Evolution__3rd_Edition

..
*
*
The word “gene” is being used in a
technical sense
Criticisms have been made
B
b
b'
b"
*
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CHAPTER 11 / The Units of Selection 309
Figure 11.4
When intragenic recombination happens between two alleles with
different heterozygous nucleotides, it breaks the gene structure.
The * indicates where the nucleotide sequence differs from the
allele. The gene sequences coming out of recombination differ
from the initial sequences.
p. 210) and builds up linkage disequilibrium between genes. The same will be true of
loci further down the DNA from the selected locus. The hitch-hiking effect is gradually
reduced with distance by recombination, but there is no clear cut-off. This poses
no problem for Williams’ definition of the gene. The neighboring allele that is hitchhiking
with the selected mutation is, in Williams’ definition, part of the gene that is
having its frequency altered.
Williams’ “gene” has a statistical reality, because shorter lengths of DNA are more
permanent and longer lengths are less permanent. The random hits of recombination
will generate a frequency distribution of genome lengths lasting for different periods of
evolutionary time. The average length that survives long enough for natural selection to
work on has been defined by Williams and Dawkins as the gene. Population geneticists
have scolded them from time to time for assuming a one-locus, zero linkage disequilibrium
view of evolution, but the dispute is a matter of definition, not substance. The
critics are identifying “gene” with “cistron.” It would be interesting to know whether
the gene in Williams’ sense is also a physical cistron; but it is a secondary question and
has nothing to do with the fundamental logic of Williams and Dawkins’ argument.
We must discuss one further matter before considering the significance of the genic
unit of selection. Critics, such as Gould (2002b), have objected that gene frequencies
change between generations only in a passive, “book-keeping” sense. The frequency
changes provide a record of evolution, but are not its fundamental cause. True natural
selection, the critics would say, happens at the level of organismic survival and reproduction.
The actual selection in the lion example happens when a lion catches, or fails
to catch, its prey. The differential hunting success drives the gene frequency changes,
and it is a mistake to identify the gene frequency changes as causal. Williams and
Dawkins, however, do not deny that whatever ecological processes are causing differential
organismic survival produce gene frequency changes within a generation. What
they deny is that this ecological interaction of organisms means that natural selection
directly adjusts the frequencies of organisms over the evolutionary timescale of many
generations.
There is an easy philosophical method of deciding whether natural selection
works on genes or larger phenotypic units. We can consider a phenotypic change
such as a new hunting skill, and ask whether natural selection can work on it if it is produced
genically and if it is produced non-genically. The case we discussed above was
genic: the advantageous new hunting behavior was caused by a genetic mutation. Now
suppose that the same advantageous phenotypic change was caused by a non-heritable