[Seedeater bird songs]
[Music] Sheela Turbek: We spent 3 years in Iberá National Park in northeastern Argentina.
The capuchino seedeaters are distributed throughout South America,
and they don’t appear to have any clear geographic barriers
that prevent them from exchanging genes.
These various capuchino species are genetically very similar,
they breed in the same habitat, and they eat the same food.
And yet, the males of these various species differ in plumage coloration and song.
Narration: Turbek and her team focused on two types of capuchinos:
the Tawney-bellied seedeater and Iberá seedeater,
which was undiscovered until 2001.
These two species, along with 8 others,
split from a single common ancestor within the last million years.
This relatively recent, rapid expansion occurred even as they shared practically the same neighborhood.
Sheela Turbek: Normally, for speciation to occur
organisms have to be confined to different geographic regions
in order to accumulate genetic differences.
We’ve seen this process occur in other radiations such as Darwin’s finches,
where from a single common ancestor they formed many different species
that are confined to different islands in the Galapagos.
Capuchino seedeaters in general,
and the Iberá seedeater and Tawny-bellied seedeater
are confined to continental South America,
and they lack any obvious spatial or temporal barriers to reproduction.
It’s unclear what kind of barriers maintain these two species
in the same geographic area and keep them distinct from one another.
We searched for nests of the Iberá seedeater and the Tawny-bellied seedeater.
We marked individual birds with a unique combination of color bands,
and we collected blood samples from adults and nestlings of these two species.
[Seedeater bird sounds]
We presented territorial males of the Iberá seedeater and the Tawny-bellied seedeater
with painted mounts and song recordings that mimicked
their own species song and plumage,
as well as that of the other capuchino species,
and recorded their aggressive responses.
Both capuchino species responded most aggressively
to the song and plumage of their own species,
and less aggressively when the song or plumage belonged to the other species.
Which suggests that both of these traits are used
to distinguish one another in territorial interactions and avoid interbreeding.
We sequenced the DNA of the two species
to determine how they differ on a genetic level,
as well as analyzed patterns of mate choice
in order to assess whether these two capuchinos ever interbreed in the wild.
To our surprise, we found that even though these species breed side by side
and they share the same resources in Iberá National Park,
the Iberá seedeater and the Tawny-bellied seedeater
only ever mated with members of their own species.
Narrator: Since the two capuchino species are so genetically similar,
Turbek’s team essentially was searching for needles in a haystack.
Sheela Turbek: The Iberá seedeater and the Tawny-bellied seedeater
are almost identical on a genetic level.
They can only be distinguished by three tiny regions of their genome
that only contain 12 genes.
Three of these genes are known to be involved in plumage coloration in other bird species
and have likely contributed to the diverse plumage patterns
that we observe in this group.
Iberá seedeater likely formed as a result of past hybridization
between capuchino lineages and
the subsequent reshuffling of genetic variation into novel combinations
that encode differences in plumage patterning and potentially male song.
Differences in song and plumage pattern alone
may be sufficient to maintain species boundaries extremely early in divergence.
[Tawny-bellied seedeater song]
Narrator: Work remains to determine if capuchino songs diverged genetically,
or if their differences are entirely learned.
[Iberá seedeater song]
It’s also unknown whether the distinct capuchino genes
associated with plumage are actively expressed.
For now, the Iberá and Tawny-bellied seedeaters will likely continue mating within their own species,
despite sharing the same turf and almost all the same genes.
As the saying goes, birds of a feather flock together.
[Music]