THE CICHLID FISH of Africa's 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--more 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.
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 "key adaptations" 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--populations of fish that can successfully carve out their own niches in a crowded lake. The cichlid's 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's 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--such as a change in diet--to 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. "Many people had written about colour," says Seehausen, "but nobody had done the experiments." 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. "Females tended to prefer larger males
rather than males of the same species," 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--the colour that predominates in this particular species--and in so doing would maintain their isolation from other species. Some females chose yellow mates--a less common colour for this species. Such behaviour could be the beginnings of speciation. "It's an incredibly exciting and testable hypothesis," 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--colour in the cichlid's case--come together in their offspring. This does seem to be the case in Africa's great lakes. Secondly, there should be little movement of individuals. "Female cichlids tend to stick around where they were born," says Turner. Seehausen believes that sexual selection could account for most of the speciation in Lake Victoria. And it doesn't 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. "It seems to be the first step on the way to speciation," he says. Kate Douglas from New Scientist, 06 March 1999 |
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--particularly mating practices.