Thursday, December 22, 2011

Black-skinned Chickens Shed Light on Evolution

With all the talk about genetic engineering and controversy regarding evolution, it is appropriate to ask what light chickens may shed on the question of how organisms change through time. A great place to witness species change is with domestic animals since people have been selecting for various traits for thousands of years, in the case of chickens about 8,000 years.

Occasionally, and probably rarely, fragments of DNA behave unexpectedly, either getting duplicated, lost, inverted, or moved to a new location. And even more rarely, several of these things happen at the same time. New research suggests that black skin in chickens results from "duplication of two different DNA sequences located in the vicinity of each other on chicken chromosome 20, where one of the copies has also been translocated and inverted."

According to National Geographic there are now about 52 billion chickens on the planet, which means in the course of the last 8,000 years there have been trillions. And a switch from white or yellow skin to black skin is a change people would notice. So even though the described duplication, translocation, and inversion of DNA fragments would be rarer still, it might have only needed to happen once and have someone notice and breed for that characteristic. 

In 1298 Marco Polo reported seeing chickens that "have hair like cats, are black and lay the best of eggs" -- an apt description of Chinese Silkies, the most popular black-skinned chicken breed. But black skinned breeds are also found in India, Indonesia, Japan, Korea, Sweden and Vietnam.

What does this all mean? Setting aside the concept of survival of the fittest in nature, it seems clear that people select for whatever traits intrigue them. 

The researchers concluded: We have examined four different chicken breeds from both Asia and Europe displaying dermal hyperpigmentation and conclude that the same structural variant underlies this phenotype in all chicken breeds. This complex genomic rearrangement causing a specific monogenic trait in the chicken illustrates how novel mutations with major phenotypic effects have been reused during breed formation in domestic animals.

In other words, they believe all domesticated black-skinned chickens result from the same DNA coding anomaly and they probably suspect that all black-skinned breeds are derived from a single ancestor. 

Chinese Silkie chick shows black skin
Photo by Jennifer F.

Read the peer-reviewed paper here. 

Here's a bonus thought: If you crossed a black-skinned breed like the Swedish Svart Hona with the Naked Neck Chickens, might you end up with a chicken that suggests a vulture?

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