Evolution__3rd_Edition

406 PART 4 / Evolution and Diversity
Tragopogon has evolved new
hybrid species in the past century
Hybrid speciation may proceed via
introgression
Britain and two in North America. The latter two examples belong to the genus
Tragopogon. Tragopogon is an Old World genus, but three species have been introduced
to North America: T. dubius, T. pratensis, and T. porrifolius (whose common name is
salsify, and whose roots can be eaten as a vegetable). All three species are found together
in regions of east Washington and Idaho, and they all first became established there in
the first two or three decades of the twentieth century. By 1950, Ownbey discovered
that two new species had appeared in this region, T. mirus and T. miscellus. Both of
them continue to thrive, and samples taken 40 years later by Novak et al. (1991) showed
that T. miscellus had become a common weed of roadsides and vacant lots in and
around Spokane, Washington, and to the east.
Ownbey showed that T. mirus and T. miscellus (each with 12 pairs of chromosomes)
are tetraploid hybrids of pairs of the three introduced species (which are diploid and
have six pairs of chromosomes). The forms of the chromosomes in the species, as well
as other characters such as flower color, revealed that T. mirus is derived from T. dubius
and T. porrifolius, and T. miscellus from T. dubius and T. pratensis.
Ownbey found many interspecies hybrids in nature, but they were all diploid and
sterile. Presumably tetraploid mutants occur in the hybrids from time to time in
nature, and have given rise to the new species. The tetraploid hybrids are fertile, and
reproductively isolated from the parental species. Subsequent work has used more discriminating
genetic markers, and has shown that the new Tragopogon species have
originated more than once. The parental species have hybridized (and the hybrids then
tetraplodized) independently in different areas. The hybrids from the different origination
events are interfertile, and all belong to the same species. Soltis & Soltis (1999)
remarked that T. miscellus may have “originated” as many as 20 times, and T. mirus 12
times, in eastern Washington in the past 60–70 years.
The diploid hybrids of Tragopogon are sterile, and the origin of a new species could
not occur until a polyploid mutant arose. In other pairs of species, the initial hybrids
are partly interfertile with one or both parental species. The hybrids then backcross to
the parents; this gene flow from parental species into hybrid population is called introgression.
Many outcomes are possible from introgression, depending on the degree of
interfertility with the parents. Often, the hybrids and parental species interbreed to
some extent for a number of generations, and the hybrid population builds up a complex
mixture of the genes of the two parental species. At some point, the hybrid population
becomes reproductively isolated from the parental species. It has then evolved into
a new species (Figure 14.11).
Many cases of hybrid speciation in plants probably involve a number of generations
of introgression, rather than an instantaneous speciation event. The difference between
introgression and simple hybridization is that in introgression the new species will have
a complex mix of parental genes, according to the history of backcrosses between
hybrid and parental species during the origin of the new species, whereas in a simple
hybrid it has 50% of its genes from one parental species and 50% from the other.
Rieseberg & Wendel (1993) reviewed introgressive speciation in plants: they listed 155
cases in which it had been suggested, and they judged that the evidence for introgression
was good in 65 of them.
One of the best examples comes from the wetlands of south Louisiana (see Plate 9,
between pp. 68 and 69). There, a number of species of attractive irises grow in the
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