Evolution__3rd_Edition

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Figure 14.11
Hybrid speciation by
introgression. The initial hybrid
individuals are interfertile with
one or both parental species
and backcross with them,
producing a hybrid population
with various mixtures of genes
from the two parental species.
At some stage, the hybrid
population may evolve far
enough to be reproductively
isolated fom the parental
species; it is then a new species.
From Rieseberg & Wendel
(1993).
Iris has also evolved new hybrid
species recently, but without
polyploidy
Figure 14.12
Where a bayou flows into
swamps in southern Louisiana,
the bayou-dwelling Iris fulva
may come into contact with the
swamp dwelling Iris hexagona.
In the intermediate regions, the
hybrid species I. nelsonii has
evolved by introgression.
Reprinted from Arnold &
Bennett (1993), after Viosca, by
permission of the publishers.
Species 1 Stabilized
introgressant
CHAPTER 14 / Speciation 407
Species 2
swamps and rivers, and Arnold and his colleagues have been using genetic markers to
reconstruct their origin. Plate 9a illustrates the three parental species in this example.
Two of them a Iris fulva, with tawny colored flowers, and I. hexagona, whose flowers
are colored in violet with yellow crests. Both species are widespread in streams in the
southeast. In southern Louisiana, I. fulva lives in water channels called bayous that are
derived from the Mississippi River, and I. hexagona lives in swamps and marshes.
Where a bayou happens to flow near a swamp, the two species may form hybrids (Figure
14.12), though at a low rate. A third parental species, I. brevicaulis, can also contribute.
I. brevicaulis is colored like I. hexagona but has a different growth habit; I. hexagona
grow up to 4 feet, whereas I. brevicaulis tend to lie flatter on the ground and curve
upwards. I. brevicaulis lives in drier habitats such as hardwood forest. These forests may
occur near bayous or swamps, and then hybrids may form from a mix of the three
parental species. Hybrids may also, to some extent, cross back to the parental species,
producing a complicated mix of genotypes in the populations where the species meet.
In the 1960s, a new species of iris was detected in the region where hybrids are found,
and was named I. nelsonii (Plate 9b). It has a morphology, including flower color,
Levee Bayou
Water table
Levee Swamp (ecotone) Freshwater marsh
Iris fulva
Iris hexagona
Hybrid