|Isle Royale, Michigan. An island of wolves.|
In the real world, population bottlenecks are not always quite as big a problem as some people imagine, nor are they quite as easy to correct as some people hope.
If that sounds like two statements in direct opposition, then you have grasped a core message of this post, which is that not all animal populations are the same, that few real-world cases line up squarely with simple theory, and that there are multiple facets to both genetic isolation and genetic rescue.
In order to understand a little more, let's look at four real-world animal populations:
exactly what happened with the lions of Southern Florida (aka, the Eastern Puma, Florida Panther, or Eastern Mountain Lion).
Cesky terrier, a breed that has never been very popular.
So what's the point?
The point is this: the issues associated with population bottle necks are not quite as simple as some imagine. Yes, the relative size of the founding population matters, but so too does the genetic health of the founding population, the total number of animals bred, the number of generations it took for the population to achieve a large size, and whether the animal in question was breeding randomly or within an organized scheme that encouraged (or discouraged) genetic diversity.
Consider, for a moment, a dog breed that was drawn into a closed registry 120 years ago with 150 dogs in its foundation registry. The breed grew rather quickly so that after 40 years it had a worldwide population of 125,000 dogs; a population it has maintained for the last 80 years. Coefficients of inbreeding in the modern population of this dog will, on the whole, be quite low, and though the apparent "effective population size" may have dropped from 150 dogs to 80 dogs due to dominant sire selection, effective population size is quite a meaningless number when you have bred well north of 1 million dogs over a 100-year span. In a situation like this diversity is not destroyed over time, but created through the sheer beat of numbers over vast distances.
Of course, there is another factor in all this, and it is an important one, and one that is too often left out.
You see, in the world of Kennel Club dogs there has always been a little "pedigree leakage."
Pedigree leakage may be small or even nonexistent in a very rare breed where everyone knows everyone else, and the dog in question is likely to look a bit odd or extreme and not have an obvious analog in another breed or cross-breed.
But what of the other more popular breeds, where there are a lot more dogs in the wind, and only a small number are likely to ever see the inside of a show ring, and analogs exist all over?
In a situation like this, instead of a reduction in effective population size, there may actually be a little growth!
After all, a Labrador Retriever with five-generation pedigree papers from the AKC may, in fact, have a Flat-coated sire, or even be whelped by a cross-bred dog of "pedigree unknown."
A Miniature Poodle may have a little Maltese coursing through its veins only a generation or two back.
And what does it matter? Not a whit if the cross was a sound one and did not add a new or heavier genetic load to the mix.
The bottom line is that there is more to population genetics than any one number, and while it's important to be worried, and to take action to increase breed genetic diversity, it's equally important to recognize that one can fall in love with less than robust genetic theory and falsely specific numbers just as easily as one can fall in love with cocked up doggy histories and the romance of breed purity. In the end, population science and genetics is about more than one number; it's about a small stack of numbers meeting a real gene pool coursing through the flesh and blood of a real population that is living in a real environment (whether wild or artificial, free-feeding or subsidized).
Yes, let us work to increase diversity, but let us also look towards Mother Nature as well.
Remember that while Mother Nature abhors closed breed pools, she also manages to work out solutions for most of those that wash up as single or paired animals on tropical islands or are introduced to foreign lands. Observe the birds in the park. How many Starlings did we start off with here in America? How many English Sparrows? How many Red Fox? How many wolves in Yellowstone? They are clearly thriving. How is that possible? And, of course, the answer is that they colonized in vast numbers over a vast space of land, and in the slow beat of numbers and space they managed forge their own genetic health.