I'd be interested to see if you can find any evidence for this in human populations. From my undergraduate classes (and in my admittedly limited exposure to pla

I'd be interested to see if you can find any evidence for this in human populations. From my undergraduate classes (and in my admittedly limited exposure to plant biology), it sounds like you're referring to increased fitness of plant hybrids. In this case, you're falling into the mindset I outlined in my other comment. At least in the simple examples presented at the college biology level, hybrids often combine true-breeding traits (hypothetically, good growth in a certain climate, resistance to a certain temperature, etc). It seems reasonable that an individual with a mixture of such traits would fare better than its true-bred counterparts. In humans, our race labels don't correspond to true-bred traits. While some genetic conditions are most common in certain racial groups, I cannot think of any set of genes where heterozygotes are only formed by mating across racial groups. Note that I'm writing in the US - these points may be less true in homogeneous countries like Norway. But in such groups, we're not seeing the effect of race as much as the effect of small groups of human founders. One interesting point, which could buttress OP's observation, is the finding that immigrants are often healthier than native born individuals[1]. This phenomenon occurs across many measures of health; unfortunately for America, a nation of immigrants, the phenomenon regresses over successive generations. 1. A discussion of weight and immigration. This is the first link I could easily find, but I have seen this discussed more convincingly with respect to low birth weight in African mothers vs Black American mothers: https://pdfs.semanticscholar.org/e645/c696508ccb09cfbd476a86b17af2562023bf.pdf https://pdfs.semanticscholar.org/e645/c696508ccb09cfbd476a86...