26 August 1999
One of my favorite paradoxes is that an inversion can confer an
apparent selective advantage even when there has been no change in the
genes it contains. Researchers have found a small but significant
excess of specimens (fruitflies) carrying one copy of the inversion. This
has puzzled me for some time, but now I think I've found a solution.
In a freely crossbreeding population with no restriction on crossovers, the genes in a given segment will segregate giving (potentially) every possible arrangement of the available alleles. Some are favored, some eliminated, while most may be acceptible when paired with certain other combinations. The various combinations may be labeled A1, A2, A3, etc., and we may observe that certain pairs of the gene-sets are more or less advantageous. For example, A1A3 may be a particularly good combination while A2A3 is adequate for survival but below average in ability to reproduce.
The good combination A1A3 may reproduce freely, but very few of its offspring will duplicate its finer qualities. Crossovers between A1 and A3 rarely if ever duplicate these combinations precisely, and even if such a specimen can be selfed it is unlikely that many of the offspring will be quite as good.
Now suppose that an inversion occurs which flips the A1 segment upside down. We may call this version A1. This A1 does not differ from A1 in the gene set, but when it is paired with any uninverted segment there can be no crossovers. So, when A1 is paired with A3, the only gametes produced will be A1 and A3. If this specimen is selfed, it will produce 3 types of offspring: A1A1 : 2 A1A3 : A3A3. That is, fully half of the offspring will precisely duplicate the advantages of the parent.
A further consequence of the inversion is that those other gene-sets which compliment it will also be favored and reproduced. So the inversion actually conditions (catalyzes?) the rest of the gene pool without directly interacting with them. In fact, the inversion works its influence because it cannot interact (crossover) with the other corresponding segments.
The inversion increases the frequency of the particular gene combinations which favor its own increase.