Journal of evolutionary biology
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Comparative Study
Differential rates of morphological divergence in birds.
There are more small-bodied bird species than there are large-bodied, even on a logarithmic scale. In birds this pattern, which is also found in other higher taxa, appears not to be due to neutral evolution. It has often been suggested that the skew of body size frequency distributions is the result of a relationship between body size and the net rate of speciation, but phylogenetic analyses so far have rejected the hypothesis that small-bodied species are subject to higher net rates of speciation. On the contrary, we show that there exists a relationship between body size and its own evolutionary variability: avian families of small body size show less interspecific variation in body size than large-bodied families of similar age and species richness.
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Although very common under natural conditions, the consequences of multiple enemies (parasites, predators, herbivores, or even 'chemical' enemies like insecticides) on investment in defence has scarcely been investigated. In this paper, we present a simple model of the joint evolution of two defences targeted against two enemies. We illustrate how the respective level of each defence can be influenced by the presence of the two enemies. ⋯ It is generally admitted that increasing the encounter rate with a second natural enemy can decrease investment in defence against a first enemy, but our results indicate that it may sometimes favour resistance against the first enemy. Moreover, we illustrate that the global defence against one enemy can be lower when only this enemy is present: this has important implications for experimental measures of resistance, and for organisms that invade an area with less enemies or whose community of enemies is reduced. We discuss possible implications of the existence of multiple enemies for conservation biology, biological control and chemical control.