Saturday, 27 March 2021

Evolution News - Rapid Evolution by Genetic Shuffling

Iberá Seedeater, Sporophila iberaensis
Endangered songbird challenging assumptions about evolution | Colorado Arts and Sciences Magazine | University of Colorado Boulder

A group of researchers led by Sheela P. Turbek of the Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO, USA have identified a rare form of evolution in a group of birds known collectively as the southern capuchino seedeaters. The process, known as genetic shuffling, had only been observed once before, in African in the cichlid fish of Lake Victoria and is believed to be behind the recent evolution of a new species, the Iberá Seedeater, Sporophila iberaensis.

In addition to speciation by gene shuffling - where a new species arises by recombining existing genes in a unique combination, this species of bird also illustrates how speciation can be maintained by pre-zygotic barriers to hybridization, that arise through natural selection because hybrids with intermediate characteristics may be at a disadvantage, having intermediate specialisations.

A Tawny-Bellied Seedeater (Sporophila hypoxantha) within the habitat it shares with the Iberá Seedeater
Photo: Sheela Turbek
I wrote about pre-zygotic barriers to hybridization in geese and how this mechanism can account for the myriad of different plumages and songs used in mating rituals in so many different bird species, some time ago. Basically, mating displays and rituals act to prevent hybridizations which would be genetically possible if the related species mated. Barriers that prevent the formation of zygotes from sperms and eggs of different species, even if these would be possible and viable, are known as pre-zygotic barriers.

This is how the new species of capucino seedeaters is maintained and not absorbed into the general population of Tawney bellied seedeaters that occupies the same environment. In fact, the Iberá Seedeater ocupies the same environment as six other closely related species of capuchinos, all of which are capable of hybridizing with it. The key to its success as a species is down to the fact that the reshuffled genes have chanced upon a unique pattern of male plumage, which has then been exaggerated by assortative mating and possibly female sex selection.

Cay Leytham-Powell in the Colorado University's arts and science magazine explains:
Not all species may travel the same path to existence, at least according to new findings from the University of Colorado Boulder and collaborators.

This new research, out today in Science, looked at a newly discovered, endangered songbird located only in South America—the Iberá Seedeater—and found that this bird followed a very rare evolutionary path to come into existence at a much faster pace than the grand majority of species.

One of the aspects of this paper that makes it so cool is that we were able to address this question of how the Iberá Seedeaters formed from multiple different perspectives. Not only did we collect on-the-ground data on who mated with one another and the identity of their offspring, but we also generated genomic data to examine how similar these two species are on a genetic level. We then zoomed out further to look at where the Iberá Seedeater fits in the context of the broader capuchino group.

Many studies will address one of these aspects or questions but not combine all of these different pieces of information into a single study.

Sheela Turbek, Lead author.
Graduate student in ecology and evolutionary biology (EBIO)
Olorado University, Boulder, CO, USA.
By comparing this bird to a closely related neighbor (the Tawny-Bellied Seedeater) in the same group (the southern capuchino seedeaters), the researchers determined that genetic shuffling of existing variations, rather than new random mutations, brought this species into existence—and their own behaviors are keeping them apart.

This species is one of only two known examples across the globe to have traveled this path, challenging the typical assumptions of how new species form.

“One of the aspects of this paper that makes it so cool is that we were able to address this question of how the Iberá Seedeaters formed from multiple different perspectives,” said Sheela Turbek, a graduate student in ecology and evolutionary biology (EBIO) at CU Boulder and the study’s lead author.

“Not only did we collect on-the-ground data on who mated with one another and the identity of their offspring, but we also generated genomic data to examine how similar these two species are on a genetic level. We then zoomed out further to look at where the Iberá Seedeater fits in the context of the broader capuchino group.”

“Many studies will address one of these aspects or questions but not combine all of these different pieces of information into a single study.”

The field work involved in collecting the assortative mating and behavioral data is extraordinarily hard, which is why these kinds of datasets rarely exist. This study and publication are a testament to Sheela's skill and hard work in the field.

Scott Taylor, Co-author
Assistant professor in EBIO
Colorado UNiversity, Boulder, Co, USA
The southern capuchino seedeaters are a group of recently evolved songbirds found throughout South America that is branching rapidly, with many of its species in the early stages of evolution. This family is best known for the dramatic variation with the males in terms of songs and plumage color, while the females are largely indistinguishable even to the most familiar researchers.

The Iberá Seedeater, the most recent member of this family, was first discovered in the remote, swampy grasslands of Iberá National Park in northern Argentina by study co-authors Adrián S. Di Giacomo and Cecilia Kopuchian from Centro de Ecología Aplicada del Litoral, Argentina, in 2001, and then described in scientific literature in 2016.

In that national park, though, are six other closely related species of capuchinos, including the Tawny-Bellied Seedeater, that breed closely beside each other. These species, despite occupying the same environment and eating the same food, rarely interbreed.

And so, researchers wondered why—and how—the Iberá Seedeater even came to be.

They explored these questions in two ways: First, they looked at how this new species may have formed by examining the ways in which its DNA differs from the Tawny-Bellied Seedeater, and second, looking at what mechanisms might be preventing it from interbreeding with the other species that occur in the park.

To do that, Turbek went down to Argentina for the breeding season for three years, staying two and a half to three months at a time, searching for and monitoring nests, collecting blood samples from adults and nestlings, and then, in the final year, performed a behavioral experiment to see whether plumage or song played a role in terms of species recognition.

“The field work involved in collecting the assortative mating and behavioral data is extraordinarily hard, which is why these kinds of datasets rarely exist. This study and publication are a testament to Sheela's skill and hard work in the field,” said Scott Taylor, an assistant professor in EBIO at CU Boulder, an author on the paper, and Turbek’s advisor.

