Evolution__3rd_Edition

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The sd gene gains an advantage for
itself, at a cost to the rest of the
body it is in
The sd gene illustrates
intragenomic conflict
CHAPTER 11 / The Units of Selection 295
The segregation distorter gene was first found in Drosophila stocks from Wisconsin
and Baja California. The gene is symbolized by sd, and we can call the other, more
normal, alleles at the locus “+”. A heterozygote for the segregation distorter is then
sd/+. The majority (90% or more) of offspring from male heterozygotes have the sd gene
because the sperm containing the + gene fail to develop. Female heterozygotes have
normal Mendelian segregation. (The segregation distorter gene in fruitflies is really a
pair of closely linked genes. However, we can discuss it as if it were a single locus: the
points of principle will be clearer that way and the facts not badly misrepresented.)
A segregation distorter gene can have a great selective advantage. The allele that gets
into more than half of a heterozygote’s offspring will automatically increase in frequency
and should spread through the population. Once the allele is fixed, the effect
would disappear, provided that segregation is normal in homozygotes. In the case of
the segregation distorter gene in Drosophila, however, other things are not equal. The
abnormal sperm are infertile, and the total fertility of male heterozygotes is accordingly
lowered. The fertility of an sd/+ male is about half that of a normal male. The effect of
the lowered fertility on selection at the sd locus is complex, and depends on whether the
reduction in fertility is more or less than 50% and what effect the reduced fertility has
on the number of offspring produced. There are, however, at least some segregation
distorter alleles that produce enough copies of themselves in heterozygotes to have an
automatic selective advantage. They produce more copies of themselves than would a
normal Mendelian heterozygote; their increase in frequency up to fixation is then
inevitable.
Segregation distortion sets up an interesting selection pressure in the rest of the
genome. On average, all other genes at other loci suffer a disadvantage because of
segregation distortion. In any case, one gene has only a 50% chance of getting into a
particular gamete because gametes are haploid whereas the individual is diploid. But
segregation distortion produces a further reduction of about 50% in the chance that
genes at other loci are passed on. In an sd/+ heterozygous fruitfly, a gene on another
chromosome has a 50% chance of entering an sd sperm, which will be fertile, and a 50%
chance of entering a + sperm, which will be infertile. Genes on other chromosomes
from the sd locus are all net losers. If they are in the same sperm as the favored sd allele,
things are normal; if they are in the shrivelled disfavored sperm, they die. Selection at
other loci will favor genes that suppress the distorters and restore the status quo.
When selection acts in conflicting ways on different genes in the same individual
body, it is called intragenomic conflict. The sd/+ fruitfly has intragenomic conflict,
because selection on the sd gene favors segregation distortion and selection on other
genes favors restoring normal segregation. Which genes win out can depend on many
factors, but the point of the example here is to show what it means for natural selection
to favor an adaptation that is the interest of a single gene (such as sd ) within a body.
11.2.2 Selection may sometimes favor some cell lines relative to other
cell lines in the same body
In organisms like ourselves, a new individual develops from an initial single-cell stage
and that single cell derives from a special cell line, the germ line, in its parents. This