Nonadaptive radiation: Pervasive diet specialization by drift in scale insects?

Area of Science:

• Evolutionary Biology
• Ecology
• Entomology

Background:

• Herbivorous insects constitute over half of metazoan species, yet their high diversity remains a puzzle.
• Adaptive host specialization is a common hypothesis for insect diversification, but nonadaptive specialization is also possible.
• Scale insects offer a unique model due to apparent lack of host-use trade-offs and potential costs of specificity via wind dispersal.

Purpose of the Study:

• To test the hypothesis that narrow host ranges in herbivorous insects can arise nonadaptively.
• To investigate the relationship between host range and diversification rates in scale insects.
• To explore the evolutionary dynamics of diet breadth in insect diversification.

Main Methods:

• Comparative phylogenetic analyses were conducted on scale insects (Hemiptera: Coccoidea).
• Diet breadth evolution and diversification rates were inferred using phylogenetic methods.
• Results were compared with existing data from butterflies (Lepidoptera).

Main Results:

• A strong positive relationship was found between host range and diversification rate in scale insects.
• Cladogenetic changes in diet breadth showed marked asymmetry.
• These findings contrast with butterflies, where a negative relationship between diet breadth and diversification exists.

Conclusions:

• Pervasive nonadaptive host specialization, driven by drift and extinction in small populations, may explain scale insect diversification.
• Nonadaptive processes can significantly influence diet-breadth evolution and taxonomic diversification in herbivorous insects.
• Differences in host specificity costs/benefits between insect groups like scale insects and butterflies can lead to contrasting diversification patterns.