232 CHAPTER 9Data from Drosophila show exactly the expected pattern. In FIGURE 9.18,
the strength of prezygotic isolation between a pair of populations or spe-
cies is plotted against the genetic difference (or genetic “distance,” based
on molecular differences) between them. Prezygotic isolation here refers to
behavioral isolation when females’ choice between males of both species
was observed in laboratory tests. The genetic distance serves as a molecu-
lar clock, indicating time since the species diverged from their common
ancestor. Sympatric pairs of species show strong prezygotic isolation at
lower genetic distances than do allopatric pairs, meaning they evolved mat-
ing discrimination faster. This pattern is what we expect if the sympatric
pairs tended to hybridize shortly after speciation, and reinforcement then
strengthened their prezygotic isolation. Further analysis reveals additional
support for the role of reinforcement. Hybridization between some pairs
of sympatric species is asymmetric: the offspring from a cross between a
female of species A and a male of species B have lower fitness than those
from the reciprocal cross (female B × male A). In these cases, sexual isolation
is stronger in the female-male combination that produces less fit hybrids, as
we predict from the hypothesis of reinforcement [108].SPECiATion BY PoLYPLoidY When a diploid species’ entire genome
is doubled (see Chapter 4), the result is a tetraploid that has four copies
of every chromosome. Tetraploids originate by the union of two “unre-
duced” gametes—both carrying a full diploid set of chromosomes—that
are formed when chromosomes occasionally fail to segregate in meiosis. The
polyploid offspring is autopolyploid if both unreduced gametes come from the
same diploid species, and allopolyploid if they come from different diploid species.
If similar events happen in tetraploid species, offspring with even more sets of
chromosomes (e.g., eight in octoploids) result. The increased number of gene cop-
ies in polyploids changes the expression (e.g., amount of gene product) of many
genes, and alters many phenotypic traits [50, 67].
Tetraploids typically have complete reproductive isolation from their diploid
ancestors, and so are distinct biological species that have arisen in one step [79].
That is because hybrids between a diploid and a tetraploid are triploid: they have one
set of chromosomes from the diploid parent and two from the tetraploid parent (in
which four homologous chromosomes generally segregate two by two in meiosis).
Triploids are largely sterile [38], because their gametes are unbalanced: they have
one copy of certain chromosomes and two copies of other chromosomes. Genome
doubling is a large mutational event: one of the very rare situations in which muta-
tions of large effect make important contributions to evolution, in this case the ori-
gin of new species.
Speciation by polyploidy is rare in animals, but it is quite common in some
groups of plants. It accounts for about 15 percent of speciation events in flowering
plants [107], and all plants have a polyploid ancestor somewhere in their evolu-
tionary past [67]. Speciation by polyploidy has occurred even very recently. For
example, hybridization among three species of goatsbeards (Tragopogon) generated
new allopolyploid species within the last several centuries, after their accidental
introduction to North America from Europe (FIGURE 9.19).
How can a new tetraploid build up a population? There is a serious obstacle.
The tetraploid species starts out with just one or a few individuals, so often the
diploid ancestor is more abundant in its habitat. This can cause the tetraploids to
hybridize most often with the diploids, producing triploid offspring with low fit-
ness and pushing the tetraploid toward extinction. A new tetraploid might increase
if hybridization were reduced by self-fertilization, vegetative propagation, higher
fitness than the diploid, or habitat segregation from the diploid [27, 84]. ManyFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_09.18.ai Date 11-21-2016Strength of prezygotic isolation
0.5 1.0 2.0
Genetic distance0.200.40.60.81.0Sympatric taxaAllopatric taxaFIGURE 9.18 In Drosophila, the strength of
prezygotic isolation increases more rapidly be-
tween sympatric pairs of species (red dots) than
it does between allopatric pairs (blue dots). The
genetic distance between members of a pair is
an index of the time since divergence began.
At small genetic distances, many sympatric pairs
show strong isolation, but allopatric pairs do not.
(After [14].)09_EVOL4E_CH09.indd 232 3/23/17 9:36 AM