggest that Bmp4 changed at an accelerated rate, as
did jaw morphology changes, during the speciation
of cichlids. Albertson et al. (2005) have confirmed the
role of Bmp4 in regulating jaw features.
Terai at al. (2002) then analyzed the specific
protein domain areas affected by these numerous
amino acid substitutions in the Bmp4 gene. They
found that all substitutions affected the portion of the
protein responsible for post-translational control of
the protein. The amino acid substitutions in cichlids
do not change the function of the protein made by
the Bmp4 gene, but instead affect how that protein
is used downstream in more complex molecular
signaling pathways. This discovery, that a small
change in the Bmp4 gene can generate bigger changes
via a cascading pathway, may have unveiled another
way in which cichlids generated so many different
adaptive jaw forms so quickly.
It is further interesting to note that it has
recently been found that the Bmp4 gene also plays an
important role in the cranio-facial development of
Darwin’s finches (Abzhanov et al. 2004; Grant et al.
2006). In fact, the investigators concluded from their
study that, “…variation in Bmp4 regulation is one
of the principal molecular variables that provided
the quantitative morphological variation acted on
by natural selection in the evolution of the beaks of
the Darwin’s finch species.” Thus, more detailed
comparisons of Bmp4 sequences, expression, and
regulation between cichlid flocks and finches should
provide future clues to the puzzle of explosive
speciation in cichlids.
Undoubtedly such examinations of the genetic
factors involved in the control and modification of
phenotype will continue to shed new light on the
puzzle of speciation processes. However, studies
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and reflection on a more macroscopic-scale are also
beneficial; one reason that there are so many more
cichlids than finches, despite so many similarities
in the mechanisms involved in their speciation,
may simply be due to time. Darwin’s finches have
been evolving for about 3 million years, whereas the
cichlids of Lake Malawi have only been evolving for
about half that time (or less). Thus, the case of cichlids
provides a kind of time-travel backward, showing
the very earliest events in speciation. As time and
evolution proceed toward higher levels of ecological
stability, the number of species may decline. Such
a pattern is suggested by comparisons between the
species of cichlids themselves. As the oldest lake,
Tanganyika’s cichlid species number is much less
than the younger Lake Malawi. Also, Tanganyika’s
cichlid species are more diverse. Such differences in
both species number and diversity suggest that after
speciation rates level off, some species out-compete
others to extinction; then interspecific competition
continues to drive a wedge between remaining
successful species. This temporal pattern in speciation
is also evident from comparisons of cichlids to
their cousins, parrotfishes (Scaridae). Cichlids and
parrotfish share components of the same ‘keyinnovation’
(i.e. pharyngeal jaws), yet there are about
ten times more species of cichlids than parrotfish.
One significant difference between them, however,
is that parrotfish originated about 40 million years
ago, and have had a great deal more time to stabilize
into a lesser number of highly diverse species.
If we could time-travel back to the early days of
parrotfish speciation, we might be met with the kind
of profusion in species that we see today in cichlids.
(Barlow 2000; Streelman & Danley 2003).
Disappearing Cichlids
Unfortunately, anthropogenic causes are
destroying cichlids, and with them the clues they
hold regarding speciation processes. Due to the
remote location of the Great Lakes, rigorous scientific
examination of these endemic cichlids has only been
undertaken within the last forty years or so. In the
meantime, the nature of threats facing cichlids has
rapidly multiplied and intensified.
Fish from Lake Malawi provide communities
there with seventy-five percent of their animal
protein; cichlids are the primary catch. The entire
fishing industry supports about two million people,
but is doing so in a non-sustainable way. Cichlids
captured are decreasing in size and quantity.
Similarly, the introduction of commercial methods of
fishing to Lake Tanganyika in the 1960s precipitated
a decline in cichlid catch after only twenty years