Encyclopedia of Evolution.pdf - Online Reading Center

gene pool
Cocos Island is too small for isolating mechanisms to break
them into different populations, a necessary step if they are
to evolve into different species (see speciation). Instead of
different populations specializing on different foods, individual
finches specialize on different foods, whether seeds, bark,
fruit, or insects, and teach younger finches how to process
these foods. Behavioral specialization on foods creates a situation
that selects some genes more than others, which in turn
will influence behavior. In this case, gene-culture coevolution
will probably never go any further than it has, since the genes
of all the birds mix together in a single population.
An example of gene-culture coevolution that has influenced
human physiology is the evolution of lactose tolerance.
Lactose is milk sugar, and juvenile mammals have digestive
enzymes that allow them to metabolize lactose. In most adult
mammals, including many humans, the genes for these enzymes
are not used. Lactose intolerance is the norm for adult mammals.
Some human societies use domesticated livestock not just
for meat but for milk as an important source of nutrition for
adults. In these societies, the cultural choice of milk as a food
source created conditions that favored the evolution of lactose
tolerance, in which the adults continued to use the genes for
the enzymes that digest lactose. Evolutionary analyses have
shown that the prevalence of genetically based lactose tolerance
in human societies corresponds closely to the importance
of livestock herding as an economic activity. For example,
lactose tolerance is rare among Oriental peoples but common
in Mongolians, and it is rare among Africans but common in
herding tribes. The Masai, although they depend heavily upon
milk, are mostly lactose intolerant, but they curdle their milk
before consuming it, which reduces the lactose content.
The selection that favors the behavioral ability need not
always be natural selection. sexual selection, in which one
sex favors members of the other sex that have certain behavior
patterns, can favor the genetic establishment of these
behavioral abilities.
Gene-culture coevolution may have been crucial in
the evolution of some important human characteristics. To
many evolutionary biologists, language is the defining ability
of the human species. Many explanations have been proposed
for the origin of language (see language, evolution
of). One thing that most of them have in common is that
the advantages of language had to be social. Language does
not help an isolated human being survive or reproduce better.
If a rudimentary form of language began as a behavioral
innovation, and provided an advantage, natural selection
would then favor those individuals with a superior genetic
ability to communicate in this fashion. The new genes would
then allow yet more innovations in communication. Genes
and culture would influence one another, in a positive feedback
spiral, until language came into existence. Note again
that the languages themselves and the ways in which they
are used are not genetically determined; they are culturally
transmitted. The brain structures that allow a person to
understand and to form language (the Wernicke’s area and
the Broca’s area of the brain, respectively) are genetically
determined, and probably evolved because of the cultural
advantage they provided to the people who possessed them.
In fact, gene-culture coevolution was probably involved in
the evolution of all aspects of human intelligence (see intelligence,
evolution of).
Further Reading
Baldwin, James Mark. “A new factor in evolution.” American Naturalist
30 (1896): 441–451, 536–553.
Cavalli-Sforza, Luigi L., and Marc L. Feldman. Cultural Transmission
and Evolution. Princeton, N.J.: Princeton University Press,
1981.
Griffiths, Paul E. “Beyond the Baldwin Effect: James Mark Baldwin’s
‘social heredity,’ epigenetic inheritance and niche construction.”
Available online. URL: http://philsci-archive.pitt.
edu/archive/ 00000446/00/Beyond_the_Baldwin_Effect.pdf.
Lumsden, Charles J., and Edward O. Wilson. Promethean Fire:
Reflections on the Origin of Mind. Cambridge, Mass.: Harvard
University Press, 1983.
Simpson, George Gaylord. “The Baldwin Effect.” Evolution 7 (1953):
110–117.
gene pool See population genetics.
genetic code See DNA (raw material of evolution).
genetic drift See founder effect.
genetics See Mendelian genetics.
geological time scale The geological time scale is the
time scale of Earth history. It is based upon the deposits of
sedimentary, igneous, and metamorphic rocks throughout
Earth history (see fossils and fossilization). No place
on Earth contains a complete column of geological deposits.
By the process of stratigraphy, which was developed
in the early 19th century by geologist William Smith (see
Smith, William), geologists can compare the deposits in
one location with those in another and, by lining them up,
reconstruct a complete geological column. Dates can be
assigned to these rocks by radiometric dating of igneous
rocks that are found between many of the sedimentary
layers.
The major divisions of the geological time scale are eons.
Eons are divided into eras. The first three eons are often
informally lumped into the Precambrian time. The Precambrian
represents almost 90 percent of Earth history. Human
civilization represents an almost unmeasurably small portion
of Earth history (see age of Earth). Eons and eras represent
major events in the history of the Earth.
Precambrian time is divided into:
• Hadean time (“hellish”; sometimes called Hadean era).
The Earth formed about 4.5 billion years ago but was
so hot that oceans formed only toward the end of the
Hadean.
• Archaean Eon (“old”; sometimes called Archaean era).
Organisms similar to modern bacteria (see archaebacteria;
bacteria, evolution of) may have lived in the
oceans almost as soon as they formed (see origin of life).
There was little oxygen gas in the atmosphere.