How important is learning in generating biological diversity?

As they prepare to head for Zambia, Dr Claire Spottiswoode and her research team explain how a group of African finches could help biologists learn more about the role of learned behaviours in promoting the evolution of novel adaptations and even new species.

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A parasitised brood of Jameson’s Firefinches. Far right: a parasitic Purple Indigobird, perfectly mimicking their mouth markings.

A remarkable evolutionary story is unfolding in the woodlands and savannahs of Africa. Announcing its presence from the top of a prominent tree, a glossy black finch sings incessantly throughout the day. Interspersed amongst its chattering and scratchy warbles are a few clearer notes, which pioneering ornithologists noticed sounded identical to the vocalisations of a local species of firefinch. These field observations precipitated an illuminating evolutionary case study that has broadened our views on how biological diversity is generated. With the support of the Leverhulme Trust, we are hoping to add a new chapter to this fascinating story.

The singing finch is a member of the genus Vidua—the indigobirds and whydahs—which, like cuckoos, are cheats that lay their eggs parasitically in other birds’ nests. Unlike cuckoos, Vidua parasitic chicks imprint on their host species’ songs and calls. As adults, the Vidua males mimic the vocalisations of their host, which the females are attracted to. These learned behaviours mean that females will only mate with males raised by the same host species, and will preferentially lay their own eggs in that host’s nests. Consequently, a female Vidua accidentally laying her eggs in the nest of a new host species can initiate a new genetically isolated lineage of Vidua, henceforth tied to that host. In an evolutionary blink of the eye, the new Vidua lineage is already well on the path to forming a new species. Such speciation via host-switching has occurred many times among the Vidua finches in the very recent evolutionary past, giving us a unique opportunity to study speciation in action.

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Purple indigobird chick

However, a major puzzle remains unsolved. Nestlings of each Vidua species perfectly mimic their host species’ elaborate and unique pattern of mouth markings (see the photo above), in a genetically-determined adaptation that aids the parasitic chick’s survival in the host nest. Yet when one of these parasitic lineages switches host, the mouths of the Vidua chicks are glaringly mismatched to those of the host. How then can new Vidua lineages successfully persist in novel host environments long enough for mimicry to evolve through natural selection?

Funding from the Leverhulme Trust will allow us to address this question through four seasons of field and aviary experiments in Africa, enabling a new direction for our long-term research on brood parasitic birds in Zambia. Our hypothesis is that learned behaviours facilitate Vidua finches’ exploitation of new host environments, overriding short-term deficiencies in mimicry. Specifically, we will test whether Vidua nestlings can alter their begging behaviour to solicit food from novel hosts, and whether hosts themselves become less discriminating against foreign chicks under certain conditions. By providing a potential window for colonisation, such behaviour could be pivotal in allowing genetic adaptation of mouth mimicry to evolve.

We will test whether a changed environment (such as a new host species) can induce novel characteristics (such as different begging behaviours), which are immediately adaptive, and which then promote selection for genetic change. This wider hypothesis has recently been gaining ground in evolutionary theory, and runs counter to the traditional view that evolutionary change always begins with genetic mutation rather than with environmentally-induced variation. We hope that Vidua finches in the African bush will help us to carry out a rare empirical test of this provocative idea in wild populations.

Dr Claire Spottiswoode, Professor Rebecca Kilner, and Gabriel Jamie
University of Cambridge.