Alternative mechanisms

First, let us think briefly about how species are supposed to form. The standard dogma is that this happens through geographic separation and subsequent differentiation. One lineage splits into two distinct ones because a spatial separation occurs, either through a dispersal event to an isolated

Fig. 1.2 The diversity of jaw morphology of Lake Victoria cichlids. Clockwise from top left they eat, snails, fish, fish larvae, algae on rocks, invertebrates on rocks, insect larvae.

new region, or through fragmentation of an existing one (vicariance). The lineages evolve in isolation, through natural selection or other processes, and eventually become distinct enough to be called new species. The differences between related, but geographically isolated species are what gave Darwin and Wallace many clues to their theory of evolution.

Could such processes be at work in the fastest vertebrate radiation? Geographic separation and natural selection have undoubtedly contributed, and a number of observations on geographic distribution and morphological divergence among species are consistent with the process. For example, closely related sister species in Lake Victoria sometimes have widely separated geographic ranges (Seehausen and van Alphen 1999); and different populations of the same species have distinct jaw morphologies that match local diets, suggesting local adaptation (Bouton et al. 1999). But there remains a dearth of special explanation: why here and why haplochromines? A growing weight of evidence suggests a role for additional mechanisms and in particular in haplochromines.

What additional mechanisms might be important? Can speciation, for example, occur without geographic isolation? There are two problems that need to be overcome. First, there has to be ecological divergence: the two incipient species have to occupy different niches to prevent them from competing and allow stable coexistence. Second, there has to be reproductive divergence, so that interbreeding does not occur. Getting these events to occur without geographic isolation is a conceptual challenge that has long occupied evolutionary biologists. In the 1990s, this question was bothering cichlid enthusiast, Ole Seehausen. Ole's hunch was that species could diverge in situ into reproductively isolated populations by assortative mating based on male coloration. Over time, mate selection by different females for different coloured males would produce two reproductively isolated species living in the same ecological niche but differing in male coloration. Once separated like this,the way would be open for natural selection to allow niche differentiation. The process could then repeat itself. The power of this mechanism is its potential speed. Initial ecological differentiation need only be small, and the constant disruptive power of female choice would drive populations rapidly apart. It was a process that seemed capable of giving rise to a multitude of species in a very short time.

What evidence supported this hypothesis? One source is patterns of geographic overlap between species. If speciation has occurred in the absence of geographic separation, there should also be groups of closely related species that overlap in range a lot. In fact, there are many such cases in Lake Victoria (Seehausen and van Alphen 1999). What about sexual selection? In the field, sympatric sister species tended to be opposite colours more commonly than allopatric pairs of species. This is consistent with the origination of new species via selection on coloration in situ. These patterns have also recently been demonstrated in Lake Malawi cichlids (Allender etal. 2003). In the laboratory, females from red species behaved preferentially towards red males, as did females of blue species towards blue males. When exposed to monochromatic light that hid the males' bright nuptial hues, females would no longer show a mate preference (Seehausen and van Alphen 1998). This was indeed assortative mating based on colour. But why should female mate choice be disruptive? One possible answer is perceptual bias: the colour-sensitive cone cells of haplochromines are particularly sensitive to red and blue parts of the spectrum, and these different sensitivities could lead females to perceive red or blue males preferentially (Seehausen et al. 1997). However, other possible mechanisms could be at work. What ever the mechanism, female haplochromines agree with Winston Churchill when he said: 'I cannot pretend to feel impartial about colours. I rejoice with the brilliant ones and am genuinely sorry for the poor browns'.

100 200 300 400 Width of the transmission spectrum (nm)

Fig. 1.3 Number of coexisting cichlid species against the clarity of the water at different sites in Lake Victoria (a wide transmission spectrum represents clear water). After Seehausen etal. (1997), with permission from AAAS.

100 200 300 400 Width of the transmission spectrum (nm)

Fig. 1.3 Number of coexisting cichlid species against the clarity of the water at different sites in Lake Victoria (a wide transmission spectrum represents clear water). After Seehausen etal. (1997), with permission from AAAS.

Another feature of haplochromine cichlids is that if mating does take place between individuals of different species, the offspring are normally perfectly viable. The only reason they can be called separate species at all is because of their fussy mate preferences. Evolutionary biologists call this 'pre-zygotic' isolation. In the field, Ole started to find rather disconcerting observations that mimicked what he was seeing in the laboratory (Seehausen et al. 1997). Where the water was murky, and that was often quite a recent phenomenon, he found few species of fish (Figure 1.3) and of dull brown coloration. In clear waters, many species coexisted together, and they were beautifully coloured. It looked as if previous mating barriers were breaking down. Turn it on its head, and mate choice in clear water seemed to have allowed divergence and maintenance of species in the first place.

Could disruptive mate choice be the reason why it is the cichlids, and not some other fish group, that have diverged in this way, and especially the haplochriomine fish that radiated in lakes Victoria and Malawi? That too appears to be the case. Comparing the incidence of mating system and male nuptial coloration in different cichlid groups, Ole showed that there was a significant association between the incidence of polygyny (where males mate with more than one female, long associated with highly selective female mate choice) and male nuptial coloration. Furthermore, the base of the radiation that gave rise to the fish 'superflocks' of Lake Victoria and Malawi, the haplochromines, was characterized by the origin of male nuptial coloration (Seehausen etal. 1999).

Could not some other fish group possessing strong sexual selection also have radiated? Put another way; is there anything else about the cichlids, which would lead to this mating system being particularly diversifying for them? Part of the answer may have to do with that second essential process of speciation without geographic isolation, ecological divergence. Some kind of novel ecological flexibility might open up new niches, making each new speciation experiment more likely to succeed. In fact cichlids have long been known to possess a novel character that would lead to such flexibility: the 'decoupled pharyngeal jaw' apparatus (Liem 1973).The bones of the mouth have been freed to evolve into specialized food-gathering implements, while the bones at the back have become very efficient grinding elements. This novelty has given the cichlids jaw-evolvability as well as behavioural plasticity. That it has played an important role in the present diversity of cichlids is a very good bet.

Therefore, much evidence points towards a role for disruptive sexual selection acting on male coloration, followed by ecological differentiation as the reason why cichlids, and particularly those in Lakes Victoria and Malawi, have diversified so rapidly and why those species are still maintained. It is a nice idea. But does a world with those simple conditions produce the desired result? Will it also work in theory? This problem was tackled by a talented undergraduate at the University of Utrecht, Sander van Doorn, who built a simulation model of the process (van Doorn et al. 1998). This step is an important one, because ultimately biologists want to put aside a small set of essential processes into a body of theory that captures the essence of reality. We have to know what processes are sufficient and important, and which are just noise.

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