This is a really beautiful story about a process that we have never seen in quite this way before. The classic and most common evolutionary model for new species is the accumulation of genetic mutations when those species are separated by a geographic barrier over perhaps millions of years. But here we found that genetic shuffling can happen quickly and without geographical isolation. It’s almost like 'instant speciation.'

Irby Lovette, Co-author.
Director of the Fuller Evolutionary Biology Program
Cornell Laboratory of Ornithology.
What they found is that the two birds are closely related genetically, only distinguishable by the genes involved in plumage coloration. As well, they found that the males responded most aggressively to songs and plumage variations aligning with their own species.

This all means that the species could very well reproduce and hybridize—they just choose not to, therefore reinforcing their own reproductive barriers.

On a broader level, though, when comparing the Iberá Seedeater to other capuchino species, the researchers found that the Iberá Seedeater shares genomic variants with other capuchinos in these regions, but the variants have been shuffled to form a unique combination, which, the researchers argue, could be an evolutionary shortcut that most likely underlies much of the diversity among the different subspecies of this family.

"This is a really beautiful story about a process that we have never seen in quite this way before," says co-author Irby Lovette, director of the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology.

"The classic and most common evolutionary model for new species is the accumulation of genetic mutations when those species are separated by a geographic barrier over perhaps millions of years. But here we found that genetic shuffling can happen quickly and without geographical isolation. It’s almost like 'instant speciation.'"
The group's findings were published today in Science:

Structured Abstract



INTRODUCTION


Organisms in the early stages of speciation provide an opportunity to understand the processes that govern reproductive isolation between taxa. Ecological or behavioral mechanisms can serve as powerful barriers to the interbreeding of co-occurring species at the onset of their divergence. Tracking mating decisions within wild populations early in speciation can improve our understanding of how behavioral isolation promotes divergence.

RATIONALE


The southern capuchino seedeaters (Sporophila) are one of the most rapid avian radiations, showing remarkably low ecological and genomic divergence. We took advantage of the recent discovery of a capuchino species, the Iberá Seedeater (S. iberaensis), to study the origin and importance of pre-mating barriers early in speciation. By combining genomic and behavioral analyses, we examined (i) the role of assortative mating in the maintenance of species boundaries, (ii) the phenotypic traits underlying species recognition, (iii) the genomic basis of such traits, and (iv) the origin of these genomic variants.

RESULTS


Sporophila iberaensis was first observed in 2001 and co-occurs with S. hypoxantha throughout its main breeding location in the northern portion of the Iberá wetlands of Argentina. Across two breeding seasons, we located nests and collected genomic samples from both species. We found extremely low genome-wide differentiation, with the exception of three narrow regions located on different chromosomes. These regions contained 12 genes, three of which are involved in plumage coloration (TYRP1, OCA2, and HERC2). Sporophila hypoxantha and S. iberaensis males differ in coloration and song, but females are indistinguishable in coloration across the avian visual spectrum. We therefore used genomic data to quantify assortative mating. Each female’s species-specific genotype always matched the genotype of her mate, demonstrating strong assortative mating despite these two species holding neighboring breeding territories, breeding synchronously, and foraging together on the same grasses. We tested the importance of divergent plumage patterning and song in species recognition and pre-mating isolation through playback experiments in the field. We presented territorial males with combinations of conspecific and heterospecific song and plumage, and assessed their aggressive behavioral responses. Each species responded most aggressively to conspecific song and plumage, confirming that both traits are used to recognize sexual competitors. Finally, we investigated the origin of the novel S. iberaensis plumage phenotype by examining genomic differentiation across the broader capuchino radiation. Although multiple species shared variants with S. iberaensis in the areas of elevated differentiation, the specific combination of these variants across the divergent regions distinguished S. iberaensis from all other capuchinos.

CONCLUSION


Our findings point to pre-mating isolation through assortative mate choice, based on both plumage coloration and song, as a primary mechanism promoting divergence between these co-occurring capuchino species. Although the ultimate fate of the incipient S. iberaensis species remains uncertain, our findings illustrate how lineages can form and quickly become reproductively isolated from co-occurring, syntopic species. Our results further suggest that the reshuffling of existing genetic variation can generate novel phenotypes that are then targeted by sexual selection. Assortative mating based on these traits may maintain species boundaries early in speciation while subsequent reproductive barriers accumulate.


Abstract


Behavioral isolation can catalyze speciation and permit the slow accumulation of additional reproductive barriers between co-occurring organisms. We illustrate how this process occurs by examining the genomic and behavioral bases of pre-mating isolation between two bird species (Sporophila hypoxantha and the recently discovered S. iberaensis) that belong to the southern capuchino seedeaters, a recent, rapid radiation characterized by variation in male plumage coloration and song. Although these two species co-occur without obvious ecological barriers to reproduction, we document behaviors indicating species recognition by song and plumage traits and strong assortative mating associated with genomic regions underlying male plumage patterning. Plumage differentiation likely originated through the reassembly of standing genetic variation, indicating how novel sexual signals may quickly arise and maintain species boundaries.

Creationists who have had the courage to read this far might like to note how, contrary to what the professional frauds in the Creation Industry tell them, the Theory of Evolution is central to this paper and the scientific understanding of how this speciation occurred. Never, anywhere in the paper, is there any suggestion that a magic man in the sky could be responsible for this speciation. Creationism, and the childish notion of intelligent [sic] design as the mechanism of speciation, has made absolutely no inroads into serious biological science at all.

The interesting thing from a biological point of view is that this was a rare mechanism by which species can occur. Since most of the genes involved related to male plumage and given the role plumage plays in the courtship and mating rituals of many birds, it is not surprising that male behaviour is the the main pre-zygotic barrier to hybridization and the means by which speciation is maintined.


Thank you for sharing!









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