Splitting in sympatry

If speciation by geographic isolation was never in doubt then the same cannot be said of sympatric speciation (Chapter 1),speciation in the absence of geographic isolation. Until very recently, there were no clear-cut empirical cases: Ernst Mayr in his 1963 book systematically attacked the best-known potential examples of the time, including the East African cichlids mentioned in Chapter 1, and another example discussed below, the apple maggot fly, as being just as (if not more) consistent with speciation in allopatry. The early theoretical work also suggested rather restrictive conditions for sympatric speciation (e.g. Maynard Smith 1966). In general, models found two problematic areas. First, it was difficult to explain assortative mating. Many models assumed strong linkage, or pleiotropy of the mating system to an ecological trait under selection, but in general pleiotropy is expected to be weak. Second, ecological differentiation is normally required to prevent competitive exclusion of one of the incipient species. Even in the presence of disruptive selection (intermediate phenotypes are selected against while extreme phenotypes are favoured), there is a tendency for just one extreme ecotype to evolve rather than two because a population will tend to move to one side or the other of the new fitness minimum. If linkage between the mating system and the ecological trait is weak, disruptive selection needs to be very strong to overcome recombination of the ecological traits.

Since then, further evidence about particular case studies has strengthened the case for sympatric speciation. In addition theoretical developments have shown that sympatric speciation is more feasible than previously thought. The theoretical advances have been (1) sexual selection leading to assortative mating (Chapters 1 and 7), and (2) adaptive dynamics leading to evolutionary branching and hence coexisting ecological polymorphism (Chapters 8 and 10) (van Doorn and Weissing 2001). Models have tended to emphasize solutions to one of these problems over the other, and hence can be termed sexual selection or ecological models.

Sexual selection models explicitly take into account the interaction of males and females and assume that males compete for mates while females exert mate choice. Normally, Fisherian runaway selection (Chapter 7) is assumed, such that male traits and female preference for those traits become genetically correlated through non-random mate choice. More extreme traits and preferences evolve until counteracted by natural selection. Mating strategies need to become polymorphic to generate reproductive isolation, and hence we need a source of disruptive selection. This might come from innate female tendencies to prefer particular divergent phenotypes, as may be the case in haplochromine cichlids. However, branching of male traits, and subsequently female preference, can also occur if rare phenotype males experience less competition for mates from other males (van Doorn and Weissing 2001, Chapter 1).

Ecological models emphasize disruptive selection on the ecological phenotype. This can readily occur in adaptive dynamic models through evolutionary branching, because the selection is not imposed externally but dependent on other resident phenotypes (frequency dependent). Under some circumstances then, evolution will drive the ecology of the species to a point where it lies at a fitness minimum and branches into a stable polymorphism (Chapter 8). Resource competition can readily do this, as long as the fitness advantage of utilizing rare resources through reduced competition outweighs the lower abundance of those resources.

So incorporation of sexual selection and ecological branching into models have shown that sympatric speciation can readily occur. In addition the evidence in favour of some of the potential examples of sexual selection has become much firmer (Chapter 1).A particularly influential case is that of the apple maggot fly Rhagoletis pomonella (Figure 12.6). The fly normally lays its eggs on developing apple fruit, in which the larvae develop, pupating once the fruit falls to the ground. In the Hudson valley of New York state, sometime during the mid-1800s, a host shift occurred and a new race evolved, apparently in sympatry, that developed on hawthorn fruit.

In the 1960s, Guy Bush, developed a verbal model of sympatric speciation via host race formation in the fly. Bush's model bears many resemblances to some of the classical sympatric speciation models. First, it assumes host-specific mating (a pleiotropic effect), such that each race mates and oviposits on the same fruit on which they fed as larvae. This would give assortative mating. Second, he assumed host-associated fitness trade-offs, such that a gene

Fig. 12.6 Apple maggot flies, R. pomonella, on an apple. Photos courtesy of Andrew Forbes.

giving a fitness advantage to development on one host was ill suited towards development on another host. Hybrids would be less fit than both parents on both hosts—a source of disruptive selection.

This model has subsequently been confirmed by data (Feder 1998). First, comparison of allozymes between the apple and hawthorn races has shown consistent differences. Races show genetic differences in host preference; females of both races prefer hawthorn but hawthorn females are more averse to apple. Once they have met, however, there is no pre-mating isolation, and no evidence of sterility or inviability barriers to reproduction. The source of the fitness trade-offs, and disruptive selection, is phenological:development is faster in hawthorn flies. Hawthorns fruit later in the year than apples and so hawthorn flies must develop quicker to complete development by autumn. Apple flies, however, are exposed to warmer weather before the winter arrives and must enter a deep diapause to prevent premature emergence before the winter sets in. Interestingly, when pre-winter temperatures are manipulated to expose hawthorn flies to 'apple'-like conditions, a selection response was detected in the alleles controlling diapause such that the flies became more like the apple race; in fact they did so almost completely in a single generation of selection. Thus, there is antagonistic pleiotropy between fitness determining loci, and this prevents gene flow between the races. It appears that Bush was right, and the apple maggot fly remains the single best example of sympatric speciation in action.

It remains plausible that sympatric speciation is common, but it will be frustrating to wait until a large number of organisms have been studied in the same degree of detail. Luckily, a broad-brush approach is available to determine the frequency of the different geographic modes of speciation: comparison of geographic range overlap across phylogenies (Barraclough and Vogler 2000). Under sympatric speciation, recently formed species should share large parts of their geographic range, whereas under allopatric speciation there should be almost no sharing of geographic range. Deeper down in the phylogeny, however, the degree of range overlap between sister taxa should depend on the degree of subsequent change in range change after the speciation event. By plotting the degree of overlap in the range against node height, we obtain a signal about the geographic mode of speciation (Figure 12.7).

In a comparison of several insect and vertebrate phylogenies, Barraclough and Vogler found that allopatric speciation was in fact the major signal, even in the Rhagoletis phylogeny where there was evidence for just a single sympatric speciation event.A further finding was that the ranges of allopatric sister taxa were frequently very different in size, suggesting that peripatric speciation is common. Broad-brush approaches like this lead us to hope that we may in a short time be able to assess the frequency of different speciation modes in a large number of taxa.

Sympatric speciation a p

Sympatric speciation a p m y s

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Node height (Ma)

Fig. 12.7 The degree of range overlap in species on a phylogeny, when they have been formed by either sympatric and allopatric speciation. Young species formed sympatrically have a high degree of range overlap but the overlap decreases with age due to range changes. Young allopatrically formed species show zero range overlap which increases with time due to range changes. After Barraclough etal. (1999).

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