Biology Reference
In-Depth Information
lated daughter species is promoted by geographical isolation and diver-
gent selection on different islands of the archipelago.These finches are tra-
ditionally considered to comprise 6 genera and 14 species. Selection has
favored divergence in feeding behavior and morphology in these birds.
Recent studies show, however, that the six species of ground finches
(
Geospiza
spp.) and the five tree finches (
Camarhynchus
spp.) are subject to
frequent hybridization and are still incompletely differentiated.Their sta-
tus is perhaps best characterized as an adaptive radiation in progress (Sato
et al. 1999; Zink 2002).
Speciation in Plants
Allopolyploidy is a frequent mechanism of origin of new plant species,
and several cases involving alien species have been documented.The
Tr ago-
pogon
example described at the beginning of the chapter stands as one of
the best examples of rapid speciation by hybridization and allopolyploidy.
As we noted in chapter 5, the origin of
Spartina anglica
on the coast of
England occurred by essentially the same mechanism. By a somewhat
more complicated process, a new species of groundsel (
Senecio
) originated
by hybridization and doubling of the hybrid chromosome number, as dis-
cussed below.
Autopolyploidy also leads to speciation of plant populations invading
new regions.The groundnut (
Apios americana
) is a plant native to eastern
North America. Populations in the southern United States are diploid in
chromosome number and reproduce sexually. Farther north, those in areas
covered by Wisconsin glaciation are triploid and reproduce vegetatively.
Tr iploid forms, differing in flower color, appear to have arisen both east
and west of the Appalachian Mountains and invaded glaciated regions
after glacial retreat (Joly and Bruneau 2002). Although diploid and
triploid forms are not classified as separate species, they are reproductively
isolated.
A similar example involves
Tolmiea menziesii
, an herbaceous plant of
the saxifrage family (Saxifragaceae). Two chromosomal races of
To l m iea
menziesii
are known, a diploid with 14 chromosomes and a tetraploid with
28 (Soltis and Rieseberg 1986). These races are morphologically identi-
cal, although the tetraploid tends to be more robust in growth form.The
tetraploid occurs on Wisconsin glaciated areas from southern Alaska
southward to central Oregon, whereas the diploid occurs in unglaciated
areas from central Oregon south into northern California. Tetraploids
show much higher levels of heterozygosity than diploids (Soltis and Soltis
