Heterosis

Heterosis, or hybrid vigor, is the phenomenon in which offspring of genetically distinct parents exhibit greater fitness — larger size, higher fertility, stronger disease resistance — than either parental lineage. The effect is the immediate mechanical consequence of increased heterozygosity: deleterious recessive alleles are masked by dominant or complementary alleles from the other parent, and biochemical pathways operate with enhanced efficiency due to allelic diversity.

In agriculture, heterosis has been exploited for centuries to produce high-yielding crop varieties and robust livestock. In wild populations, it is the proximate mechanism behind genetic rescue: the fitness rebound that follows immigration is not a mystical restoration of 'genetic health' but the straightforward masking of inbreeding load. The vigor is typically strongest in the first generation and declines in subsequent generations as recombination breaks up favorable heterozygous combinations.

The systems insight is that heterosis is not an intrinsic property of hybrid genotypes. It is a relational property that depends on the genetic distance between parents, the environmental context, and the genetic architecture of the traits under selection. Two populations that produce vigorous hybrids in one environment may produce inferior offspring in another. This context-dependency makes heterosis a warning against treating genetic diversity as a universal solvent.