Tuesday, 16 January 2018

Birds of a Feather - New Species Evolve

Large cactus finch (Geospiza conirostris)
Study of Darwin's finches reveals that new species can develop in as little as two generations

Birds of a feather: U of T researchers discover Amazon bird to be rare hybrid species

This week we have a couple of examples of new species arising - that thing that creationists insist can't happen. The evolutionary process that created them was hybridisation which occurs very much faster than the more usual (at least in vertebrates - speciation by hybridisation is common in plants) divergence by a combination of genetic drift and natural selection. And in these cases, the species are birds.

The first is from those devil figures for creationists - Darwin's finches or Galapagos finches. This was originally reported and commented extensively upon last November and revealed an observed incidence of apparent speciation when a vagrant male appeared on the island of Daphne Major. This was much larger and had a larger, more robust beak than any of the other three species present on the island. It also sang with a different song.

This individual proceeded to mate with a female medium ground finch (Geospiz fortis) giving rise to a new lineage. The vagrant male was later identified as a large cactus finch (Geospiza conirostris) from Española island, about 62 miles in the southeast.

The offspring inherited a large beak and a 'strange' song quite different to those of the 'native' species and so failed to attract a mate from the local species, leaving no alternative but to mate within their own lineage and so establishing breeding isolation.

What is not known from this study is whether the hybrid lineage could and would have interbred with the founder male's species since they never came into contact. It is quite possible that this would have lead to eventual breeding out of the hybrid features, so it's not possible to say that this is a real new species or one produced by population isolation.

Golden-crowned manakin (Lepidothrix vilasboasi)
A better case for a real new species was reported a few days ago by a team from Toronto University from amongst the manakins of Brazil. Through a series of genetic and other test, they have shown that the golden-crowned manakin (Lepidothrix vilasboasi) is a hybrid between the snow-capped (Lepidothrix nattereri) and opal-crowned (Lepidothrix iris) manakins.

While hybrid plant species are very common, hybrid species among vertebrates are exceedingly rare... A hybrid species forms when two parental species mate to produce a hybrid population, which then stops being able to freely interbreed with the parental species. In this case, the two parents are the snow-capped manakin, named for its bright, snowy-white crown feathers, and the opal-crowned manakin, named for its brilliant, iridescent crown feathers.

Jason Weir, senior author, Associate professor, Department of Biological Sciences, University of Toronto.

Their findings are published in PNAS:

Significance
Hybridization between species can produce reproductively isolated lineages by combining parental genotypes in novel ways. Here, we used thousands of genetic markers to demonstrate that the recently rediscovered golden-crowned manakin represents an avian hybrid species from the Amazon basin. This hybrid species has a unique golden-colored crown patch used for display, which differs from the brilliant white coloration of the parental species. We used microscopy to demonstrate that, despite its unique coloration, the crown has intermediate color-producing morphological features at the nanoscale. We propose that these intermediate features disrupted the high reflectivity of the parental species, resulting in a dull hybrid population. Selection then sequestered carotenoids to the crown to compensate for its low reflectivity.

Abstract
Hybrid speciation is rare in vertebrates, and reproductive isolation arising from hybridization is infrequently demonstrated. Here, we present evidence supporting a hybrid-speciation event involving the genetic admixture of the snow-capped (Lepidothrix nattereri) and opal-crowned (Lepidothrix iris) manakins of the Amazon basin, leading to the formation of the hybrid species, the golden-crowned manakin (Lepidothrix vilasboasi). We used a genome-wide SNP dataset together with analysis of admixture, population structure, and coalescent modeling to demonstrate that the golden-crowned manakin is genetically an admixture of these species and does not represent a hybrid zone but instead formed through ancient genetic admixture. We used spectrophotometry to quantify the coloration of the species-specific male crown patches. Crown patches are highly reflective white (snow-capped manakin) or iridescent whitish-blue to pink (opal-crowned manakin) in parental species but are a much less reflective yellow in the hybrid species. The brilliant coloration of the parental species results from nanostructural organization of the keratin matrix feather barbs of the crown. However, using electron microscopy, we demonstrate that the structural organization of this matrix is different in the two parental species and that the hybrid species is intermediate. The intermediate nature of the crown barbs, resulting from past admixture appears to have rendered a duller structural coloration. To compensate for reduced brightness, selection apparently resulted in extensive thickening of the carotenoid-laden barb cortex, producing the yellow crown coloration. The evolution of this unique crown-color signal likely culminated in premating isolation of the hybrid species from both parental species.


Using coalescent modelling, the team have shown that this new species diverged from the two parent species about 300,000 years ago. By Amazonian standards, this is very recent, most species having diverged from their most recent relative around 1.5 to 4 million years ago.

A single crown feather from a golden-crowned manakin
Photo credit: Don Campbell
By examining the crown feathers, which are duller in the golden-crowned manakin than in either of its parent species, the team showed that this resulted from a mixture of the two different keratin structures found in the parents. These two species have highly reflective keratin which helps males display in the darkness of the Amazon forest with either a brilliant white or iridescent yellow crown. By contrast, the golden-crowned manakin has much less reflective crown plumage. It is believed that the original hybrid probably had a grey crown which has since evolved a yellow colour by female sex selection.

The Amazonian manakins each occupy a territory separated from others by wide rivers which they are reluctant to cross so maintaining genetic isolation. It is believed that the small founding population soon became isolated due to climate change and forest contraction as rivers widened. Had they not done so, it is likely that they would not have remained a separate species but would have either died out due to competition or by being 'bred to death' by one or other of the parent species.

So here we have one almost certain example of the recent evolution of a new species by hybridisation and probably two. No doubt creationists will still insist their dogmatic insistence that this can't happen is the best explanation of the observable evidence of it happening.

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