Evolution__3rd_Edition

310 PART 3 / Adaptation and Natural Selection
Genes are actively, not passively,
part of natural selection
The issues are scientifically
important
phenotypic change instead, such as individual learning or some developmental accident
in the lion’s nervous system. The thought experiment provides a test case between
the organismic, phenotypic and the genic accounts of evolution. In the genic case, we
know, natural selection favors the improved hunting type and the gene for it increases
in frequency. But what happens in the phenotypic case? The answer is too obvious to
labor over. The individual lion with improved hunting ability will survive and produce
more offspring than an average lion, but no evolution or natural selection, in any interesting
sense, will occur. The trait will not be passed on to the next generation. Natural
selection cannot directly work on organisms.
The change in gene frequency over time, therefore, is not just a passive “bookkeeping”
record of evolution. Genes are crucial if natural selection is to take place.
The need for inheritance, and the fact that acquired characters are not inherited, gives
the gene a priority over the organism as a unit of selection. Whenever a gene is being
selected, it produces a phenotypic change and the frequency of different organismal
types will change along with the gene frequency. But the change in organism frequency
is a consequence of the change in gene frequency: it is the gene frequency that natural
selection is actually working on and this is why Williams and Dawkins maintain that
the gene is the unit of selection.
Why does the argument matter? Its importance is to tell us what entities adaptations
exist for the good of. Evolutionary biologists work on particular characters (like
banding patterns in snails and sex), trying to work out why the characters exist. The
ultimate, abstract answer is that any adaptation exists because it increases the reproduction
of the genes encoding it, relative to that of the alleles for alternative characters.
The genes that exist in nature are the genes that in the past have out-reproduced
alternative alleles. Natural selection will always favor a character that increases the
replication of the genes encoding it.
It is important to know what the ultimate beneficiaries of adaptations are. When we
are trying to explain the existence of particular characters, we need to know whether a
proposed explanation is correct. The argument that genes are the units of selection
provides the fundamental logic that is used to find out. We imagine different genetic
forms of the character, and the correct explanation must specify how the genes for the
observed form of the character will out-reproduce other genetic types. In practice,
there may be several possible hypotheses, and they can be tested between using the
methods of Chapter 10, but before those methods are applied we have to insure that the
hypotheses make theoretical sense. We can rule a hypothesis about adaptation out
before the practical testing stage if it contradicts the theory of gene selection.
11.4 The two senses of “unit of selection” are compatible:
one specifies the entity that generally shows
phenotypic adaptations, the other the entity whose
frequency is generally adjusted by natural selection
We have now specified what the unit of selection is in two different senses. They have
sometimes been confused, but many evolutionary biologists now appreciate the distinction.
The two have been given names; Hull (1988), for instance, distinguishes
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