THE SUBSPECIES MISCONCEPTIONS AND PROBLEMS WITH TAXONOMIC CLASSIFICATIONS MAINTAINED BY ZOOS
Legislation is often to blame, not because legislators are bad people but because they may act on partial or misleading information and are unfamiliar with the total picture.
-Dr. Jim Sandeson
Founder, Small Cat Conservation Alliance
A subspecies is a taxonomic designation made to aid the classification of and management of species. There are several criteria that are required to justify a subspecies including unique geographic range, some level of unique genetic variants, usually some level of local adaptation, and some level of isolation. The reason this wording is vague and uses such loose terms as “some level” is that there is a gradient in what constitutes a subspecies. The term subspecies itself is open to debate and controversy in scientific circles. There is no accepted level of differences that can be used across all species to determine what constitutes a subspecies because various types of animals have different mutation rates, variation, dispersal, behavior, and plasticity in their traits. As populations become isolated and accumulate genetic differences through time these may become large enough to be separate subspecies. Many of the subspecies classifications we have today were made in the 19th century based on very few collected specimens that are now archived in zoos. Later after subspecies designations were made, it was found that the level of variation within species is often much higher than appreciated leading to the conclusion that many original subspecies are not valid. Finally, genetic analyses that directly examine differences between subspecies often show many do not represent unique subspecies but should be classified together.
Subspecies Change Through Time
Further, one must keep in mind that species, ecological boundaries, and the environment all change in time. Therefore, “subspecies” is a classification that also changes through time. Dispersal rates can change between areas, habitats can change, climactic conditions change; all this affects whether subspecies diverge or again merge together. Subsequently, subspecies may either continue to diverge and become new species, or become reconnected with other subspecies and merge into one large population. Because of these many factors, it is not correct to manage species strictly based on subspecies designations. It is much more important to manage and conserve species based on the factors most import for conserving the species as a whole. The most important criteria are to maintain large population size, maximize adaptive variation, preserve rare variants, and reduce inbreeding and genetic drift, before managing for subspecies. Whether a species should be managed at the subspecies level needs to be scientifically determined on a case by case basis.
Tigers were divided into 8 subspecies by numerous naturalists as specimens were collected throughout Asia and studied from 1758 to 1968. The criteria used to define them were primarily coat color, body size, and the geographic region from which they originated. The subspecies include tigris (Bengal), virgata (Caspian Sea region), altaica (Amur), sondaica (Java), amoyensis (China), balica (Bali), sumatrae (Sumatra), corbetti (Southeast Asia). Three of these subspecies are now extinct. However, in more detailed studies that examined tiger morphology in a large number of animals some of these subspecies were not clear classifications. In addition, the range boundaries between them were not well defined. A detailed genetic study conducted by Luo in 2008 suggested that today there are 6 living subspecies (this study divided the corbetti into two different subspecies - corbetti and jacksonii). However, there are problems with this study and reasons that some of these designations may not be valid at this point and require further evidence.
Problems with Tiger Subspecies Classification
Sample collection and population reductions can artificially create differences that can then be misinterpreted as different subspecies. The Luo study in 2008 used specimens from zoos that either originated from a known location in the wild or whose parents came from the wild. The subspecies analysis was done using microsatellite markers, mitochondrial DNA genes, and the disease resistance-related MHC gene DBR region. The study found no major differences among most tiger subspecies in the functional MHC gene. The genetic variants used to show differences between tiger subspecies were the neutral microsatellite and mitochondrial genes. These variants are considered neutral because they have no known effect on fitness. This is often done because neutral variation provides information on isolation and population connectivity. When populations are isolated the frequency of genetic variants changes between them. The level of difference in the frequency of the variants can be used to determine how genetically different populations are, and to assess subspecies validity.
The problem is that this study used a very small number of samples to represent some of the most widely distributed tiger subspecies. For example, there were only 2 tigers from China. In India, only 2 locations were sufficiently sampled within that entire subcontinent. In addition, the samples used were those from the 20th century, after tigers were extirpated from many areas and the remaining populations were isolated. For example, the Amur tiger samples came from a relatively small region relative to its historical range. Because of the limited sampling, this study did not examine all of the diversity present within the range of the original tiger subspecies. It also sampled tigers after the human-induced reductions and population fragmentation of the early 20th century. The genetic drift in the remaining small tiger populations would shift allele frequencies, which would artificially inflate the differences among subspecies they tested. Therefore, the conclusions of this study are informative, but do not provide enough data to test the validity of subspecies in wild tigers. The Luo subspecies designations should only be used as a guide, and not as hard, concrete classification scheme for the tiger breeding program.
Breeding Subspecies with Small Founders Leads to Genetic Drift and Inbreeding
A program that advocates the breeding of only subspecies with a limited number of founders, and small population size, leads to genetic drift that causes loss of variation and also increases inbreeding which increases the frequency of genetic diseases. Both of these increase susceptibility to infectious diseases and decrease adaptive variation. As these tiger subspecies are further bred in zoos, they accumulate additional differences between them that then ensure that they will come out as separate subspecies when their genetic variation is analyzed. This is similar to how differences in dog breeds are created. If you have a pool of animals, and you take a subset and breed them in different groups, you will create changes that hide the original structure. In addition, by closing a population for breeding, you increase frequency of disorders, just as pure breed dogs have more health problems than mixed-breed dogs.
The Scientific Approach to Tiger Breeding
A sound scientific approach to tiger management and breeding recommends a program that maintains all genetic variation that is currently present in the captive tiger population. This leads to increased effective number of breeders, which is the single most important factor for increasing the likelihood of population survival and reducing extinction risk. This is the strategy that the Species Survival Trust is using for their captive tiger management.
There is a noble but ill-informed idea that big cats and other endangered species should not be kept in captivity at all but should be allowed to live in the wild. Where is the wild? It’s gone.
Former host of Mutual of Omaha’s Wild Kingdom