Singing a different song: Incipient speciation in the Small Tree Finch

Two island populations are diverging genetically and acoustically.

Speciation – the origin of new species – is one of the best pieces of evidence for evolution. If species were created by some whimsical designer, we would expect to see clear boundaries between them. Instead, we observe populations at different stages of the speciation process. Some populations clearly belong to distinct species while others freely interbreed and cause headaches among taxonomists that try to pigeonhole all this diversity. In addition, the rate at which populations diverge often differs between traits. Genetic changes tend to accumulate over long periods of time whereas behavioral differences can quickly arise (see for example crossbills). Bird song is a beautiful example of a behavioral trait that can kickstart the speciation process. Several species of songbird learn their song from a tutor (mostly the father singing near the nest) and might make mistakes during the learning process. These mistakes can give rise to local dialects and potentially isolate neighboring populations because they do not recognize each others songs. A recent study in the Journal of Evolutionary Biology documented this process in the Small Tree Finch (Camarhynchus parvulus) on the Galapagos Islands.

Genes and Syllables

Diane Colombelli-Négrel and Sonia Kleindorfer focused on two populations of the Small Tree Finch on the islands of Santa Cruz and Floreana. Genetic analyses – based on microsatellites – indicated that these island populations are diverging. The researchers wondered whether these birds have also developed different songs. Studying more than 900 recordings from 112 males revealed 10 syllable types, of which only 4 were shared between the islands. Hence, the Small Tree Finches from Santa Cruz and Floreana are becoming genetically and acoustically different. But do these differences also affect their behavior? When a bird from Santa Cruz ends up on Floreana, will it be able to find a partner and mate?

The Small Tree Finches on Santa Cruz and Floreana differ genetically (left graph) and in the songs that they produce (right graph). Notice that each song is composed of one syllable type. From: Colombelli‐Négrel & Kleindorfer (2021).

Playback

To assess the behavioral response of the birds, the researchers turned to playback experiments. They tested the response of 91 males – 40 on Santa Cruz and 51 on Floreana – to songs from the two islands. These experiments showed that “males had a stronger response to the intruder song from their own geographical area.” This finding suggests that male Small Tree Finches are able to discriminate between local and foreign songs. Whether females are also capable of telling the difference between males from their own island and accidental visitors remains to be tested. If so, it would point to some level of reproductive isolation. Given enough time, we might end up with two distinct species of Small Tree Finch.

Playback experiments showed that male Small Tree Finches responded most to songs from their own island (in black) compared to songs from another island (in grey). From: Colombelli‐Négrel & Kleindorfer (2021).

References

Colombelli‐Négrel, D., & Kleindorfer, S. (2021). Behavioural response to songs between genetically diverged allopatric populations of Darwin’s small tree finch in the Galápagos. Journal of Evolutionary Biology, 34(5), 816-829.

Featured image: Small Tree Finch (Camarhynchus parvulus) © Mike Comber | Wikimedia Commons