Deserts are among the harshest environments on Earth. The multiple ages of different deserts and their global distribution provide a unique opportunity to study repeated adaptation at different timescales. Here, we summarize recent genomic research on the genetic mechanisms underlying desert adaptations in mammals. Several studies on different desert mammals show large overlap in functional classes of genes and pathways, consistent with the complexity and variety of phenotypes associated with desert adaptation to water and food scarcity and extreme temperatures. However, studies of desert adaptation are also challenged by a lack of accurate genotype-phenotype-environment maps. We encourage development of systems that facilitate functional analyses, but also acknowledge the need for more studies on a wider variety of desert mammals. Deserts: Natural Laboratories for Studies of AdaptationDeserts (see Glossary) are the driest environments on the planet and cover at least 33% of the land surface on Earth [1]. Although mainly characterized by aridity and water scarcity, deserts also experience daily and annual extreme thermal amplitudes, and intense UV radiation [1]. Deserts have long been seen as natural laboratories for investigating how biological design is challenged by different aspects of the environment, and how organisms have adapted to these challenges [1,2]. They also offer unique opportunities to study convergent evolution at distinct points in time and space, given their well-documented geological age and diverse geographical origins [1,2].

One of the most celebrated textbook examples of physiological adaptations to desert environments is the unique ability that desert mammals have to produce hyperosmotic urine. Commonly perceived as an adaptation mainly observed in small rodents, the extent to which urine‐concentrating ability has evolved independently in distinct mammalian lineages has not previously been assessed using modern phylogenetic approaches. We review urine‐concentrating ability data from the literature in 121 mammalian species with geographic ranges encompassing varying climatic conditions. We explicitly test the general hypothesis that desert‐dwelling mammals have evolved greater ability to concentrate urine than non‐desert species, controlling for body mass, phylogenetic affinity and other covariates. Ancestral state reconstruction across our dataset’s phylogeny shows that the ability to produce hyperosmotic urine, measured as maximum urine osmolality, has evolved convergently in mammalian species with geographic ranges characterised by low mean annual aridity index. Phylogenetic generalised least‐squares (PGLS) models show that the mean annual aridity index of a species’ geographic range largely predicts its urine‐concentrating ability, even when accounting for body mass differences, phylogenetic correlations, the specific condition under which urine osmolality was measured, the method used to measure urine osmolality, and the species’ diet. In contrast, we find much weaker correlations between mass‐adjusted basal metabolic rate and environmental variables when analysing 84 of the species included in the urine osmolality analysis. Taken together, our results not only show that desert mammals effectively concentrate more urine than non‐desert mammals, but further suggest that aridity is likely to have been one of the main selective pressures leading to increasing maximum urine‐concentrating ability and driving its repeated evolution in different desert mammalian lineages.

The evolutionary history of African ungulates has been explained largely in the light of Pleistocene climatic oscillations and the way these influenced the distribution of vegetation types, leading to range expansions and/or isolation in refugia. In contrast, comparatively fewer studies have addressed the continent's environmental heterogeneity and the role played by its geomorphological barriers. In this study, we performed a range-wide analysis of complete mitogenomes of sable antelope (Hippotragus niger) to explore how these different factors may have contributed as drivers of evolution in southcentral Africa. Our results supported two sympatric and deeply divergent mitochondrial lineages in west Tanzanian sables, which can be explained as the result of introgressive hybridization of a mitochondrial ghost lineage from an archaic, as-yet-undefined, congener. Phylogeographical subdivisions into three main lineages suggest that sable diversification may not have been driven solely by climatic events affecting populations differently across a continental scale. Often in interplay with climate, geomorphological features have also clearly shaped the species' patterns of vicariance, where the East Africa Rift System and the Eastern Arc Mountains acted as geological barriers. Subsequent splits among southern populations may be linked to rearrangements in the Zambezi system, possibly framing the most recent time when the river attained its current drainage profile. This work underlines how the use of comprehensive mitogenomic data sets on a model species with a wide geographical distribution can contribute to a much-enhanced understanding of environmental, geomorphological and evolutionary patterns in Africa throughout the Quaternary.

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