COAT COLOR GENETICS AND OTHER NATURAL, YET RARE, IMPORTANT VARIANTS
A tiger from Sumatra and a tiger from south China have less genetic distinctiveness between them than a person from Ireland and a person from India
-Stephen J. O’Brien
Chief of the Laboratory of Genomic Diversity
Coat Color Variation in Species, Complexity, and Environment
- Coat color can be adaptive in some environments and not in others
- Coat color can be linked to development genes
There is great coat color variation between species, yet much color differences also occur within populations of the same species. This is partly because coat color can be adaptive in some environments and not in others. A good example is deer mice. Hoekstra’s lab at Harvard has shown that there are both light and dark mice and whether or not the color is most common in a population is dependent on the background of the environment. The coat color can be a complex trait that is affected by multiple genes. Its appearance can therefore change through modification by various mutations. Therefore, even though we observe a certain coat color today, it is likely that in the future it may be further changed by other mutations, potentially making it more adaptive. In addition, there is added complexity when it comes to coat color. During the development of the embryo the genes that affect coat color also affect other cells that develop into other tissues, therefore the genes can also influence other traits not linked to coat color. This can be negative; some genes that affect color can also cause neurological disorders. However, it can also be positive. A good example is in wolves. Wolves in Yellowstone that have the black phenotype have a higher fitness, yet detailed study has found that this is not linked to better camouflage from the color. It appears that the genes, which are causing the black color, provide an unknown fitness benefit to the wolf that we are yet to understand.
White Coat Color in Tigers is a Natural Trait
- White tiger color is from wild populations in India
- Caused by variant in gene (SCL45A2) that does not appear to cause diseases
- Most white tigers today are not inbred and have diversity similar to orange
The white coat color is a natural variant that was first found in wild tiger populations of India. There are misconceptions that the white color was developed in captivity but this is not true. A recent study by Xu found that this color is caused by a change in the sequence of a protein called SCL45A2, which does not appear to cause any health disorders. Genetic analysis of white tigers by Janecka has shown that levels of diversity of white tigers compared to standard orange tigers are the same, and that most white tigers are not inbred as often claimed.
The Need to Protect all Variants that are Present in Species
- We cannot predict what variants will be adaptive in the future
- Need to protect all diversity in a species
We cannot predict how the environment will change in the future so even if a particular color appears bad today for camouflage, in the future it may be adaptive. We also cannot predict what other benefits these variants may have on the fitness of individuals beyond their appearance. If the coat color variant is present in a population at a low frequency, it may enable the species to adapt to new conditions in the future through natural selection. This is why we need to preserve all of the natural variation in the tiger species, including rare coat color variants such as the white, snow white, and tabby patterns. The white color is indeed a natural variant in tigers, although it was, and still is, rare in the wild. Some organizations such as the Association of Zoos and Aquariums and its member zoos are trying to eliminate white tigers. This is short-sighted policy should be abolished. In contrast, the Species Survival Trust takes the stand that all tiger variants should be protected and maintained, including the white tiger.