Simulated lakes and simulated radiations

Theoretical models consist of assumptions, a best guess about how things work in nature, and predictions,which are the model results.A good model will make a few biologically reasonable assumptions and result in predictions that bear a strong resemblance to reality, hence isolating the important mechanisms.

Van Doorn and colleagues started by assuming that individual fish can be characterized by a colour preference (of females), a pigmentation (of males), and by their niche use (represented for simplicity by a single number: think of it as prey size, or water depth). Individuals compete, and are more likely to die if their niche use is similar to that of other individuals. This keeps the population size limited. Fish are born by sexual reproduction, which is dependent on female mate preference, male pigmentation, and the degree of niche overlap (similar niches increases the probability of mating). Mate preference, male pigmentation, and niche use are also heritable, so that offspring resemble their parents, but imperfectly, so small random changes (mutations) are created in each generation. Finally, the more brightly coloured males are, the lower their survival as a result of natural selection (such as predation). So far, so good.

One other important assumption is that females have peaks in perceptual ability at both ends of the colour spectrum. Perceptual ability relates the pigmentation of males to the colour perceived by females. In perfectly clear water, there is a near-perfect match between the two, although females perceive very bright pigments (at either end of the colour spectrum) slightly better than others. In very murky water, all pigments appear brown to females. In slightly murky water, only pigments that are close to female's perceptual peaks are perceived to be coloured.

The model was run by starting off a small population of a single species in clear water and letting mating, reproduction, and death take its course. Species were defined as groups of individuals that, because of their niche, colour, or preference, were very unlikely to mate, and could hence evolve independently of the others. After 2000 generations, five species were coexisting from the original species in this simple and tiny virtual lake. The process could clearly work. How exactly does it happen?

The key is female mate choice. As a result of the biased perception of red and blue, on average females prefer males that have more-extreme-than-average pigmentation. As a result, both pigments and preference become more extreme over time (Figure 1.4). The process is a familiar concept in

Fig. 1.4 Speciation via sexual selection in the van Doorn et al. (1998) model. Individuals of different species are represented by different symbols. The curved line represents female preference for male colour and is biased towards red and blue (females on average prefer males that are bluer or redder than the population average). Because of this bias, brown species gradually split into two, one redder and one bluer, as can be seen with the species represented by the open squares. After van Doorn et al. (1998), with permission from the Royal Society of London.

Fig. 1.4 Speciation via sexual selection in the van Doorn et al. (1998) model. Individuals of different species are represented by different symbols. The curved line represents female preference for male colour and is biased towards red and blue (females on average prefer males that are bluer or redder than the population average). Because of this bias, brown species gradually split into two, one redder and one bluer, as can be seen with the species represented by the open squares. After van Doorn et al. (1998), with permission from the Royal Society of London.

sexual selection theory and is known as a 'runaway' process. Species that have neutral colours and preferences, neither red nor blue, will split into two species with slightly brighter (redder or bluer) colours. Once incipient species no longer interbreed, their niches diverge as a result of competition (this can not happen in a single species because interbreeding stops the niche changing). The amount of niche space present limits the number of species that can coexist, and it is for this reason that the model only produces a few species. If species have different niches, they can have more similar colours without losing their integrity as species. That of course is exactly what we see in Lake Victoria: lots of species with the same nuptial colour.

The final triumph of the model is what happens when the water is made turbid. Species cannot diverge, or any longer remain sexually isolated because all males appear the same to females. Species number crashes, just as in nature. The model is successful because by using the small pieces of biology gathered so far, it successfully predicts many of the important patterns in nature: it is a good conceptual cartoon for what goes on in nature.

However, the model appears not to be the last word in cichlid speciation. Species in the model form from brown fish gradually splitting into slightly less brown ones. In fact, individual species in nature often display a male red/blue colour polymorphism, suggesting that speciation and colour change are much more instantaneous. Thus, the model is in some respects only a rough cartoon of some of the actual processes. In addition, there is a second type of colour polymorphism within some species in which females vary in colour and are associated with a rather interesting genetical system (Seehausen and van Alphen 1999; Seehausen etal. 1999). Something different must be going on in those.

Teaming up with theoretician Russ Lande, Seehausen devised a model that incorporates these 'instant' novel female colour morphs with the strange genetics in a sympatric speciation scenario (Lande et al. 2001). They showed that given the way novel colour morphs and other traits are inherited together, rapid speciation is likely to result even without ecological differentiation. The female colour polymorphism is due to a gene that causes sex reversal from male to female and is associated with a distinct colour pattern (Seehausen etal. 1999) (Figure 1.5).

Imagine then that novel colours are only seen in females. Unusual males that prefer, or do not discriminate against, this colour now have high mating success for two reasons; they are rare male phenotypes, so get all the mating with unusual coloured-females that normal males pass by. In addition, if the sex-reversal gene is widespread, they will also be the rarer sex, so get more mates anyway. This process, which favours the novel males through rarity of the male sex, is called sex ratio selection. We will encounter this process again in Chapter 5. An association between the new colour morph and preference

Fig. 1.5 A cichlid, Paralabidochromis chilotes, from Lake Victoria (length 15 cm). Blotchy morphs like these are, in most populations, female, and include sex reversed males that may play a role in speciation by sex ratio selection. Photo courtesy of Ole Seehausen.

for that colour morph builds up. Over only a few dozen generations, a new reproductively isolated species has arisen in situ.

It appears likely that at least two in situ processes can account for colour-diverse haplochromine species richness: sexual selection and sex ratio selection. Both these processes can cause speciation with geographic separation, but they can also do it in the absence of geographical separation. The processes appear bizarre and extraordinary at first sight. However, both processes are not unexpected in a wider context; we will come across them again later in the book. What then has the cichlid story taught us?

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