Narrow specialization should be necessary for the survival of the species but at the same time it may cause its extinction
According to Van Valen’s Red Queen hypothesis (Van Valen 1973), every species has to run in evolutionary races with connected species as fast as it can to avoid its extinction. This leads to kind of „arms races“ (Dawkins and Krebs 1979): The effect of every advantageous adaptation is subsequently degraded by contrary adaptation developed by connected species. Narrow resource specialization seems to be necessary for pace keeping in these arms races, even though it is traditionally viewed as evolutionary dead end (Cope 1903). Specialists are favored over generalist because they don’t need to face the problems of antagonistic adaptations to different resources (Futuyma and Moreno 1988). For this reason, they can respond to contrary adaptations of other species more effectively. There is no place for generalist in this system. Nevertheless they still exist. Their advantage could be greater resource availability and therefore, they spend shorter time by searching of an appropriate resource (Jaenike 1990; Begon et al. 2006). The principal question is, whether this generalism is just a temporary condition, which inevitably results to the specialization, or an evolutionary stable strategy.
Cuckoo bees can tell us new facts about evolution of resource specialization
There are relatively lot of studies focused on evolution of resource specialization of herbivorous insect or typical parasites (living on or in the host, on which they feed, harm him, but don’t kill him (Poulin 2007)). However it is problematic to generalize their results onto another ecological relationships. We extend the research to cuckoo bees, which can be another appropriate model group for studying evolution of resource specialization. Most of them are specialist in one host or few closely related hosts. Nevertheless, generalist species with many hosts from different genera and families are also know in cuckoo bees (Bogusch 2003; Bogusch et al. 2006). Genus Sphecodes, on which we are focused, is appropriate model due to relative good knowledge of their hosts and occurrence of several types of host specialization (Bogusch and Straka, in press).
Generalism is highly derived strategy and host switches commonly occur in cuckoo bees
Using the combined data set of five genes we reconstructed a phylogeny of tribe Sphecodini. Subsequently we mapped two characters onto phylogenetic tree of genus Sphecodes: ancestral host specifity (specialist/generalist) and ancestral hosts.
According our result it is clear that the most recent common ancestor of genus Sphecodes was specialist and also vast majority of Sphecodes ancestors were specialist to (Fig. 1). So it seems that generalist species arose from specialist ancestors only recently. How is it possible that they are able to compete with specialist? Solution may be the specialization at the individual level. This strategy was proved in two generalist species S. monilicornis and S. ephippius (Bogusch et al. 2006) and it is possible that also other generalist species are specialist at the individual level. Therefore, generalism is in this case highly derived and stable strategy.
The results also show that host switches commonly occur and even specialist can switch to another host (Fig. 2). Hence in cuckoo bees specialization isn’t the evolutionary dead end in cuckoo bees.
Figure 1: Character state reconstruction of host specifity of cuckoo bees genus Sphecodes. The big pie charts show posterior probabilities obtained by Bayesian analysis using the Distribution II (main analysis). The smaller pie charts show the results (posterior probabilities in Bayesian analyses or possible character states in Maximum Parsimony analyses) of other analyses if there have a different meaning from the main analysis.
B1: Bayesian analysis using the Distribution I, MP1: Maximum Parsimony analysis using the Distribution I, B3: Bayesian analysis using the Distribution III, MP3: Maximum Parsimony analysis using the Distribution III.
Figure 2: Character state reconstruction of ancestral hosts of cuckoo bees of genus Sphecodes. The possible hosts obtained by Bayesian analysis are shown above the nodes. The possible hosts obtained by Maximum Parsimony analysis are shown below the nodes. Black spots indicate host switch.
Begon, M., C. R. Townsend, and J. L. Harper. 2006. Ecology: from individuals to ecosystems. Blackwell Publishing, Oxford.
Bogusch, P. 2003. Biologie vybraných druhů kleptoparazitických včel. 96pp. Department of Zoology. Charles University in Prague, Prague.
Bogusch, P., L. Kratochvil, and J. Straka. 2006. Generalist cuckoo bees (Hymenoptera : Apoidea : Sphecodes) are species-specialist at the individual level. Behavioral Ecology and Sociobiology 60:422-429.
Bogusch, P., and J. Straka. in press. Review and identification key to the Sphecodes (Hymenoptera: Halictidae) cuckoo bees of central Europe. Zootaxa.
Cope, E. D. 1903. The primary factors of organic evolution. Open Court Publishing Company, Chicago.
Dawkins, R., and J. R. Krebs. 1979. Arms Races between and within Species. Proceedings of the Royal Society of London Series B-Biological Sciences 205:489-511.
Futuyma, D. J., and G. Moreno. 1988. The evolution of ecological specialization. Annual Review of Ecology and Systematics 19:207-233.
Jaenike, J. 1990. Host Specialization in Phytophagous Insects. Annual Review of Ecology and Systematics 21:243-273.
Poulin, R. 2007. Evolutionary Ecology of Parasites. Princeton University Press.
Van Valen, L. 1973. A new evolutionary law. Evolutionary Theory 1:1-30.