Evolution__3rd_Edition

308 PART 3 / Adaptation and Natural Selection
Figure 11.3
(a) Three genes along a
chromosome. Loci A and B are
heterozygous. The * indicates
where the nucleotide differs
from the other allele. Now
consider the effect of
recombination on the structure
of the gene at the B locus.
(b) Intragenic recombination
does not affect the structure of
the A genes; (c) nor does
recombination in the
neighboring B/b locus. The
same pair of A and a gene
sequences come out of
recombination as were
present before.
The longevity of a genetic unit
matters relative to the time
evolutionary change takes
(a)
A B C
(b) (c)
a b C
(b) (c)
*
A b C
*
* *
A b C
a B C a B C
(Figure 11.3c). Recombination within the gene does not usually alter the outcome
(Figure 11.3b). If the locus was homozygous before the mutation, all the gene except
for the mutant base pair will be identical in the original and mutant forms. Intragenic
recombination therefore produces exactly the same result within the gene as no recombination;
it only alters the combinations of genes.
Intragenic recombination can destroy the heritable information in a gene in one
special circumstance. If the locus was heterozygous before the mutation and recombination
occurs between the mutant site and the other site that differs between the
two strands, the products of recombination differ from the initial strands (Figure 11.4).
Clearly, this could happen. When it does, the length of DNA whose information is
inherited is shorter than a gene. For this reason, if we take a long enough view, the only
finally permanent units in the genome are nucleotide bases; recombination does not
alter them. However, this long view is of little interest. We are concerned with the
timescale of natural selection. It takes a few thousand generations for a mutation’s
frequency to be significantly altered (Section 5.6, p. 107) and, over this time, genes, but
not genomes or phenotypes, will be practically unaltered. Genes will then act as units of
selection and will be permanent enough to have their frequency altered by natural
selection.
Williams defined the gene to make it almost true by definition that the gene is the
unit of selection. He defined the gene as “that which segregates and recombines with
appreciable frequency.” The gene in this definition need not be the same as a cistron
(i.e., the length of DNA encoding one protein, or polypeptide). It is instead the length
of chromosome that has sufficient permanence for natural selection to adjust its frequency:
longer lengths are broken by recombination and shorter lengths have no more
permanence that the gene (for the reason shown in Figure 11.3). The gene in Williams’
definition is what Dawkins calls the replicator. In practice, the replicator (or Williams’
gene) does not consistently correspond to any particular length of DNA.
When selection is taking place at one locus, a cistron at a neighboring locus will to
some extent (depending on the amount of recombination) have its frequency adjusted
as a consequence. In a population genetic sense, this is hitch-hiking (Section 8.9,
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