Replicate adaptive radiations occur when lineages repeatedly radiate and fill new but similar niches and converge phenotypically. While this is commonly seen in traditional island systems, it may also be present in host-parasite relationships, where hosts serve as islands. In a recent article in BMC Biology, Johnson and colleagues have produced the most extensive phylogeny of the avian lice (Ischnocera) to date, and find evidence for this pattern. This study opens the door to exploring adaptive radiations from a novel host-parasite perspective.

Adaptive radiations occur when lineages speciate ecologically, and are considered to be responsible for much of Earth's staggering diversity [1]. There are several classic examples of adaptive radiations: Darwin's finches in the Galapagos Islands, Anolis lizards in the Caribbean, Cichlids in East African lakes, and Drosophila, Tetragnatha spiders, and silverswords, all found in the Hawaiian Islands. Adaptive radiations are profoundly important to studies of evolutionary biology, and inform our understanding of how species and communities form. However, the definitions and subsequent identification of adaptive radiations have been variable [2,3]. Despite the slow progress towards a consensus definition for adaptive radiation, Johnson et al. [4] make a compelling case that feather lice (Phthiraptera: Ischnocera: Philopteridae) are an emerging model for the study of adaptive and convergent radiations.

While adaptive radiations have typically been reported from classic island settings such as those mentioned above, there are other lifestyles that mimic the conditions of islands, such as parasitism. Parasitism is a remarkably successful life history strategy and has evolved numerous times across the tree of life [5]. Metazoan parasites comprise nearly 50% of all animal species and dominate food web links [6]. Despite this ecological and evolutionary dominance, there has been rampant convergent evolution in morphology, ecology and host exploitation strategies across metazoan parasites [7]. This suggests that a common set of environmental factors has selected for common phenotypes, a hallmark of adaptive evolution. For example, Clay [8] pointed out that in feather lice of birds, the 'fundamental similarity of all the Philopteridae makes it difficult to recognize whether the external characters common to two groups are due to parallel or convergent evolution or to phylogenetic relationships.' Heretofore, there have been few robust tests for adaptive radiations within parasitic clades. Parasites are often soft-bodied, and their fossil record is sparse, hampering efforts to determine the mode and tempo of evolution in parasitic species empirically. Furthermore, it has been difficult to sample enough taxa or molecular characters to robustly reconstruct the evolutionary history of an entire parasitic lineage hypothesized to be an adaptive radiation. Johnson et al. have presented the most inclusive and robust phylogeny of the Ischnocera to date, representing a major step forward in understanding the evolution of this radiation.

Johnson and coauthors have shown that avian lice display one of the more common patterns associated with adaptive radiations: morphological convergence between species adapted to similar environments, or ecotypes. This is seen in many famous examples where diverse community assemblies are made of closely related, but morphologically divergent, species filling similar or the same niches. The authors show that the feather lice represent a lineage that has radiated, repeatedly, into different host niches and many different host lineages. They provide compelling evidence that the highly divergent ecotypes based on which feathers the lice exploit - body, head, wing or generalist - have evolved over and over again as new hosts with multiple vacant niches are colonized (Figure ​(Figure1a).1a). There is also a less commonly observed pattern of vicariant speciation, where species with similar ecologies diversify in host breadth, but not in ecology (Figure ​(Figure1b).1b). This could also be a form of adaptive radiation, though, as preening behaviors differ among bird species and may force the lice to adapt in other ways. The study by Johnson et al. provides compelling evidence that avian lice have adaptively radiated; however, further investigation will be needed to test this.

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.