The Hitchhiking Gene

[Morphoeclectic]

Despite genetic recombination, it is theoretically possible that a harmful gene
can "ride on the coat tails" of a beneficial one favored by selection if it is near
the locus of the latter and so avoid separation by chromosomal crossovers.

While very rare, it does also apply to the non-recombinational sex Y chromosome.
Thus, Muller's ratchet still holds true for sexually reproductive societies.

Hitchhiking could pose a real problem for Evolution by natural selection if
the deleterious alleles are brought to fixation: If they are lethal in
homozygous form, they will impede selection altogether. A steady
accumulation of harmful alleles could tip a population into mutational
meltdown especially if their harm is exacerbated by a change in
selective pressure.

Moreover, physical location to a selected gene on a chromosome may
not be necessary in achieving fixation by a slightly deleterious allele.

It can still hitch for a ride before being dropped off by recombination.
The balance of probabilities should ensure that at least one such allele
should reach near fixation due to repeated hitching and alighting. As
the negative selection on a slightly deleterious gene is only slight or
even negligible, this is perfectly possible.

Many genes ,harmful from a human but not evolutionary perspective,
ie those undetected by selection due to causing late onsetting
disorders ( cancer, Huntington's etc), may also be inadvertently
promoted by selection.

Thus, the result of selection is the gradual increase of fitness
in some areas, with the loss in others.

If the cumulative losses are serious, there is only one outcome:
extinction.