Encyclopedia of Evolution.pdf - Online Reading Center

gymnosperms, evolution of
began periods of extremely rapid growth, with the result that
human overpopulation has threatened many human societies
and the world as a whole. In recent decades, however, almost
every country in the world has experienced a decline in population
growth, as a result of education and economic development
rather than evolutionary changes. If the human species
avoids a devastating population explosion, it may well be the
first species that has ever limited its own population growth
by a nongenetic version of group selection.
Further Reading
Eldredge, Niles. Reinventing Darwin: The Great Debate at the High
Table of Evolutionary Theory. New York: John Wiley, 1995.
Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge,
Mass.: Harvard University Press, 2002.
Sober, Elliott, and David Sloan Wilson. Unto Others: The Evolution
and Psychology of Unselfish Behavior. Cambridge, Mass.: Harvard
University Press, 1999.
Wilson, David Sloan. Darwin’s Cathedral: Evolution, Religion, and the
Nature of Society. Chicago: University of Chicago Press, 2003.
gymnosperms, evolution of Gymnosperms constitute a
broad category that includes all extant seed plants that are
not flowering plants (see angiosperms, evolution of), as
well as many extinct forms. Seed plants (gymnosperms and
angiosperms) have the following characteristics:
• Vascular tissue. Vascular tissue consists of xylem and
phloem. Xylem tissue conducts water up from roots into
stems and leaves. Phloem tissue conducts water with sugar
and other organic molecules, usually down from the leaves
where the sugar is made (see photosynthesis, evolution
of) to the roots. Some seedless plants (see seedless
plants, evolution of) such as ferns also have vascular
tissue. Roots, stems, and leaves are defined partly by the
presence of vascular tissue. Mosses and their relatives do
not have vascular tissues and do not have true roots, stems,
or leaves. Because of vascular tissue, seed plants are well
adapted to life on dry land.
• Pollen. Pollen grains contain one or more sperm nuclei
inside a hard protein coat that protects the sperm and
other male cells while they are transported through the air
by the wind or by pollinators (see coevolution). In gymnosperms,
pollen grains usually develop inside of small
conelike structures. In seedless plants, sperm must swim
through layers of water from one plant to another. Because
pollen carries sperm through the air, seed plants are well
adapted to life on dry land.
• Seeds. Seeds contain an embryonic plant and a food supply,
surrounded by a protective coat. Because the embryo
is already partly grown, and has a food supply, a plant
can germinate from a seed and grow quickly, whereas
seedless plants must begin their growth from single spores
on the soil. In gymnosperms, seeds usually develop inside
of conelike structures. Seeds of gymnosperms (Greek
for “naked seed”) develop without being surrounded by
parental tissue.
The first gymnosperms evolved during the Paleozoic
era. The earliest gymnosperms were Elkinsia and Archaeosperma,
which appeared late in the Devonian period.
Gymnosperms remained a relatively minor component of
the forest vegetation during the Carboniferous period
when seedless plants grew to a very large size. During the
Permian period, as cooler and drier conditions spread,
the large seedless trees declined in abundance, and the forests
were dominated by gymnosperms. Among these gymnosperms
were the trees known as cordaites. Two modern
groups of gymnosperms, the conifers and the cycads,
evolved during the Permian period. Also common in the
late Mesozoic era were seed ferns (pteridosperms) that
were not ferns and were probably not related to modern
gymnosperms.
In the cooler, drier conditions of the Mesozoic era, gymnosperms
(especially conifers and cycads) dominated the forests
of the Triassic period and the Jurassic period. Many
of the conifers resembled the modern Norfolk Island pine,
which is not really a pine. The Wollemi pine, which is also
not really a pine, was thought to be extinct until it was discovered
in Australia in 1994. True pines and their relatives
evolved during the Mesozoic also.
Another group of Mesozoic gymnosperms was the bennettitalean
plants. The details of their leaf and wood anatomy,
as well as of their reproductive structures, suggest that
they may have been the ancestors of the flowering plants.
Some bennettitalean plants had female conelike structures
surrounded by male reproductive structures inside of bracts,
which is very similar to the structure of a flower. When the
flowering plants evolved at or before the beginning of the
Cretaceous period, they displaced the gymnosperms from
the tropical areas.
Modern gymnosperms spread during the Cenozoic era
to their present habitats. Modern gymnosperms include these
groups:
• Conifers. Most conifers have stiff needle-like or scale-like
evergreen leaves. Some primitive conifers, such as the podocarps
of New Zealand and South America, and the auracarians
of Chile, are found in the Southern Hemisphere.
Most modern conifers are junipers, cypresses, pines, spruces,
firs, hemlocks, and larches. Pines evolved during the
Mesozoic era but have spread extensively since, especially
in cool regions with poor soils.
• Cycads. Cycads look like small palm trees.
• Ginkgoes. There is only one modern species of ginkgo (the
maidenhair tree, Ginkgo biloba) which closely resembles
ginkgoes that lived millions of years ago (see living fossils).
This species may have been saved from extinction
by Chinese monks who grew them in monasteries.
Ginkgoes have leaves with veins that diverge rather than
branching out. They produce seeds singly on stalks rather
than inside of cones.
• Gnetales. This is a loosely defined group of plants with
gymnosperm reproductive structures, but some of which
have thin, flat leaves that resemble those of flowering