Encyclopedia of Evolution.pdf - Online Reading Center

that the human embryo actually goes through a fish stage.
Recapitulation is a visible pattern that results from other processes:
• Because embryonic development proceeds from simple to
complex, it is inevitable that young embryos of any vertebrate
species will resemble one another more than will the
older embryos. Zoologist Karl Ernst von Baer formulated
this hypothesis, still called von Baer’s law, in the 19th century.
• Because embryonic development occurs in a sheltered, wet
environment, such as inside of an egg or a uterus, embryos
of vertebrate species may resemble one another because of
their adaptation to a similar environment.
• The early development of the spinal cord means that it will
be larger than, and project from, the rest of the embryo,
creating the appearance of a tail.
• Some genes have in fact been retained from earlier stages of
evolution (see developmental evolution). The human
embryo does not literally have gill slits during early development
but expresses some of the same genes that begin
the production of gill slits in fishes.
Although recapitulation is not an evolutionary process,
it does reveal some interesting analogies between evolution
and embryology. Animal evolution began with single-celled
protists (see eukaryotes, evolution of). The earliest animal-like
multicellular protists may have been hollow balls of
relatively unspecialized cells, resembling the blastula state of
embryonic development. The earliest true animals may have
resembled cnidarians (see invertebrates, evolution of)
with an external tissue layer and a tissue-lined gut with a single
opening, which also closely resembles the blastula state of
embryonic development. Past these stages, however, the comparison
between evolution and development is unclear. The
best examples of recapitulation may therefore have been at
far earlier stages than those that Haeckel illustrated.
The emphasis that Haeckel placed upon recapitulation
in his popularization of evolution may have had a major
influence upon the theories of psychologist Sigmund Freud.
Freud claimed that cultural evolution paralleled the developmental
stages of children. Therefore, people in primitive
societies were more childlike than people in more advanced
societies. Freud considered modern human neuroses to
be holdovers of earlier evolutionary stages. Belief in the
inheritance of acquired characteristics (see Lamarckism)
provided Freud with a mechanism by which cultural experiences
could be imprinted on the genes that controlled development.
Freud’s disciple Sándor Ferenczi took the analogy
even further. He interpreted the penis as a fish that was
heading back into the primeval ocean of the womb, and the
postcoital relaxation as a return to the tranquillity of the
Precambrian ocean. Most scientists consider the analogies
used by Freud and Ferenczi to be even less realistic than that
used by Haeckel.
Further Reading
Gould, Stephen Jay. Ontogeny and Phylogeny. Cambridge, Mass.:
Harvard University Press, 1977.
———. “Freud’s evolutionary fantasy.” Chap. 8 in I Have Landed:
The End of a Beginning in Natural History. New York: Harmony
Books, 2002.
———. “Abscheulich! (Atrocious).” Chap. 22 in I Have Landed:
The End of a Beginning in Natural History. New York: Harmony
Books, 2002.
———. “The great physiologist of Heidelberg.” Chap. 27 in I Have
Landed: The End of a Beginning in Natural History. New York:
Harmony Books, 2002.
reciprocal altruism See altruism.
red queen hypothesis
red queen hypothesis The red queen hypothesis was proposed
by evolutionary biologist Leigh Van Valen in 1973 as
an important component of evolutionary theory. Van Valen
found, from a study of the fossil record, that extinction
within evolutionary lineages (not counting mass extinctions)
occurred at constant rates. From the viewpoint that
evolution produces a continuous improvement in the adaptation
of a lineage, this seemed puzzling: Modern lineages
were no better at surviving than ancient ones. Van Valen
interpreted this to mean that the environment was always
changing, with the result that evolutionary lineages could
never finish the process of adapting to them. This is mainly
true of the biological environment, the other species with
which each species interacts (see coevolution).
In writer Lewis Carroll’s Alice in Wonderland, Alice and
the Red Queen must continually run as fast as they can just
to stay where they are, because the world is moving past them
just as rapidly. This metaphor reflects what is happening in
the process of evolution. When species evolve in response to
one another, evolutionary changes in one species may select
for evolutionary changes in other species. This may result in
two types of evolutionary change:
• Directional change. If a species of herbivore evolves the
ability to eat certain plants despite their defenses, the plant
species benefits from evolving more effective defenses.
If predators evolve the ability to detect prey despite their
camouflage, the prey benefits from evolving more effective
camouflage. Over time, directional change occurs as the
defenses of plants and prey accumulate in response to herbivores
and predators. This explains why plants have such
an astounding array of defensive chemicals.
• Fluctuating change. Pathogens have very short generation
times and can evolve rapidly; when a pathogen evolves the
ability to infect a host that had previously resisted it, the
host species benefits from evolving new defenses against it.
Some members of a population of a vertebrate host may
evolve the ability, through their immune systems, to defend
themselves from a pathogen; in response, mutants arise in
the pathogen population that can elude the host defenses.
Defenses can be gained and lost, resulting in fluctuating
changes.
In the red queen evolutionary scenario, there is never
“enough” evolution; the plants and the prey never arrive at
the point when they have enough defense, and the host never
evolves a permanent defense. The herbivores, predators, and