Radford Neal wrote:
> Here's one possible scenario: Imagine that a man carries a highly
> detrimental gene on his Y chromosome (which, however, must obviously
> not be so detrimental as to be invariably fatal). This gene could
> cause high mortality for male embryos that he fathers, leading to most
> of his children being female.
>
> Another possibility: The man's X chromosome could carry a "meiotic
> drive" gene which (a) causes sperm carrying it produces a toxin, and
> (b) confers immunity to this toxin on the sperm carrying it. The
> result is that the sperm carrying the X chromosome kill off the sperm
> carrying Y chromosome, leading to all his children being female. Such
> meiotic drive genes have been observed in various animals, though not
> yet in humans, to my knowledge.
There would be heavy selection against the first of these; the second
would have no direct evolutionary advantage as it would be passed on with
the usual probability (50% from female single carrier, 100% from male or
from female double carrier) as a neutral trait on the X chromosome, so
Hardy-Weinberg equilibrium would apply. (The fact that the male carrier only
passes it on to female children is (perhaps surprisingly) irrelevant.)
There might of course be an indirect evolutionary advantage if the skewed
sex ratio were reflected in a polygamous culture, which could maintain a
higher birth rate. In a monogamous culture the birth rate would be reduced,
as there would be fewer females with male partners. However: a question
that I've wondered about for a long time is this: what mechanism prevents
"own-sex selection" traits?
A sex-linked mutation leading either to a male begetting mostly males
would be carried to the next generation about twice as often as would be
expected. This is, in the short term, an enormous evolutionary advantage. In
the long run, of course, it could lead to the extinction of the species, but
natural selection is not "intelligent" about long-term results and only
"learns" by experience. One can imagine an interesting SF plot based on the
discovery of a town in which the people are all perfectly normal except that
there are no girls among the 80% of the population descended from Old Grampa
Sloane, and the teenagers are all leaving town to look for romance...
Curiously, the "all-females" scenario doesn't work: if one X chromosome
of a female prevents the birth of male offspring, it will still be passed on
to 50% of her daughters, who will still only have one copy of it. So that's
the same rate at which a specific X chromosome would be inherited anyway.
So, the question is, what are the mechanisms that stop this from
happening? Did sexual reproduction develop many times before this bug was
removed, with males out of control? (I realize that there are
parthenogenetic insects and lizards, and fish that are almost all female but
occasionally change sex to provide one male in the school ("Now, I'll be
Frank..."); and I *think* I recall reading somewhere about a lizard or fish
or something that is always female and needs a male of another closely
related species to reproduce... but I may have imagined that one.)
Presumably, something about the reproductive process makes it very
robust against this sort of thing. But I don't have any idea what.
-Robert Dawson
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