Biology Reference
In-Depth Information
(
Acyrthosiphon pisum
), which was introduced to North America from
Europe in the late 1800s (Via 1999). In North America, the pea aphid is
considered a pest of alfalfa (
Medicago sativa
) and red clover (
Tr ifolium
pratense
).Via (1999) showed that populations of aphids on these two for-
age plants are genetically divergent and that gene flow between them is
reduced. She concluded that incipient barriers to successful interbreeding
exist and that alfalfa and red clover biotypes of pea aphids are on the road
to becoming reproductively isolated species.
Divergence of host-related populations of crop insect pests is probably
not unusual. In France, for example, the European corn borer (
Ostrinia
nubilalis
) has evolved host races on mugwort (
Artemisia vulgaris
), a wild
plant, and maize (
Zea mays
), a crop plant native to the Americas (Martel
et al. 2003). The frequency of interbreeding between the two forms
appears to be less than 1%. Several factors apparently contribute to the
strong genetic isolation of the two forms (Thomas et al. 2003). Popula-
tions on mugwort complete their larval development and emerge about
10 days earlier than those on maize, so the chances of mating between the
two forms are greatly reduced. Sex pheromones produced by the two
races also differ, reinforcing the tendency for mating of individuals of the
same race. Selection for populations on maize also appears to be related
to an enemy-free environment. A parasitoid wasp usually kills more than
50% of corn borer larvae overwintering on mugwort but does not attack
larvae overwintering on maize plants.
Over the longer term, good evidence exists for full speciation by these
sorts of processes. When Polynesian colonists reached Hawaii in about
400 CE, they brought with them a variety of food plants, including
banana (
Musa paradisiaca
) and coconut palm (
Cocos nucifera
). Zimmerman
(1960) noted that since the arrival of these plants, five species of moths of
the genus
Hedylepta
have evolved to use banana and one to use coconut
palm. This genus of moths includes 23 species endemic to Hawaii; the
remaining 17 species feed primarily on various monocot plants. The
ancestral form of
Hedylepta
apparently used a native Hawaiian palm of the
genus
Pritchardia
.
Animals occupying insular environments of relatively recent origin or
recent colonization demonstrate that speciation can easily occur within a
few thousand years. Many examples of this process have been docu-
mented for fish in lakes and coastal lagoons in regions covered by the last
Pleistocene glaciers (Schluter 1997, 2000). Many of these lakes contain
pairs or trios of closely related species that show character displacement
