Outbreeding depression:
A friend has directed me to a marvelous review article, Suzanne Edmands, Between a Rock and a Hard Place, Molecular Ecology, vol. 16, no. 3 page 463 published online November 15, 2006. The article is concerned with maintaining species in the wild.
Suzanne Edmands takes the position that inbreeding depression and outbreeding depression are both real, and that this has practical implications when dealing with wild populations in danger of extinction. It might seem desirable to undertake deliberate hybridization of a population in order to rescue it from inbreeding depression but this may entail the danger of outbreeding depression. Both forms of genetic misadventure can be traced in principle back centuries. However, inbreeding depression has received far more attention. In fact my word processor recognizes “inbreeding” but not “outbreeding” as a valid English word.
The article reports more than 10 times as many hits on a web search for the one word as compared with the other. And yet both effects are real, and indeed their relative importance seems roughly comparable. There may be a relatively higher risk in the first generation of inbreeding, but the outbreeding over inbreeding fades rapidly over subsequent generations.
There is also a question of what “depression” means. When studies look at either inbreeding or outbreeding they can distinguish between “morphological traits,” what the animal looks like, and “life history traits,” more or less how lucky it is getting along. These latter traits are more sensitive to inbreeding or outbreeding.
So far, it sounds very familiar. The first generation boost in fertility followed by a later reduction is demonstrated both in our model and in field data. And when it comes to comparing life history effects, human fertility is indeed dropping while humans really don’t look all that bad.
However, the time it takes hybrid incompatibility to develop, how long two lines must be separate before bringing them back together is harmful varies a great deal. This is not the impression we have looking at effective population sizes, which seem remarkably constant. That is not a serious problem for the model, but it does seem unexpected.
Problems with hybrid incompatibility seem to arise more rapidly in animals where the X chromosome is large and the Y (or more accurately the single sex chromosome in one of the sexes) chromosome small. That describes us humans. (John H. Gillespie Population Genetics, The Johns Hopkins University Press, Baltimore, 2004 page 179.
Birds seem to retain crossbreeding interfertility for something like a 1,000,000 years, while the Iceland study shows humans running into serious problems in 10 generations. Compare that with the fact that the effective gene pool size for birds is only 20 or 30% larger than the size for mammals (Gillespie page 17 chart), and there is something to puzzle over.
The bad news is that the article points out that you can have inbreeding depression and outbreeding depression going on at the same time. That is absolutely true of the computer program. The two are produced by completely independent mechanisms, and there is nothing about having depression caused by too large a population size that protects the model individual from the effects of being too closely related to a mate.
This has serious practical ramifications. The reproductive rate of the more developed regions of the world has dropped significantly below 2 children per woman. At the extreme, consider what happens if there is only 1 per woman. Not only does the individual have no siblings, but in time no first cousin and then no second cousin. All too soon, in historical time, there is no cousin close enough to provide a suitable mate at all.
We are not there yet, but this does eliminate that notion, “Oh it will all work itself out in the end.” It won’t. Action will need to be taken. As I have mentioned I cannot give specific advice, but this does require immediate rapt attention.
All of this is somewhat off the point. The model assumes, and I think the demographic facts are consistent, that the issue is not bringing together populations that have long been separated. One could, with great labor, go about creating a series of virtual populations that have been separated for a time and then bring them together to see what happened. The computer program will let you do that. When I have tried, I found it so tedious that I could not trust myself to be error free, so whatever turned out, I would not have given much faith to it myself.
Perhaps if the day comes when I can make the program available, and if that does not require eliminating the models ability to store and then recover populations, someone with sufficient skill will find a reason to try it out and get to the bottom of it.
Meanwhile, the paper clearly documents that there is such a thing as too much genetic diversity. Those who maintain otherwise no longer have a defensible position. Of course that is on top of ample proof of the same thing that I have already presented.
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