Evolution__3rd_Edition

120 PART 2 / Evolutionary Genetics
. . . or rate of gene frequency
change between generations ...
. . . or other methods
genotypes have the same fertility. These assumptions can all be tested by further work.
For instance, survival can be measured at the other life stages too, and fertility can also
be assessed. In a few cases, lifetime fitnesses have been measured comprehensively, by
tracing survival and reproduction from birth to death.
The second method is to measure changes in gene frequencies between generations.
We then substitute the measurements into the formula that expresses fitness in terms
of gene frequencies in successive generations (equation 5.6). Both methods have been
used in many cases; the main problems are the obvious difficulties of accurately measuring
survival and gene frequencies, respectively. Apart from them, in the examples we
considered there were also difficulties in understanding the genetics of the characters: we
need to know which phenotypes correspond to which genotypes in order to estimate
genotype fitnesses.
We shall meet a third method of estimating fitness below, in the case of sickle cell
anemia (see Table 5.9, p. 126). It uses deviations from the Hardy–Weinberg ratios. It
can be used only when the gene frequencies in the population are constant between the
stages of birth and adulthood, but the genotypes have different survival. It therefore
cannot be used in the examples of directional selection against a disadvantageous gene
that we have been concerned with so far, because in them the gene frequency in the
population changes between birth and adult stages.
We have discussed the inference of fitness in detail because the fitnesses of different
genotypes are among the most important variables a perhaps the most important
variables a in the theory of evolution. They determine, to a large extent, which genotypes
we can expect to see in the world today. The examples we have looked at, however,
illustrate that fitnesses are not easy to measure. We require long time series and large
sample sizes, and even then the estimates may be subject to “other things being equal”
assumptions. Therefore, despite their importance, they have been measured in only a
small number of the systems that biologists are interested in. (That does not mean that
the absolute number of such studies is small. A review of research on natural selection
in the wild by Endler in 1986 contains a table (24 pages long) listing all the work he had
located. Fitnesses have only been measured in a minority a an unknown minority a of
those 24 pages’ worth of studies of natural selection, but the number could still be
non-trivial.) Many unsolved controversies in evolutionary biology implicitly concern
values of fitnesses, but in systems in which it has not been possible to measure fitnesses
directly with sufficient accuracy or in a sufficiently large number of cases. The controversy
about the causes of molecular evolution in Chapter 7 is an example. When
we come to discuss controversies of this sort it is worth bearing in mind what would
have to be done to solve them by direct measurements of fitness.
5.10 Natural selection operating on a favored allele at
a single locus is not meant to be a general model
of evolution
Evolutionary change in which natural selection favors a rare mutation at a single
locus, and carries it up to fixation, is one of the simplest forms of evolution. Sometimes
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