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As seen in the photos, the gene pool can have different outcomes in the same litter of cubs. In this article I will explain how the gene pool works in large cats.
Because we are not scientists, we will not go into the subject of genetics in depth. We will, however, offer some definitions to help explain the general idea.
There are three main ways of describing why the genetic diversity possessed by a species is essential to its long term survival:
Heterozygosity is positively related to fitness. The rate of evolutionary change able to occur in a group of organisms is dependent on the amount of genetic variation present in the gene pool.
The global pool of genetic information represents the "blueprint" for all life.
Although evidence has not produced unanimous results, there appears to be a correlation between the average heterozygosity of a group of animals and their average fitness. Fitness can be defined by the organism's ability to perform a long list of biological functions, and can be used as a measure of how succesful an organism is at exploiting its particular niche. Typically, organisms with a high fitness rating are very successful, and have many healthy offspring, while organisms with a low fitness value may not.
Heterozygosity is a measure of genetic difference within a population, and to some degree is a measure of the populations ability to withstand disaster. For example, the worlds entire human population is extremely diverse, and would have a high heterozygosity.
Breeding closely related animals reduces the level of heterozygosity in the offspring. When inbreeding occurs within a population rare genes can be lost, and the frequency of deleterious genes can increase or even become fixed, and overall genetic variability is reduced.
A classic example of this is the cheetah. All cheetahs are extremely closely related, and the level of heterozygosity within the entire cheetah population is of a similar order to that of brothers and sisters. Cheetahs are almost clones of each other. In fact, the entire cheetah population existing today is believed to be descended from one pregnant female that survived the last glacial period around five thousand years ago.
A typical outcome of inbreeding is called "inbreeding depression". Most organisms carry many deleterious alleles, but the affect of these are covered, or masked, by the individual also carrying a fully functioning copy of these alleles. In diverse population the chance of both parents giving a deleterious allele of the same gene to their offspring is minimal. When inbreeding occurs offspring may recieve deleterious alleles of a gene from each parent. Having two deleterious alleles for that gene means that they do not have a working copy of that gene, and this can reduce fitness or even be fatal.
Inbreeding depression is where deleterious alleles increase in frequency in the population, and variability decreases. The effects manifest as decreased fitness. Less offspring are born, and these have a lower chance of survival than previous generations, generally due to birth abnormalities. Inbreeding depression can, and generally does, lead to and cause extinction.
It appears that cheetahs have survived their period of inbreeding depression not by an influx of new individuals bearing genetic diversity (as there are none), and not by mutation causing increased variability (because not enough time has passed to allow their gene pool to naturally diversify to original levels), but by natural selection removing the deleterious genes from the gene pool. Individuals with a heavier lode of deleterious genes are outcompeted for food and mates by their healthier comrades, and do not pass their deleterious genes on.
It appears that cheetahs have survived the dangerous period of inbreeding depression, and, as a consequence, are now able to inbreed fairly succesfully, without as much danger of deleterious alleles manifesting. It seems that the cheetah population was recovering from the inbreeding event mentioned, but are again under threat as their numbers decrease.
One pitfall of low heterozygosity is the low genetic variability at immune loci. There are some diseases that the entire cheetah population have no resistance to, and when an individual contracts such a disease, it will die unless helped. In effect, the cheetah population is similar to a crop monoculture: we can protect them with some medicines, but they are vulnerable.
Where two animals are closely related, but different species mate, it is the male that will be sterile. This being the breeding of a hybrid animal. When two animals mate, the offspring that are homogametic sex will be fertile (XX females), while offspring that are heterogametic will be infertile.
"The opposite of inbreeding depression is outbreeding depression. This is where the animals involved aren't closely related enough, and there are compatability problems ... too much diversity. Examples of this are the mule, a cross between a horse and a donkey, and also goats and sheep, lions and tigers, and kangaroos and wallabies can be crossed. Different species can be crossed when they are closely related. The hybrid animal is only fertile if it is the homogametic XX female. Hybrids of the heterogametic sex (XY males) are always sterile, probably due to imcompatability of the sex chromosomes.
There have been many cases where endangered animals have been "rescued" by outbreeding. Populations with low heterozygosities can be given a genetic refreshment by introducing an individual from a distant population of the same species. Such individuals tend to have a sufficiently different genetic makeup to increase variation without causing outbreeding depression. Animal breeders do this often. There are many fancy chicken breeds, and these are created by people breeding unique looking animals with their siblings, so as to end up with a small population that is purebreeding for the desired trait. If a line becomes too inbred, they will cross it with something new, to refresh the gene pool, and then cross the best hybrid offspring back to the original stock for a few generations. They end up with an animal that looks like what they wanted, but has a broader gene pool.
All dog breeds were made this way, too. Every dog you see is actually an artifact of a human breeding program. All dogs came from the wolf, which had a huge population of great diversity. That doesn't mean that wolves looked like Great Danes and Chi Hua Hua's, it just means that the potential was there for them to develop that way. People simply inbred them until different alleles started manifesting, and unique looking individuals started appearing, and then concentrated on developing that unique trait. Inbreeding depression was avoided by occasional outbreeding.
As human development encroaches on the living space of all other large vertebrates we find that population numbers are always decreasing. This means that heterozygosity will be decreasing, as there are fewer individuals alive at any given time to carry that population's genetic diversity. As a result, diversity is lost, and inbreeding occurs.
Postulates at minumum population numbers have been made, but these vary with the characteristics of each species. Suggestions have been made that 500 individuals is the absolute lowest number a population can fall to, and still have diversity enough to withstand disaster (fire, flood), epidemic, and inbreeding depression successfully.
Many of the worlds great animals have populations lower than that, so breeding programs have been created to increase population numbers, and ensure that inbreeding is minimised. Outbreeding to increase diversity is not the only answer, though, because when outbreeding occurs the offspring is a hybrid. The identity of the hybrid is that of neither parent. It takes many generations of back crossing to regain the features of the species being "protected". Such programs are long term, and involve the danger of losing the species integrity. The answer is to never let population numbers get so low that such extreme measures need to be taken, but for many species it is too late for that. I guess the answer is complicated. There is a worldwide cooperative effort toward creating successful breeding programs which maintain genetic diversity, using captive animals from zoos and wild animals. Also there now exists a library of gametes (sperm and egg) from many species, frozen in liquid nitrogen for artificial insemination at "some later date", but this is not enough. Why have a population represented as gametes in a freezer when it could, and should be running about in the wild? Zoos and freezers do not make it O.K. to destroy habitat.
The cat population structure as it relates to territories; As far as cats go with their territories and the way females have smaller territories and males have larger ones, and how they overlap, and the fact that their lands are large, somtimes huge, it all leads towards inbreeding anyways. But they can handle it, it's the way they've been for millions of years. Cats generally have a lower heterozygosity equal to that of other animals it would mean that the males would have to wander vast territories, and offspring would have to be sired by males from either end of the continent.
Genetic variation enables a species to adapt and evolve to new circumstances. Alleles have been developed by the process of mutation and natural selection.
A special thanks to Brendan Duffy, a PhD student in molecular genetics from Melbourne, Australia. His input regarding this article and definitions have been very helpful.