Encyclopedia of Evolution.pdf - Online Reading Center

0 angiosperms, evolution of
• Modern salamanders have legs yet many of them live
underwater. They use their legs for walking underwater on
rock surfaces against the current. It is possible that the first
legged amphibians evolved in rushing water.
• Early amphibians may have used their legs to drag themselves
around in shallow water, where they would be safe
from deep water predators.
• The shallow water in which early amphibians lived may
have been deficient in oxygen due to decomposition of leaf
litter. If the amphibians lifted themselves up and breathed
air, they could overcome this problem.
• Evolutionary biologist Robert A. Martin suggests that legs
assisted in clasping during sexual reproduction, a function
they still possess in many modern amphibians.
It is likely that legs proved useful for several different
functions over a long period of time. Whatever combination
of advantages may have selected for the evolution of legs, it
had to be something that worked in a primitive condition.
The earliest amphibians with legs could scarcely lift themselves
at all.
The evolution of limbs would not require the acquisition
of many new genes. Hox genes control the pattern of body
part development in most animals (see developmental evolution).
Some of these Hox genes (numbers 9–13) control
limb development in mice, from shoulder to feet. Analogs of
these genes are found in all vertebrates. Activation of specific
Hox genes can produce limbs; evolution would then work out
the structural details of these limbs, rather than produce them
from scratch.
While the arm and leg bones have analogs in some
fishes, the digits were an amphibian invention. The earliest
tetrapods had more than five digits (Ichthyostega had seven,
Acanthostega had eight). Though many tetrapods today have
fewer than five digits (horses, for example, have just one; see
horses, evolution of), all surviving tetrapods have a fivedigit
fundamental pattern.
Many fossil species that are intermediate between fishes
and modern amphibians have been discovered. In addition,
amphibians (as their name implies) are themselves intermediate
between fishes and fully terrestrial animals.
Further Reading
Benton, Michael. “Four feet on the ground.” In The Book of Life:
An Illustrated History of the Evolution of Life on Earth, edited
by Stephen Jay Gould, 79–126. New York: Norton, 1993.
Clack, Jennifer. Gaining Ground: The Origin and Evolution of Tetrapods.
Bloomington, Ind.: Indiana University Press, 2002.
———. “Getting a leg up on land.” Scientific American, December
2005, 100–107.
Martin, Robert A. “Fishes with fingers?” Chap. 10 in Missing Links:
Evolutionary Concepts and Transitions through Time. Sudbury,
Mass.: Jones and Bartlett, 2004.
Shubin, Neil H., et al. “The pectoral fin of Tiktaalik roseae and the
origin of the tetrapod limb.” Nature 440 (2006): 764–771.
angiosperms, evolution of Angiosperms (the flowering
plants) are one of the largest groups of plants, with at least
260,000 living species in 453 families. Angiosperms live in
nearly every habitat except the deep oceans. They range in
size from large trees to tiny floating duckweeds. Despite their
tremendous diversity, angiosperms are a monophyletic group,
which means that they all evolved from a common ancestral
species (see cladistics).
Most species of eukaryotes have life cycles in which meiosis
alternates with fertilization. Meiosis eventually produces
haploid eggs and sperm that fuse together during fertilization
(see Mendelian genetics). Plant life cycles differ from those
of animals, fungi, and most protists (see eukaryotes, evolution
of) by having a multicellular haploid phase. The multicellular
haploid structures produce eggs and/or sperm. The
haploid structures of the simplest land plants, which evolved
earliest, live in water or moist soil (see seedless plants,
evolution of). In seed plants, however, the male haploid
structures are pollen grains, which contain sperm and travel
through the air from one plant to another; and the female
haploid structures remain within the immature seed. Seeds
contain, feed, and protect embryonic plants. Angiosperms
and gymnosperms are the seed plants (see gymnosperms,
evolution of).
The shared derived features of angiosperms include flowers,
fruits, and double fertilization, which all angiosperms
(and no other plants) possess. Flowers produce fruits and
consist of the following parts:
• Sepals. These are leaflike structures that protect unopened
flowers
• Petals. These attract pollinators (see coevolution). In
some cases, the petals cannot be distinguished from the
sepals.
• Stamens. Pollen develops inside of anthers, each of which
has two pairs of pollen sacs. Filaments hold the anthers up
from the base of the flower. Stamens are the male component
of a flower.
• Carpels. Tissue of the female parent completely surrounds
the seeds during their development, forming a carpel. Pollen
grains attach to the stigma, which is a surface at the top of
the carpel; the immature seeds are protected within an ovary
at the base of the carpel. Carpels may be fused together into
pistils. The carpel and pistil tissue develops into a fruit,
which assists in the dispersal of the seed to a new location.
Carpels are the female component of a flower.
Not all flowers have all of these parts. A flower may lack
sepals or petals or both. A flower may have only stamens or
only carpels, rather than both.
The other unique feature shared by all angiosperms
is double fertilization, in which each pollen grain contains
two sperm nuclei. One of the sperm nuclei fertilizes the egg
nucleus, and the other fertilizes female polar nuclei that
develop into endosperm, a nutritive tissue inside the seed.
The earliest undisputed fossils of angiosperms date
back about 130 million years (see Cretaceous period).
Some researchers interpret a few earlier fossils to be those
of angiosperms. Unlike the dinosaurs, the angiosperms as a
group survived and recuperated from the asteroid impact of
the Cretaceous extinction, perhaps because their seeds