THE CICHLID FISH of Africa鈥檚 great lakes have rather particular tastes. One
species will confine itself to a certain rocky promontory, another might live in
a sandy bay, and a third will be found only in the deep waters. Their dietary
preferences are equally restrictive: to the point where one type dines
exclusively on scales that it strips, using specially adapted mouthparts, from
the left sides of other cichlids, while another, which looks like the
mirror-image of the first, eats only scales from right flanks. And, while most
female cichlids are plain brown, when it comes to choosing a mate, colour is
all. Those that go for blue males spurn suitors with yellow or red colouring,
even if they look like their blue counterparts in every other way. Females that
prefer yellows or reds are just as prejudiced.
This cliquey behaviour has remarkable consequences. Between them, Lakes
Malawi, Victoria and Tanganyika contain around 1700 species of
cichlid鈥攎ore than double the total of all the freshwater fish species in
Europe and North America. Each of these species must retain a degree of
isolation if it is to avoid treading on the fins of the others. Without their
idiosyncrasies the fish might interbreed and biodiversity would be lost. The
lakes would become huge melting pots, instead of being home to the most
explosive proliferation of new species known.
Why are there so many cichlid species? The question has intrigued biologists
for decades, and it took a new twist in 1996, when geological data revealed that
Lake Victoria was completely dry during most of the last ice age. DNA analysis
had already indicated that the 500 or so cichlid species now living there share
a common ancestor. So, this dazzling variety has evolved in no more than 12 500
years. Whatever it is that causes speciation, cichlid fish must have it in
spades.
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Exactly what constitutes a species is hotly debated. But for practical
purposes, if two distinct populations exist that cannot or will not breed with
one another, they can be considered separate species. A new species evolves only
when two events combine: creation of novelty, and construction of a barrier to
reproduction with other species. The novelty comes with random genetic mutation.
Isolation can be achieved in a variety of ways: in space, time and
behaviour鈥攑articularly mating practices.
Most mutations are harmful, some have no effect on survival, and very few
lead to new species. In the course of evolution, however, genetic change has
produced a handful of so-called 鈥渒ey adaptations鈥� which have greatly increased
the options for diversification. Multicellularity, air-breathing and
warm-bloodedness, for example, are all at the root of explosive surges in
evolution. Karel Liem, from Harvard University, has suggested that cichlids
underwent their own unique key change. By mechanically uncoupling their upper
and lower pharyngeal jaw, the fish have hugely increased their ability to adapt
to specialist diets. As a result, adaptive radiation is common. In other words,
evolution keeps coming up with new variations on a theme鈥攑opulations of
fish that can successfully carve out their own niches in a crowded lake.
The cichlid鈥檚 isolationist lifestyle helps to keep these evolving groups
separate from their close relatives. George Turner from the University of
Southampton and his colleagues used DNA analysis to measure the relatedness of
cichlid populations living near adjacent rocky headlands in Lake Malawi. Their
results showed that a 700-metre-wide sandy bay is enough to create an effective
barrier to breeding between neighbouring populations.
But geographical isolation cannot be the whole story, because many cichlids
do not have well-defined home bases. In Lake Victoria, over 300 species
intermingle along the shorelines or in deep waters, without physical barriers to
prevent interbreeding. So how can evolving species find the isolation they need?
According to Ole Seehausen, from Leiden University in the Netherlands, it鈥檚
mostly down to picky females and colourful males.
If female fish are very choosy about the colour of their mate, then
proto-species need not be physically isolated from each other to make the leap
to speciesdom. All that is required is for adaptive radiation鈥攕uch as a
change in diet鈥攖o go hand in hand with colour change. Female preference
does the rest, because if just a few females consistently choose to mate with,
say, blue males, and pass this preference on to their offspring, then any
adaptive change will become isolated in this new interbreeding population. One
clue that this might be happening comes from the observation that where pairs of
closely related species live together, the males of one tend to be blue and the
other either red or yellow. 鈥淢any people had written about colour,鈥� says
Seehausen, 鈥渂ut nobody had done the experiments.鈥� So he did.
Fickle females
In the lab, Seehausen mixed together fish from two species that looked very
similar except that one had blue males and the other red. In normal light the
females went for males of their own species. But when he altered the light to
disguise the colour of males, there was a change in behaviour. 鈥淔emales tended
to prefer larger males rather than males of the same species,鈥� says
Seehausen.
He has since charted the preferences of individual females within a single
population of a species with variable male coloration. A few fish were not
choosy. Most consistently picked blue males鈥攖he colour that predominates
in this particular species鈥攁nd in so doing would maintain their isolation
from other species. Some females chose yellow mates鈥攁 less common colour
for this species. Such behaviour could be the beginnings of speciation.
鈥淚t鈥檚 an incredibly exciting and testable hypothesis,鈥� says Turner. He has
been working with Mike Burrows from the Dunstaffnage Marine Laboratory, Oban,
Scotland, on computer simulation models to find out whether sexual selection
really can drive speciation, even when there are no geographical barriers. Their
models suggest that the theory can work in practice but only under certain
circumstances. First, females must see many males before they mate, maximising
the chance that genes for fussiness and genes for the preferred
trait鈥攃olour in the cichlid鈥檚 case鈥攃ome together in their offspring.
This does seem to be the case in Africa鈥檚 great lakes. Secondly, there should be
little movement of individuals. 鈥淔emale cichlids tend to stick around where they
were born,鈥� says Turner.
Seehausen believes that sexual selection could account for most of the
speciation in Lake Victoria. And it doesn鈥檛 stop there. Sexual selection is also
responsible for the origin of new species among insects, birds and perhaps even
primates, although nowhere so fast or so frequently as in the cichlids,
according to Turner. 鈥淚t seems to be the first step on the way to speciation,鈥�
he says.