What is a gene pool?
A gene pool is a hypothetical collection of all the variations of genes in a population. This could be a population of rabbits in a field, fish in a pond, or dogs in a breed. In a closed population, such as pedigree dogs, the numbers of gene variants is unlikely to increase, unless new dogs are brought into the breed, or mutations occur (which is rare and usually harmful). A gene pool can, and most likely will, get smaller when genes are lost through complete chance (i.e. not passed on to any descendants), or when dogs do not reproduce.
Sometimes an animal having a certain trait can influence how likely it is to survive and/or reproduce, this could be a faster rabbit evading a fox, a better camouflaged fish not being seen by its predators, or a pet dog having a good temperament and being chosen for breeding. All of these selection pressures can, over time, shape a population, making some genes associated with these benefits more common, while others become rarer or are lost from the gene pool.
How does selection impact a gene pool?
Dog breeders will choose carefully and select dogs that possess specific desirable traits, such as an excellent level of health and good temperamen. By applying a selection pressure, (or a breeding criteria), to a breed, it makes some traits, and the genes that control them, more common, while others which control less desirable traits become rarer.
Dogs with desirable traits are likely to be bred from more frequently, while others that do not possess these traits may not be used for breeding at all. Over time, the gene variants associated with these popular dogs become common in the breed, while those associated with the less desirable dogs may be lost and disappear forever. These lost genes may include those that controlled the less desirable traits, but may also include other genes that just happened to be found in the less desirable dogs.
For example, if a longer coat is desirable, then dogs with a long coat are more likely to be bred from and pass on their genes. Dogs with a short coat may not be bred from at all and so will not pass on any of their genes. These lost genes may include those that produce a shorter coat, but also includes all of the other genes that contributed to the rest of the dog, i.e. its eye colour, leg length, quality of hips, temperament, etc.
What is the impact a shrinking gene pool can have on a population?
If a population is made up of 100 dogs and there are 50 different variations of each gene, then the likelihood of finding two dogs with the same genes is small. If over time the number of dogs stays as 100, but the number of gene variants shrinks down to 10, then the likelihood of finding two dogs with same genes is much higher. These dogs will have inherited their similar genes from an ancestor that featured in both their pedigrees and so they are, to some degree, related. Therefore, as the gene pool shrinks, the likelihood of two related dogs mating increases. The mating of related dogs is known as inbreeding. As inbreeding increases, so too can the risk of health problems occurring within the population.