Biology Reference
In-Depth Information
flower calyces of plants tended to be hairier than those of plants in pas-
ture populations and to remain attached to the fruits after they fell from
the plant. This was correlated with higher seed germination, but lower
seedling survival, in the following season. In greenhouse experiments
under identical conditions, roadside plants also tended to flower earlier
and to produce about 22% more flowering heads.Thus, over a period of
only about 20 yr, evolutionary differentiation of pasture and roadside
populations of rose clover had occurred. Subterranean clover (
Tr ifolium
subterraneum
) in Australia, introduced both deliberately and inadvertently,
similarly shows a number of strains distinct from planted varieties (Cocks
and Phillips 1979). Some of these may have originated by interbreeding
and natural selection in Australia.
In California, slender wild oat (
Avena barbata
), an alien introduced
from Europe, has evolved distinct races. One race is adapted to the semi-
arid grasslands and oak woodlands of the Central Valley and lower
foothills and the other to the valleys of the coastal mountains and the
higher foothills of the Sierra Nevada Mountains (Clegg and Allard 1972).
Populations in the grassland and oak woodland habitat are characterized
by black, hairy lemmas and a series of five enzyme loci that are
monomorphic. Coastal mountain and higher Sierran foothill populations
show paler, less hairy lemmas and high levels of polymorphism for
enzyme loci.
Many other annual forbs and grasses show genetically based patterns
of adaptation to local habitat conditions, especially in characteristics
related to reproduction. In Europe, common groundsel (
Senecio vulgaris
)
shows differentiation of ruderal and cropland populations, with cropland
plants having larger leaves and more flowering heads (Leiss and Müeller-
Schärer 2001). Charlock mustard (
Sinapis arvensis
) and corn poppy
(
Papaver rhoeas
) exhibit population differences in seed dormancy (Garbutt
and Whitcombe 1986; Lane and Lawrence 1995). Canada thistle (
Cirsium
arvense
), a noxious weed in many temperate regions, also shows ecotypes
differing in phenology and disease resistance (Donald 1994). Among
annual grasses in North America, red brome (
Bromus rubens
) populations
differ in flowering phenology and seed mass in different habitats (Wu and
Jain 1978) and cheatgrass (
Bromus tectorum
) varies in flowering phenology,
seed mass, and the response of seed germination to temperature (Rice and
Mack 1991; Meyer and Allen 1999).
Forage plants, both forbs and grasses, have been transported worldwide.
These plants exhibit a complex pattern of genetic variation due to both
artificial and natural selection. Many artificially selected cultivars and
