SPECiES And SPECiATion 225
rearrangements are inversions and reciprocal translocations (see p. 90). Especially
in the case of translocations, heterozygotes have reduced fertility compared with
homozygotes for either the original or the derived (new) arrangement. For this rea-
son, populations with different chromosome arrangements are nearly or entirely
monomorphic, and may form narrow hybrid zones where one “chromosome race”
meets and interbreeds with another (FIGURE 9.14). The fertility of heterozygotes
for chromosome rearrangements may be low either because the rearrangements
carry different alleles that create Dobzhansky-Muller incompatibilities, or because
of mispairing of chromosomes in meiosis produces gametes that lack certain chro-
mosome regions.How fast does reproductive isolation evolve?
The time required for reproductive isolation to become strong, after it has started
to evolve, varies greatly. The origin of a new species by polyploidy, which is espe-
cially common in plants, requires only one or two generations (see p. 232). IfFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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Evolution4e_09.13.ai Date 01-23-20170.450.40
Parental
linesFemale
(population A)Male
(population B)F 1
hybridF 3
hybridPaternal
backcrossMaternal
backcross0.500.550.600.65SurvivorshipAu: Should label “F 3 hybrid” be “F 2 & F 3 hybrid”? This might be a good gure to insert
a balloon caption to explain a key point in the graph?FIGURE 9.13 Crosses show that the low fitness
of hybrids between populations of the cope-
pod Tigriopus californicus is caused by a genetic
mismatch between mitochondrial and nuclear
genes. Maternally inherited mitochondria (circles)
and nuclear chromosomes inherited from both
parents (rods) of populations A and B are colored
red and blue, respectively. Crosses produce F 1
hybrids with population A mitochondria. These F 1
offspring have slightly higher survival, showing
“hybrid vigor.” Crosses then produce F 2 and F 3
hybrids, with recombined nuclear genes. The pa-
ternal backcross is produced by mating F 3 females
with population B males. These offspring have
low fitness, because most of the nuclear genes
come from population B and are mismatched
to the mitochondrial genes from population A.
In contrast, offspring of the maternal backcross,
in which most of the nuclear genes come from
the same population as the mitochondria, have
normal, high survival. (After [9].)Futuyma Kirkpatrick Evolution, 4e
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Troutt Visual Services
Evolution4e_09.14.ai Date 01-23-2017Frequency of Novosibirsk type0.81 o g k i h n m p
o g k i h n m p(A) (B)0.410 20
Transect distance (km)Novosibirsk Hybrid Tomsk50 150.60.20FIGURE 9.14 wo “chromosome races” T
of the common shrew (Sorex araneus)
form a very narrow hybrid zone in Siberia.
(A) The Novosibirsk and Tomsk races dif-
fer by the fusion of some single-armed
chromosomes (e.g., o and p in Tomsk) into
double-armed chromosomes (e.g., o and
g in Novosibirsk). In meiosis in hybrids, the
multiple rearrangements cause a chain of
nine chromosomes to form, and irregular
segregation produces many unbalanced
gametes and low fertility. (B) A transect
from Novosibirsk to Tomsk shows a cline in
the frequency of the Novosibirsk chromo-
some arrangement less than 9 km wide.
The chromosome configuration of either
race cannot increase within populations of
the other race, probably because meiosis
in F 1 hybrids produces gametes that lack
some chromosomal regions. (A after [73];
B after [74].)09_EVOL4E_CH09.indd 225 3/23/17 9:36 AM