Creationism in Crisis - How a 'Macro-Evolutionary' Change can be Produced by Changes to a Few Genes

Creationism in Crisis

How a 'Macro-Evolutionary' Change can be Produced by Changes to a Few Genes.

A transient change in expression of one gene (Shh) can produce a cascade of developmental events leading to the formation of feathers instead of scales

© UNIGE / Cooper & Milinkovitch

Sablepoot bantams

Scales or feathers? It all comes down to a few genes - Medias - UNIGE

Lurk a while in any Evolution vs Creationism group in the social media and you can guarantee a creationist will try to argue that a given example of evolution which they have probably just deamnded be provided, and which conforms to the scientific definition of evolution - any change in allele frequency over time - is not real evolution because it wasn't 'macro-evolution' which they will define as a change in 'kind' or the evolution of a new species, even the evolution of new structures - whichever definition they think stands the best chance of winning.

I've been at this for long enough to remember how, when the early internet 'bulletin boards' on CompuServe had morphed into thriving debate 'forums', creationists would simply argue that there was no such thing as evolution, full stop! It simply never happened; not in the slightest. No Way! Everything was created exactly as it is today during 'Creation Week' a few thousand years ago! Any evidence to the contrary was a forgery by evil scientists or had been planted by Satan.

Then, in the face of so much evidence, particularly the very large number of living and extinct species and the impossibility of them all being collected together in a few days, herded onto a wooden boat and surviving for a year, they began to concede that there had indeed been evolution, but only limited to evolution withing species, and evolution moreover at a phenomenal rate, far exceeding anthing proposed by biologists, so as to produce the millions of species from a few 'kinds' that survived the genocidal flood their putative creator had inflicted on its creation, just a few thousand years ago.

What creationists are doing is moving the goal-posts by redefining a well-understood scientific term to place it beyond what science actually claims, so they can demand evidence of something that no evolutionary biologist ever claimed - that a single mutation produced a new species or that one species gave birth to another in a single event, ignoring the fact that evolutionary biologists understand the evolution is normally a slow process which takes place in the species gene pool, with the instances of new species arising by, for example, hybridization, being rare exceptions, but nevertheless, natural processes, not requiring supernatural entities to explain.

So, ask a creationist now wedded to the notion that real evolution needs to be 'macro-evolution' to define a reptile 'kind' and a bird 'kind' and they will normally define a reptile 'kind' as having scales and a bird 'kind' as having feathers, ignoring the fact that paleontologists have discovered several feathered dinosaurs (reptiles).

Now, present a creationist with an example such as the one in this research paper, where changes to a small number of genes resulted in what they would define as 'macro-evolution', i.e., a change in 'kind' because that change in chickens results in them growing feathers where they normally grow scales, showing the feathers are simply evolved scales.

The research was carried out by Professor Michel Milinkovitch, of the Department of Genetics and Evolution at the Faculty of Science in the University of Geneva, Switzerland.

The University of Geneve press release takes up the story;

Scales, spines, feathers and hair are examples of vertebrate skin appendages, which constitute a remarkably diverse group of micro-organs. Despite their natural multitude of forms, these appendages share early developmental processes at the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how to permanently transform the scales that normally cover the feet of chickens into feathers, by specifically modifying the expression of certain genes. These results, published in the journal Science Advances, open new perspectives for studying mechanisms that have enabled radical evolutionary transitions in form among species.

The skin of terrestrial vertebrates is adorned with diverse keratinized appendages, such as hair, feathers, and scales. Despite the diversity of forms within and among species, the embryonic development of skin appendages typically begins in a very similar way. Indeed, all of these structures develop from cells that produce a localized thickening on the skin surface and express particular genes. One of these genes, called Sonic hedgehog (Shh), controls a signaling pathway - a communication system that allows the transmission of messages within and between cells. Shh signalling is involved in the development of diverse structures, including the neural tube, limb buds and skin appendages.

A common ancestor

The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution at the Faculty of Science of the UNIGE, is interested in the physical and biological processes that generate the diversity of skin appendages in vertebrates. In particular, his group has previously demonstrated that hair, feathers and scales are homologous structures inherited from a reptilian common ancestor.

Feathers of the chicken embryo are used by scientists as a model system to understand skin appendage development. While it is known that certain breeds of chickens, such as the ‘Brahma’ and ‘Sablepoot’ varieties, exhibit feathered legs and dorsal foot surfaces, the genetic determinism of this trait is not fully understood.

A transient modification for a permanent change

As the signaling pathways responsible for this transformation have not been fully determined, Michel Milinkovitch’s group investigated the potential role of the Shh pathway.

We used the classic technique of ‘egg candling’, in which a powerful torch illuminates blood vessels on the inside of the eggshell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream.

Dr. Rory Cooper, co-author
Post-doctoral researcher Laboratory of Artificial and Natural Evolution (LANE) Department of Genetics and Evolution
University of Geneva, Geneva, Switzerland.

Our results indicate that an evolutionary leap - from scales to feathers - does not require large changes in genome composition or expression. Instead, a transient change in expression of one gene, Shh, can produce a cascade of developmental events leading to the formation of feathers instead of scales.

Professor Michel Milinkovitch, co-author
Laboratory of Artificial and Natural Evolution (LANE) Department of Genetics and Evolution
University of Geneva, Geneva, Switzerland.

The two scientists observed that this single stage-specific treatment is sufficient to trigger the formation of abundant juvenile down-type feathers, in areas that would normally be covered with scales. Remarkably, these experimentally-induced feathers are comparable to those covering the rest of the body, as they are regenerative and are subsequently and autonomously replaced by adult feathers.

After comparison with embryos injected with a ‘control’ solution (without the active molecule), RNA sequencing analysis showed that the Shh pathway is both immediately and persistently activated following injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales into feathers.

This research, initially focused on the study of the development of scales and feathers, therefore has important implications for understanding the evolutionary mechanisms generating the enormous diversity of animal forms observed in nature.

The abstract to the duo's research paper gives more details:

Abstract

Vertebrate skin appendage early development is mediated by conserved molecular signaling composing a dynamical reaction-diffusion–like system. Variations to such systems contribute to the remarkable diversity of skin appendage forms within and among species. Here, we demonstrate that stage-specific transient agonism of sonic hedgehog (Shh) pathway signaling in chicken triggers a complete and permanent transition from reticulate scales to feathers on the ventral surfaces of the foot and digits. Resulting ectopic feathers are developmentally comparable to feathers adorning the body, with down-type feathers transitioning into regenerative, bilaterally symmetric contour feathers in adult chickens. Crucially, this spectacular transition of skin appendage fate (from nodular reticulate scales to bona fide adult feathers) does not require sustained treatment. Our RNA sequencing analyses confirm that smoothened agonist treatment specifically promotes the expression of key Shh pathway–associated genes. These results indicate that variations in Shh pathway signaling likely contribute to the natural diversity and regionalization of avian integumentary appendages.

Rory L. Cooper Michel C. Milinkovitch
Transient agonism of the sonic hedgehog pathway triggers a permanent transition of skin appendage fate in the chicken embryo.
Sci. Adv.9,eadg9619 (2023).DOI:10.1126/sciadv.adg9619

Copyright: © 2023 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

The question now is not how creationists will change their minds in view of this evidence of a 'macro-evolutionary' change being caused by a few genetic changes, but what strategy they will adopt to avoid having to do so. In all probability, they will attempt to redefine 'macro-evolution' again, to exclude the embarrassing examples of it that science keeps finding.

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