Evidence for assumptions and predictions

Most of the evolutionary models for fundamental niche evolution assume antagonistic pleiotropy, in other words negative genetic correlations between fitness in different habitats. Some negative fitness trade-offs have been shown, and these undoubtedly enhance the process of specialization. For example, pea aphids with a genetic propensity for high fecundity on clover have a genetic propensity for low fecundity on alfalfa, and vice versa (Hawthorne and Via 2001). Rather excitingly, performance on those plants is also correlated with preference for those plants, and could represent an incipient stage in speciation. In general, however, most genetic correlations are not significantly negative and most are close to zero, indicating that high fitness in one environment is relatively independent of fitness in other environments.

There are numerous potential reasons why this result may be rather arte-factual, to do with the difficulties of conducting flawless and powerful experiments to measure genetic correlations. On the other hand, the fact that most genetic correlations are near zero, rather than positive, implies that different genotypes favour different environments, rather than a single genotype being best in all environments. The former is needed for the alternative 'neutral' models. In addition, deleterious mutations with habitat-specific effects are known in Drosophila, which is an assumption of many of the neutral models (Kawecki 1994).

There is plenty of evidence that variation in resource abundance can favour specialization. Several examples of rapid niche evolution have occurred as introduced species have invaded new habitats and become abundant (Thompson 1998). In most cases this involves evolution of preference for the new habitat. One of these species is the Edith's Checkerspot butterfly Euphydryas editha (Figure 9.4), which has been the subject of a long-term study in and around the state of California (Singer etal. 1993). Two populations, 'Rabbit' and 'Schneider' have shown long-term changes in preference to lay eggs on novel host plants that have become more abundant in these habitats due to human activity. In both cases the preference differences were heritable, and at Schneider this preference was also associated with elevated growth rates on the preferred host (Singer et al. 1988). In addition to such genetic changes, which represent changes in the fundamental niche of a population, there is also plenty of evidence from this and other phytophagous insects that selection of host plants on which performance is poor depends on the abundance of hosts on which performance is high (Mayhew 1997).

Recently, Prinzing (2003) has shown that arthropod species living on tree trunks in Germany are more likely to occupy a large number of microhabitats if they have long generation times, move faster, and spent more of their life

Fig. 9.4 Edith's Checkerspot butterflies, Euphydryaseditha, mating. The butterfly has rapidly evolved to use new host plants as the abundance of hosts in the environment has shifted. Photo courtesy of Camille Parmesan.

on the trunks, giving them better search capabilities and opportunities. The trends might have come about via evolution of the fundamental niche or from plastic behaviour: both sets of theory make the same prediction and only genetic or behavioural studies could separate the two possibilities. Similar correlations have been found between the variability of a habitat and host plant range across herbivorous insect species (Brown and Southwood 1983).

The evidence that interspecific competition causes niche evolution comes from studies of well-defined morphological characters that are functionally linked to resource useā€”such as the beaks of Darwin's finches (see Figure 12.4). The majority of cases (there are at least 75) cite as evidence that the characters are more divergent when species are in sympatry than when they are allopatric. However, in most cases the case for character displacement by competition is far from complete. Perhaps the very best evidence comes from experimental work by Schluter (2001) on body size and shape of three-spined sticklebacks. In lakes where one species exists, the species is generalized and feeds both in the water column and on the lake bed. When two species coexist, however, they specialize with a thin species in the water column, and a larger rounded species on the lake bed (Figure 9.5).In an elegant series of experiments in artificial ponds, Schluter was able to show selection operating in the predicted direction when an intermediate form was made to coexist with specialists of either type: if it faced benthic competition, the more benthic-like of the intermediate

Fig. 9.5 Specialization under competition. The picture shows sticklebacks, Gasterosteus spp., from Enos lake on Vancouver Island, British Columbia, Canada. From top to bottom are shown: limnetic male, limnetic female, benthic female, benthic male. The benthic forms are larger and have deeper bodies than the limnetic forms. Photo courtesy of Dolph Schluter.

Fig. 9.5 Specialization under competition. The picture shows sticklebacks, Gasterosteus spp., from Enos lake on Vancouver Island, British Columbia, Canada. From top to bottom are shown: limnetic male, limnetic female, benthic female, benthic male. The benthic forms are larger and have deeper bodies than the limnetic forms. Photo courtesy of Dolph Schluter.

phenotypes had reduced success, thus selection favoured evolution towards a more pelagic existence, and the opposite occurred when a pelagic competitor was introduced. This disruptive selection is a likely mechanism of speciation (see Chapter 12).

In general, the list of character displacement examples, although admittedly preliminary, is remarkable for the absence of character convergence, for the fact that the species involved are closely related, and for the fact that carnivores are very well represented. These may, of course, be artefacts of observer and publication bias, but they may also have a biological basis, a possibility that is intriguing. Can other interspecific interactions cause specialization? There are some intriguing pieces of evidence. There is a cross-species association between host plant range and sequestration of nasty chemicals which serve as defences against predators. This is consistent with predation-driven selection of specialization (Dyer 1995).

There is little evidence that intraspecific competition within habitats can widen fundamental niche use, but certainly the realized niche tends to be greater when densities are high in a large number of species. For example, Pemphigus aphids preferentially occupy leaves of their host plants with a good vascular supply which they defend despotically. Subsequent aphids are faced with having to occupy less productive locations on the plant, and at higher densities the range of microhabitats used increases (Whitham 1980).Similar observations could be quoted for almost any territorial species.

In general then, a number of extrinsic ecological and intrinsic biological forces are both predicted to favour restriction of the fundamental and realized niches, and theory and evidence are coming together. It cannot be claimed that we have a good understanding of the relative importance of these forces yet, but a series of hypotheses with some support bodes well for the future.

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