Specialization and generalization in the diversification of phytophagous insects: tests of the musical chairs and oscillation hypotheses

Abstract

Evolutionary biologists have often assumed that ecological generalism comes at the expense of less intense exploitation of specific resources and that this trade-off will promote the evolution of ecologically specialized daughter species. Using a phylogenetic comparative approach with butterflies as a model system, we test hypotheses that incorporate changes in niche breadth and location into explanations of the taxonomic diversification of insect herbivores. Specifically, we compare the oscillation hypothesis, where speciation is driven by host-plant generalists giving rise to specialist daughter species, to the musical chairs hypothesis, where speciation is driven by host-plant switching, without changes in niche breadth. Contrary to the predictions of the oscillation hypothesis, we recover a negative relationship between host-plant breadth and diversification rate and find that changes in host breadth are seldom coupled to speciation events. By contrast, we present evidence for a positive relationship between rates of host switching and butterfly diversification, consonant with the musical chairs hypothesis. These results suggest that the costs of trophic generalism in plant-feeding insects may have been overvalued and that transitions from generalists to ecological specialists may not be an important driver of speciation in general.

Coupling of diversification and pH adaptation during the evolution of terrestrial Thaumarchaeota

“…3), our data lend support to this idea in terms of terrestrial niches associated with variable ecological characteristics. Another interesting possibility is that high ongoing diversification rates in Thaumarchaeota are associated with high levels of niche switching without extensive specialization, which has been suggested for certain eukaryotes (45)(46)(47), or high levels of energy source switching by assimilating different substrates such as inorganic or organic nitrogen compounds (48,49). Our work therefore demonstrates how approaches that combine contemporary molecular and environmental data can be used to compare evolutionary processes between prokaryotes and eukaryotes, and raises important questions about the similarities and differences in the long-term drivers of diversification across the tree of life.…”

Adaptation to different host plant ages facilitates insect divergence without a host shift

“…Shifts to novel host plants have been identified as a critical factor in speciation among phytophagous insects [2,4,8,[10][11][12][15][16][17][18][19][20]], yet about half of speciation events occur without a host shift [22,23]. Non-ecological processes such as genetic drift in allopatry could easily explain diversification with host conservatism, but examples particularly from galling insects suggest that ecological mechanisms could also be acting [15,[25][26][27].…”

Evolutionary ecology of specialization: insights from phylogenetic analysis

“…Hardy & Otto [38] raise the interesting point that the question of whether specialization influences speciation depends on how specialization is defined: 'One grey area is how to define the relevant niche with respect to diversification, as generalists along some axes (e.g. resource use) may be specialists along others (e.g.…”

“…Specialization and generalization in the diversification of lepidopterans were examined for evidence of the musical chairs versus the oscillation hypotheses [38]. Following a 'musical chairs' model we might see that specialist clades were more often transitioning between hosts, but remaining specialized, whereas in the 'oscillation' model we would predict that niche-breadth shifts (e.g.…”

“…First, transition from specialization to generalization is possible and even prevalent in certain ecological contexts. Second, specialization in traits related to foraging or reproduction can be associated with increased evolutionary success of some specialist clades, especially in specialist clades that experience greater transition rates to different specialist states ('musical chairs' pattern described above [38]). Specialization also need not by itself be the driver of speciation.…”