Gregory Cochran has an interesting post up about the possible advantages of hybid vigor (heterosis), i.e. the possible advantages of offspring from two genetically dissimilar parents.
I think the advantages of hybrid vigor might be overstated. While hybrids could possibly introduce beneficial genes, what is more important than producing hybrids is selection pressure for favorable genes. Hybrids might at one point been a mechanism to introduce adaptive traits, but in the near future such traits could be introduced via genetic engineering and then artificially selected for, as with domesticated animals or plants.
Cochran is right that whatever advantages hybrid vigor might bring are basically “a first-generation effect.” One sees this with canine mutts, where after mutts interbreed for multiple generations, especially in the wild, mutts will often develop pariah-like characteristics, losing their traits of domestication. In short, there is no (artificial) selection pressure on mutts for (beneficial) domesticated traits.
Cochran also states that hybrids among more genetically distant populations could possibly produce more beneficial traits, but there are plenty of counterexamples to this.
For instance, the genetic distance between Europeans and Amerindians is significant, but mestizos do not in anyway seem remarkable. Other than the donkey show and taco, mestizos haven’t contributed much to humanity. Mestizos are fairly mediocre both academically and athletically, as evidenced both by Mexico’s poor academic performance and low medal count in the Olympics, and mestizos don’t really seem to excel at much, other than the recent honor of being the most obese people on the planet.
Mestizos are a walking and breathing counterexample to the benefits of hybrid vigor.
Eccles & Mount: “Cross-breed Dogs Have More Health Problems than Purebred Dogs” (For “Pariah Dogs,” see entry here.)