To test whether a lineage displays the previously mentioned characteristics of an adaptive radiation, studies must identify how species are related and whether a change in speciation rate is due to adaptive or non-adaptive forces. Finding evidence of phenomena indicative of an adaptive radiation, such as adaptive convergence and an increased rate of morphological evolution strengthens a case for adaptive radiation. Even in the absence of an extensive fossil record, comparative phylogenetics and population-level adaptation studies are useful for testing these hypotheses. Phylogenetic methods can test lineages for patterns consistent with adaptive radiations. However, they are less effective when sampling is incomplete and results are complicated by the difficulty in inferring a supported and robust phylogeny in recent and rapid radiations. Adaptation experiments strengthen phylogenetic results by testing how these patterns arose in contemporaneous species. By combining these two approaches future research will further illuminate the nature of this radiation.
While the study by Johnson et al. is by far the most rigorous phylogenetic treatment of the Ischnocera to date, denser taxon sampling is required to rigorously test whether replicate ecological radiations or vicariant diversification across hosts is more prevalent among avian lice. If sampling is increased in even a few feather lice lineages with multiple phenotypes or hosts, the pattern of speciation will be more evident. While the authors conclusively show that head, body and wing ecotypes have repeatedly evolved independently, quantifying how over- or under-dispersed divergent lineages are would identify how important replicate radiation was to avian lice diversification. Another avenue of research would be to test if certain ecotypes increase the chance of dispersal to new hosts, or constrain the ability to switch to other hosts. By more exhaustively sampling taxa from multiple host lineages it should be possible to explore whether a complete community assembles from a common ancestor every time a new host is colonized, and how long this process takes. This additional information will be important for evaluating and comparing the mode of diversification in the ischnoceran lice in light of other radiations.
It may also be possible to perform microevolutionary experiments within this group. Identifying what variation in head or body shape of the lice is present within populations and whether there is Mendelian or quantitative genetic variation underlying it would be useful. This information would aid the design of parasite transfer experiments to test if ecotypes will re-evolve in the presence of empty niches. For simplicity's sake, this would be especially informative if identified in young radiations endemic to a single host family, where shared variance may be due to identity by descent. The feather lice found on the Tinamous are exceedingly diverse and may present just such a system for future work [10
]. Clay [8
] noted (p. 143) 'The genera of the Heptapsogaster
-complex found on the Tinamidae are believed to be descended from a single ancestral stock on this group which has branched out to fill the different ecological niches on the body of the bird; the species now have a superficial resemblance to the unrelated occupants of similar niches on other orders.'