Encyclopedia of Evolution.pdf - Online Reading Center

isolating mechanisms
from one another, these two groups of plants have apparently
started on the road to separation, perhaps eventually into two
different species.
Natural selection often reinforces the dependence on different
methods of pollination. For example, flowers that have
characteristics intermediate in their appeal to hummingbirds
and to bumblebees will probably not be pollinated very well
by either hummingbirds or bumblebees.
Differences in timing of reproduction. Each species has
its own reproductive season. Mutations that cause some
individuals in a population to have a different reproductive
season will prevent those individuals from interbreeding
with others. Early in the evolution of alder trees (genus
Alnus), populations diverged into some that pollinated in
the autumn (subgenus Clethropsis) and those that pollinated
in the spring (subgenus Alnus). These subgenera can no longer
interbreed. In North America Rhagoletis pomonella flies
whose maggots ate wild hawthorn berries, some populations
evolved a preference for apples. An immediate reproductive
isolation occurred because apple fruits mature three to four
weeks earlier than those of hawthorns, with the result that
the flies that hatch from apples are ready for reproduction
weeks earlier than those that hatch from hawthorns.
Differences in animal mating behavior. Animal populations
may be reproductively isolated because of differences in
mating rituals. Evolutionary biologists Peter and Rosemary
Grant have extensively studied the beak sizes and mating preferences
of several species of Darwin’s finches on some uninhabited
islands in the Galápagos Islands. They have found
that hybrids between finch species can occur, but occur rarely.
Young male birds learn their mating songs from their fathers,
and females use male mating songs to guide their choice of
mates. Occasionally, a young male bird overhears and learns
the song of a different species of finch nearby. When the
young male bird grows up and sings, he attracts females of the
wrong species.
Hawaii has hundreds of species of fruit flies (genus Drosophila),
all of them unique to the Hawaiian Islands, and all
of them having recently evolved there (the islands are from
five million to one-half million years old). There may be
scores of species living on a single island within the archipelago
and close enough to one another to readily interbreed.
They do not interbreed, however, because they have evolved
different mating dances. The females of one species will not
recognize the males of another species as being potential
mates. Laboratory investigations of fruit flies (some species of
which are the most intensively studied animals in the history
of biology) indicate that small genetic mutations can cause
major changes in mating behavior. Therefore the reproductive
isolation between two groups of flies may have occurred
quickly after a relatively minor genetic change.
Subsequent to the reproductive isolation caused by differences
in mating dances in the Hawaiian flies, the populations
evolved yet other differences, making them into recognizably
different species. The species now differ in the anatomy of
mating and reproductive anatomy, and other characteristics
that make them look different to one another (for example,
one species has its eyes on long stalks). These anatomical dif-
ferences between species may or may not have anything to do
with adaptation to their environments.
Specialization on different food resources. Specialization of
animals on different food resources will result in reproductive isolation
only if individuals crossbreed primarily with other individuals
that have the same specialization. The most famous examples
of this are of insect populations within the same species that specialize
on new food plants. They not only eat the plants but live
and mate upon them, isolated from the insects on a different kind
of food plant. Rhagoletis pomonella flies (see above), native to
North America, lay their eggs on the fruits of hawthorns, a wild
bush that produces fruits that resemble small crabapples. When
Europeans and white Americans began planting apples in eastern
North America, some populations of this fly species began to lay
their eggs in apples. The populations of flies that lived in apple
trees could potentially interbreed with the populations that lived
on hawthorns but seldom did so, even if wild hawthorns and
cultivated apples were side by side. After a couple of centuries
of isolation, the apple flies and the hawthorn flies will no longer
interbreed even when given the opportunity to do so in the laboratory.
This process was observed by naturalist Benjamin Walsh,
an American acquaintance of Charles Darwin, in the 19th century,
and was more fully investigated in the 20th century.
Another example of reproductive isolation occurring
on newly introduced plant species involves the soapberry
bug. Soapberry bugs have snouts about 0.36 inch (9 mm) in
length that penetrate the fruit and consume contents from the
seeds of balloon vines, native to the southern United States.
However, in southern Florida, golden rain trees have been
imported from Asia and used as ornamentals in the vicinity
of balloon vines. Golden rain tree fruits are smaller, thus a
smaller snout is needed to penetrate it and get to the seed.
Some populations of soapberry bugs have specialized on
golden rain trees and seldom interbreed with the populations
that still live on balloon vines. The populations of bugs living
on the golden rain trees have begun to evolve shorter beaks,
about 0.28 inch (7 mm) in length. This process occurred
recently, as golden rain trees were not extensively planted in
Florida until the 1950s. In another example, fruit flies that
resulted from the hybridization of a species of fly that lives
and reproduces on blueberries and a species that lives and
reproduces on snowberries have become reproductively isolated
from both parental species by living on Japanese honeysuckle,
which has been in North America for less than three
centuries (see invasive species).
Sometimes the isolation of populations on different
food resources can result from just a small number of genetic
changes. Two species of flies in the Seychelles archipelago
of the Indian Ocean are Drosophila simulans and Drosophila
sechellia. The first species avoids the poisonous morinda
fruit, while the second species is attracted to it, for laying
their eggs. Researchers found that the species differed by only
a small number of genes—just enough to produce tolerance
of and attraction to morinda fruits in D. sechellia.
Examples are not, however, limited to insects that live
on and eat plants. Marine stickleback fishes invaded freshwater
lakes of British Columbia after the most recent of the ice
ages. Some of them had a tendency, apparently genetically