■■Natural selection is any consistent difference in
fitness among different phenotypes or geno-
types. Evolution caused by natural selection has
been observed directly many times.
■■f tness is the number of offspring that an individ-i
ual leaves to the next generation, or the average
number that an allele, genotype, or phenotype
leaves. Selection causes evolution when there is
a correlation between a phenotype and fitness,
and a correlation for that phenotype between
parents and offspring.
■■The rate at which a beneficial allele spreads
through a population is determined by how
strongly it is favored, measured by its selection
coefficient, and by the amount of genetic varia-
tion at that locus in the population. Even if it is
favored by selection, a new beneficial mutation
can be lost by chance when it is still rare.
■■An allele that has no effect on fitness can spread
if it is associated (in linkage disequilibrium) with
an allele at another locus that is favored by selec-
tion. one consequence is that a selective sweep
reduces genetic variation in the region of chro-
mosome near the selected locus.
■■Several kinds of selection can act to maintain
genetic variation. one is overdominance, the
situation in which heterozygotes have high-
est fitness. other kinds are negative frequency
dependence, multiple niche polymorphism, and
spatial variation in selection.■■Positive frequency-dependent selection occurs
when fitness increases with the frequency of
genotypes or phenotypes in a population. un-
like balancing selection, this situation eliminates
variation. Which allele becomes fixed depends
on the initial allele frequency.
■■With underdominance, heterozygotes have
lower fitness than both homozygotes. under-
dominance does not preserve genetic variation,
and one of the alleles will either be lost or spread
to fixation, depending on its initial frequency. An
allele that is underdominant can spread when
rare only if some evolutionary factor other than
selection is at work.
■■The mean fitness of a population evolves as al-
lele frequencies change. fisher’s fundamental
theorem of natural selection states that selection
causes the mean fitness to increase. in Wright’s
adaptive landscape, allele frequencies change in
the direction that increases mean fitness. These
conclusions hold only under certain conditions.
Evolution can cause a population’s mean fitness
to decrease when fitnesses are frequency-de-
pendent.
■■Deleterious mutations occur frequently. They
are maintained in populations by mutation even
though selection acts to remove them. Their
combined effects across the entire genome con-
tribute to senescence.TERMS AND CoNCEPTS
absolute fitness
adaptive landscape
artificial selection
balancing selection
deleterious mutation
evolutionary trade-
off
fitness
fitness componentfixed, fixation
frequency-
dependent
selection
fundamental
theorem of natural
selection
genetic correlation
hitchhikinglinkage
disequilibrium
mean fitness
multiple niche
polymorphism
mutation load
overdominance
polymorphic
equilibriumpositive selection
purifying selection
relative fitness
selection coefficient
selective sweep
standing genetic
variation
underdominanceSuGGESTioNS foR fuRTHER READiNG
Many of the concepts discussed in this chapter
were first explored by the three people who
pioneered the subject of population genet-
ics in the early twentieth century: R. A. fisher,
Sewell Wright, and J. B. S. Haldane. The title of
this chapter is taken from that of a book writ-
ten by fisher in 1930 (The Genetical Theory ofNatural Selection, oxford university Press, ox-
ford). it is a brilliant but difficult work that is still
an inspiration for researchers. W. B. Provine’s
The Origins of Theoretical Population Genetics
(university of Chicago Press, Chicago, 1971) is a
wonderful history of the period leading up to
the rise of population genetics.SuMMARy
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