Encyclopedia of Evolution.pdf - Online Reading Center

0 convergence
bee-pollinated flowers into red, hummingbird-pollinated
flowers and resulted in an immediate reproductive isolation
of two populations, resulting in the evolution of two
separate species (see isolating mechanisms; quantitative
trait loci).
• Different lineages of plants have evolved adaptations for
wind pollination, in which the petals are reduced in size or
lost, pollen production is increased, and the pollen-capturing
surfaces of the flowers increase in size.
Plant defenses. Plants have evolved convergent adaptations as
defenses against herbivores.
• Spines and thorns have evolved many times in separate lineages
of plants. Some plants have seeds with sharp spines
that entangle animal fur, causing the animal to disperse the
seeds to a new location. Strikingly similar structures for ani-
T
Monocot and dicot angiosperms have had separate lineages of evolution
for about 00 million years. A monocot (the sandbur grass, genus Cenchrus)
(labeled C) and a dicot (the puncture vine, Tribulus terrestris) (labeled
T) have independently evolved spines around their seeds that puncture
animal skin and entangle animal fur, allowing the seeds to be carried to
new locations. (Photograph by Stanley A. Rice)
C
mal fur dispersal have evolved in Tribulus terrestris, a dicot,
and genus Cenchrus, a grass (see photo at left). Grasses
and dicots evolved from a common ancestor, without spiny
seeds, that lived over a hundred million years ago.
• The use of latex (a sticky, milky sap) as a defense against
herbivores and as a way of sealing damaged areas has also
evolved separately in several plant families. As a result,
rubber latex is produced by a spurge (Hevea brasiliensis),
a fig (Ficus elastica), and the dita bark tree Alstonia in the
dogbane family.
• Chemical defense in plants has evolved thousands of times
(see coevolution). Usually, each lineage of plant uses a different
set of chemicals, which is why the plant kingdom produces
such an astounding variety of pharmaceutical and spice
substances. Some plant chemicals such as limonene reappear
over and over, producing the lemon scent separately in lemons
(a citrus), lemon basil and lemon balm (mints), lemon
grass, and the conifer Thuja standishii. Onion scent has
evolved separately in onions (Allium cepa), which are monocots,
and in the salmwood tree, a dicot (Cordia alliadora).
Other examples of convergent evolution in plants include the
following:
• Parasitism has evolved separately in several lineages of
flowering plants. These plants have lost their chlorophyll
and obtain food by growing on other plants. Examples
include plants in the heath family (genera Pterospora and
Monotropa), relatives of snapdragons (genus Epifagus),
and plants in the orchid family (genus Corallorhiza).
• Within the genus Calochortus, the same flower shapes
appear to have evolved in several different lineages: the
cat’s ear form, which is hairy and has open petals; the mariposa
form, which is cuplike; the fairy lantern form, which
hangs down, with the petals together; and the star tulip
form, with smooth spreading petals.
• At least three lineages of plants have separately evolved
mutualisms with bacteria in their roots that convert nitrogen
gas into nitrogen fertilizer: the legume family Fabaceae,
alders (in the birch family Betulaceae), and Vitex keniensis
(in the verbena family Verbenaceae). The bacteria
involved in these mutualisms are different: Legumes have
coevolved with bacteria such as Rhizobium, while alders
have coevolved with Frankia bacteria.
• Strikingly similar leaf forms, so similar as to be almost
indistinguishable except by experts, have evolved in Jacaranda
mimosifolia (family Bignoniaceae) and the mimosa
(Albizzia julibrissin of the Mimosoideae).
Animal Convergences
Eyes. Eyes have evolved several times in the animal kingdom.
• Among the earliest eyes were the crystalline eyes of trilobites,
which evolved separately from the compound eyes
found in other arthropods and the calcitic eyes of certain
echinoderms.
• Most arthropods have compound eyes, but some spiders
(the jumping spiders that rely on acute vision) have a lens