Encyclopedia of Evolution.pdf - Online Reading Center

0 Devonian period
gene from an invertebrate chordate amphioxus was inserted
into its DNA.
Hox genes do not control the structure of the body parts
but makes them develop in their proper places. One example
of this is the Hox gene Antp. This gene normally controls the
development of legs in the thorax of flies. A mutation in this
gene can cause legs to grow out of the fly’s head, where the
antennae would have been, a mutation called antennapedia
(antenna, plus Greek pedia for foot). A missing Hox gene
product causes a structure, such as a leg, to misunderstand its
location in the developing animal. The normal gene, like the
mutation, does not affect the structure of the legs but their
location.
Homeotic genes other than Hox genes are essential to
the order of development in animal bodies. For example, the
sog gene in flies promotes the development of a nerve cord
along the bottom or front (ventral) surface, while the chordin
(chd) gene in vertebrates promotes the development of a
nerve cord along the top or back (dorsal) surface. Molecular
biologists E. M. DeRobertis and Y. Sasai reported in 1996
that these two genes were derived from a common ancestor.
Experiments have confirmed this. If RNA transcripts from
either a fly sog gene or a frog chd gene are injected into an
embryo, they induce the normal development of the nerve
cord: on the ventral surface in flies, on the dorsal surface in
frogs. Thus, it appears, the embryonic development of arthropods
is inverted with respect to that of vertebrates. This was
the first scientific confirmation of the theory presented by
Geoffroy St.-Hilaire in the early 19th century that vertebrates
developed as upside-down arthropods (see invertebrates,
evolution of; Cuvier, Georges).
Another homeotic gene (not a Hox gene) in flies is the
hedgehog gene. This gene got its name from a mutant form, in
which the fly was covered with prickles. The normal function
of the hedgehog gene is to control the front-to-back order of
development within each of the segments defined by the Hox
genes. When this gene was also discovered to be present in
vertebrates, it was given the name sonic hedgehog (shh). The
shh gene is involved in the development of vertebrate limbs.
It is expressed for a brief period in the development of fish
fins; its prolonged expression allows the development of legs
and feet in four-legged vertebrates (see amphibians, evolution
of). Within each of the vertebrate limbs, the genes are
expressed in order.
Other homeotic genes (not Hox genes) found widely
among animal genomes are the pax and tinman genes. The
first gene stimulates the development of eyes. The structure of
the eye is very different in flies and vertebrates, but both are
stimulated by versions of the same pax gene inherited from
an ancestor that may have had simple eye spots. The tinman
gene specifies the development of a heart, although the form
of the heart is different in each animal.
Most of the studies of MADS genes in plants have
involved the development of flowers from buds. Plants
retain embryonic tissues in their buds, which are free to
develop in different ways, like the stem cells of animals. A
bud resembles an embryo in its developmental possibilities.
Different MADS genes influence the development of differ-
ent flower parts: the sepals on the outside, the petals inside
of them, then the male stamens, and the female carpels
in the middle (see angiosperms, evolution of). Mutations
in the MADS genes can cause unusual developmental
patterns, such as the development of sepals and petals
where stamens and carpels would normally develop, or vice
versa. A single mutation of a homeotic gene in a flowering
plant can change the entire shape of the flower, which will
change its relationship with pollinators (see coevolution).
For example, a single mutation changes snapdragon-like
flowers (Linaria vulgaris), which are bilaterally symmetrical,
into the Peloria version, which is radially symmetrical.
Genes similar to the MADS genes can be found in plants
related to the ancestors of the angiosperms (see gymnosperms,
evolution of), even though these plants do not
have flowers.
Some biologists, such as the botanist Sonia Sultan, have
pointed out that it is not only necessary to understand the
genetic processes of development but also to understand the
ecological consequences of development (such as phenotypic
plasticity; see adaptation), in order to understand the process
of evolution. This approach has been called “eco-devo.”
Further Reading
Amundson, Ron. The Changing Role of the Embryo in Evolutionary
Thought: The Roots of Evo-Devo. Cambridge, U.K.: Cambridge
University Press, 2005.
Carroll, Sean B. Endless Forms Most Beautiful: The New Science of
Evo Devo and the Making of the Animal Kingdom. New York:
Norton, 2005.
Carroll, Sean B. The Making of the Fittest: DNA and the Ultimate
Forensic Record of Evolution. New York: Norton, 2006.
Coen, Enrico. The Art of Genes: How Organisms Make Themselves.
New York: Oxford University Press, 1999.
Davidson, Eric H., and Douglas H. Erwin. “Gene regulatory networks
and the evolution of animal body plans.” Science 311
(2006): 796–800.
DeRobertis, E. M., and Y. Sasai. “A common plan for dorsiventral
patterning in bilateria.” Nature 380 (1996): 37–40.
Freeman, Scott, and John C. Herron. “Development and evolution.”
Chap. 18 in Evolutionary Analysis. 3rd ed. Upper Saddle River,
N.J.: Pearson Prentice Hall, 2004.
Maynard Smith, John. Shaping Life: Genes, Embryos, and Evolution.
New Haven, Conn.: Yale University Press, 1998.
Shubin, Neal, C. Tabin, and Sean B. Carroll. “Fossils, genes, and the
evolution of animal limbs.” Nature 388 (1997): 639–648.
Zimmer, Carl. “A fin is a limb is a wing: How evolution fashioned its
masterworks.” National Geographic, November 2006, 110–135.
Devonian period The Devonian period (410 to 360 million
years ago) was the fourth period of the Paleozoic era
(see geological time scale). The first significant advances
of life onto land had occurred during the preceding Silurian
period. During the Devonian period, plants filled the wet
areas of the Earth, making large areas of the Earth’s surface
green for the first time.
Climate. The climate in the equatorial regions was warm
and wet. Portions of the southern continent, over the South