Sexual colour dimorphism drives evolutionary radiations in African reed frogs

Interested in evolutionary radiations and colourful African frogs? Then take a look at this important new paper published by Dan Portik and collaborators from 16 different countries across Africa, Europe and the US. 

Sexual dichromatism (when females and males show striking colour differences) is potentially thought to drive rapid evolutionary radiations in some groups. Though it is quite rare in amphibians, present in only around 2% of species – in Hyperoliidae (African reed frogs) it is particularly prevalent. Unlike most other vertebrates, hyperoliid females tend to be the ones showing off their ornate colours, whereas the males stay as low-key greens or browns. Until now, we have been unsure of the origins of this very conspicuous colour dimorphism, or if it is even associated with rapid bursts of evolution in frogs.

Illustration of (a) several Hyperolius species in the predominately sexually dichromatic Clade 1 and (b) several Hyperolius species in Clade 2 that exhibit multiple transitions to secondary monochromatism. Males are positioned in the top rows, with females below, and the phylogenetic relationships among species are depicted (though not all taxa have been included) Photo credits: Daniel Portik, Jos Kielgast, Bryan Stuart, Andrew Stanbridge.

Using years of collaborative Pan-African field sampling, Dan developed over 1000 sequence capture loci for a large proportion of the African Hyperoliidae (254 samples from 12 genera) and built a new high-resolution phylogeny to investigate the relationships between species. He then used this to show that dichromatism evolved once, followed by multiple reversals to monochromatism in some species, and demonstrated that increased net diversification rates are shown in sexually dichromatic species – on average twice the rates shown by monochromatic species!

Though sexual dichromatism is clearly linked to rapid evolution in hyperoliids, its exact function remains unclear, and so it presents a very interesting and novel model system for future research into how sexual selection and natural selection may interact across evolutionary timescales.

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