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Evolution

I wanted to know about coat colour genetics and inheritance in the horse because Kaimanawa Horses are so plainly brown much more so than historical accounts of wild horse colours that included grey, roan, blue, piebald and skewbald. As it turns out compared to other animals, horse coat colour is a fairly simple genetic problem.

Although selective breeding can produce incredibly diverse coat colours in horses, white, grey, black, red (chestnut) and bay horses are the basic colour types [1]. Which basic type a horse is-is governed by just four genes, each with their own site, called a locus, in the horse’s genome. There is one gene and locus each for white and grey horses, and two others determine the chestnut, bay and black hues.

Four genes can produce five different basic colours because each gene for coat colour has two different forms, called alleles. The allele at each loci can be either dominant or recessive. A recessive allele’s influence on coat colour is weakened or prevented entirely if there is a dominant allele at one of the other three loci. And so different combinations of dominant and recessive alleles at the four loci determine the coat colour of a horse.

coat colour graphic III

Crudely put, dominant alleles rule such that horses are black or chestnut if they are not white or grey. Dominant alleles at loci for white or grey make white and grey horses. A horse is black because it has a dominant allele at the loci for blackness but does not have a dominant red allele at its loci for redness. Chestnut horses do not have a dominant black allele, and so the red colour is uninhibited. Bay horses are an interesting combination of the two. They have a dominant allele at both the loci for redness and blackness, causing much of the black hue to be restricted to their extremities – mane, tail and feet.

After those four loci for the basic colour types, four other subordinate loci on the genome govern the different hues of black, bay and chestnut, such as palameno, buckskin, and dun. And another four loci govern coat colour patterns, like roan, leopard spotting, and tabiano (pinto, pie- or skewbald). But alleles at these eight loci for hue and pattern have no influence if the alleles for black and red are not also present. Different shades and patterns, therefore, are variations of fundamentally black, bay or chestnut horses.

Knowing how horse coat colour is derived and inherited, we can turn back to the question why are Kaimanawa Horses to plain brown? Perhaps they are not afterall? – my next post.

Bibliography

  1. Thiruvenkadan AK, Kandasamy N, Panneerselvam S. 2008. Coat colour inheritance in horses. Livestock Science 117: 109-129.

The evolution of horses has fascinated us for a long time – perhaps because horses contributed so importantly and in so many ways to our societies for several millennia, but also because many of us still have relationships with horses. We know that to understand an animal better we need to know where it came from and what it came from.

The ‘Evolution of Horses’ display at the American Museum of Natural History, New York.

The evidence for horse evolution has been the most illustrated evolutionary story in the history of school and university textbooks, and museum display. It was the first popularised evolutionary series.

The horse has also played a central role in the development of evolutionary science. How the evidence for horse evolution has been interpreted and explained over the last 150 years is an interesting case-study of science in our society. It illustrates how science grows knowledge and changes us and our communities, but also how we influence science and limit knowledge.

A post introducing evolution is required early in this blog because evolution is central to modern biology, including our understanding of horse physiology, behaviour and ecology.

In the minds of many, evolution and adaptation are the same but they are not. Evolution is simply biological – living and inherited – change through time. Evolution is a fact – populations and species do inherit changes through time. That evolution has occurred and continues to occur is indisputable. With simple tools and a keen eye, you and I can measure evolution as it occurs.

For example, when populations of fish are harvested in ways that consistently remove the largest fish subsequent generations reach sexual maturity faster and at smaller sizes. With more technical expertise we could measure rapid evolution in greater detail. For example, animal diseases have evolved and now infect people, like human immunodeficiency viruses – HIV – and fish become resistant to poisons.

Gary Sutter and the fossil horse he found – prepared for display by the Sierra College Natural History Museum.

More dramatic evolutionary changes over many millennia can be measured using modern comparative genetics and seen in the preserved remains of once living things – fossils.

Scientific debates about evolution are not about whether or not evolution occurred or occurs. The evidence for evolution is accepted. The scientific debates are about HOW evolution occurs or, for specific living things, WHAT caused their evolution.There are many mechanisms that cause evolution. Some are sudden and dramatic like the global catastrophe of a meteor impact or rapid climate change that cause extinction of species and populations. Some mechanisms will be more subtle and occur slowly, like the differences that emerge between two populations of the same species when they are prevented from exchanging members. Our goal is to try and understand how changes occur and what causes those changes.

rhino skeletons

Rhino skeletons at Paris Natural History Museum. Photo by Colin Burns.

In future posts under the category Evolution I will describe the evolution of horses and other Perissodactyla. I will investigate what is understood about how they evolved and what caused their evolution. I will examine how the accumulating evidence for horse evolution modified scientific thinking. Perhaps more interesting still, I will consider how scientific thought has been constrained by the interaction of that evidence with the expectations and prejudices of contemporary society. This will not just be a journey in science, but science in society.

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