How To BE FiT 291
individuals that develop on the abundant plant but not of individu-
als that develop on the rare plant (FIGURE 11.19A). A mutation at
another locus, B, improves fitness of individuals that develop on
the rare plant but not on the common plant. Both mutations are
advantageous, but the mutation at locus A increases in the whole
population faster than the mutation at locus B, because the abun-
dant plant supports and produces more moths. So although the
average fitness in both habitats increases, adaptation to the abun-
dant plant will increase faster than adaptation to the rare plant
(FIGURE 11.19B). There will therefore be selection for mutations
at other loci that increase the insects’ preference for the abundant
plant, because offspring of females that lay their eggs on that plant
will be more likely to survive (FIGURE 11.19C). A specialized popu-
lation will evolve [21, 26].
Moreover, as the population becomes more and more lim-
ited to the majority habitat, mutations that disable adaptations to
the minority habitat become nearly neutral and may increase by
genetic drift, resulting in mutational decay [31]. For this reason, the
ability to feed on less common plants may be lost. Due to the accu-
mulation of mutations, fitness-related traits may display greater
variation among individuals reared in the minority environment
than in the majority environment [61].Experiments on niche evolution
Many investigators have compared genetic changes in laboratory
populations, especially of bacteria, in constant and variable envi-
ronments [30]. Populations of bacteria maintained in variable envi-
ronments evolve broader niches, both by the evolution of generalist
genotypes and by the maintenance of diverse specialized geno-
types. In contrast, specialist genotypes usually become prevalent
in constant environments. Under these conditions, both negative
correlations in fitness and mutational decay can occur. In popula-
tions of E. coli that were propagated in Richard Lenski’s laboratory
for 20,000 generations with glucose as the only energy source, the
ability to metabolize other substrates declined, apparently because
of antagonistic pleiotropic effects [13]. In another study, an unused
character, swimming motility, declined in Pseudomonas fluorescens
bacteria at a rate that depended on how well fed the bacteria were.
When resources were plentiful, mutations that degraded motility
were selectively neutral, but when resources were limiting, such
mutations were advantageous, probably because less active bac-
teria saved energy [25]. Thus, unused features may be lost either
by selection or by mutation and genetic drift, depending on the
population’s resources.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_11.19.ai Date 11-22-2016FitnessHabitats or
resources
X
Y(A)(B)A 1 A 1 A 1 A 2 A 2 A 2 A 1 A 1 A 1 A 2 A 2 A 2FitnessB 1 B 1 B 1 B 2Time (generations)B 2 B 2Mean tness (—w)B 1 B 1 B 1 B 2 B 2 B 2—w
x—w
y(C)Time (generations)10Frequency of
preference alleleFIGURE 11.19 Adaptation and the evolution of specializa-
tion in a population that inhabits a common environment
(X) and a rarer environment (Y). (A) Variation at locus A and
locus B affects fitness in environment X and environment Y,
respectively. Equal selection coefficients are assumed for
both loci. (B) Because the more abundant environment X
contributes more individuals to the population in each gen-
eration, selection increases the frequency of allele A 2 faster
than the frequency of B 2 , so mean fitness in environment X
(w–X) increases faster than mean fitness in environment Y (w–Y).
(C) Because the population has become better adapted to
environment X than environment Y, selection increases the
frequency of an allele that inclines individuals to prefer to
use environment X.Go to the
Evolution Companion Website
EVoLUTioN4E.SiNAUER.CoM
for data analysis and simulation exercises,
quizzes, and more.11_EVOL4E_CH11.indd 291 3/22/17 1:11 PM