Encyclopedia of Evolution.pdf - Online Reading Center

isotopes
lar to them. Traits that encourage mating among individuals
that have very similar characteristics are called specific mate
recognition systems. Specific mate recognition systems may
allow individuals to avoid wasting their reproductive efforts on
crossbreeding with an incompatible population.
Natural selection causes evolutionary changes within
populations, as Charles Darwin (see Darwin, Charles) first
explained (see origin of species [book]). However, Darwin
did not adequately explain how one species could diversify into
more than one. In order for one species to evolve into more
than one, isolation is essential; and if isolation occurs, speciation
may be inevitable. Evolutionary scientists since the time of
Darwin have sought, and found, numerous examples of reproductive
isolation, often resulting from isolating mechanisms.
Further Reading
Eldredge, Niles. “Of genes and species: Modern evolutionary theory.”
Chap. 5 in The Pattern of Evolution. New York: Freeman, 1999.
Malausa, Thibaut, et al. “Assortative mating in sympatric host races
of the European corn borer.” Science 308 (2005): 258–260.
Schilthuzen, Menno. Frogs, Flies, and Dandelions: The Making of
Species. New York: Oxford University Press, 2001.
Stanton, Maureen L., and Candace Galen. “Life on the edge: Adaptation
vs. environmentally mediated gene flow in the snow buttercup,
Ranunculus adoneus.” American Naturalist 150 (1997): 143–178.
Weis, Arthur E., and Tanya M. Kossler. “Genetic variation in flowering
time induces phenological assortative mating: Quantitative
genetics methods applied to Brassica rapa.” American Journal of
Botany 91 (2004): 825–836.
isotopes Isotopes are atoms of the same element that have
different numbers of neutrons in the nucleus. The smallest
(lightest) isotope contains about the same number of neutrons
as protons. The number of protons defines the element.
For example, all carbon atoms have six protons. Carbon-12
( 12 C) has six protons and six neutrons, while 13 C has seven
neutrons, and 14 C has eight neutrons.
Isotopes (see table) are useful in evolutionary studies for
two reasons:
• Many isotopes are radioactive—that is, the extra neutrons
destabilize the nucleus, which ejects particles and changes
into another kind of atom at a constant rate. This makes
radioactive isotopes useful for determining the ages of some
rocks (see radiometric dating). 14 C is radioactive and is
the basis of radiocarbon dating.
• Nonradioactive isotopes can be useful as indicators of
environmental conditions or biological activity in ancient
deposits, fossils, or remnants of organisms. 13 C is an example
of a nonradioactive isotope.
Mass spectroscopy separates isotopes of one kind of
atom from one another and measures them separately, allowing
an isotope ratio to be calculated. The isotope ratio can be
compared to standards to allow interpretation.
Carbon isotopes. Carbon dioxide (CO 2) in the air contains
both 12 CO 2 and a very small amount of 13 CO 2. CO 2
reacts with calcium in seawater to produce calcium carbonate,
which is limestone. Limestone can therefore be inorganic
in origin, although massive deposits of calcium carbonate can
be produced by microorganisms. The ratio of 13 C to 12 C in
the air is defined as zero. When photosynthetic organisms
remove CO 2 from the air to make it into sugar, the enzyme
rubisco prefers 12 C. All of the organic compounds of the
photosynthetic organisms, and the entire food chain of animals
and decomposers, come from this sugar. Thus organic
material has more 12 C, and less 13 C, than the air. Inorganic
limestone, in contrast, tends to have a little more 13 C than
does the CO 2 in air, because the heavier isotope sinks. Zero
or positive numbers for the ratio (called δ 13 C, in parts per
thousand) are inorganic, while negative numbers are organic.
In the fossil record, inorganic limestone has a δ 13 C of about
zero, while for organic matter it is -25 parts per thousand.
The Isua formation in Greenland (sedimentary rocks that are
3.8 billion years old, almost as soon as the Earth was cool
enough for oceans to form) contains no fossil cells but does
contain graphite with a significantly negative δ 13 C ratio, suggesting
it was organic in origin. This is the earliest evidence
of life, and indicates that life originated very rapidly after the
oceans formed (see origin of life). Frequently, a decrease
in the δ 13 C ratio precedes a period of glaciation, perhaps
because photosynthesis removes a great deal of CO 2 from the
air and reduces the greenhouse effect. Abrupt decreases
in δ 13 C are not, however, always associated with glaciations.
A moderate δ 13 C ratio may result from the photosynthesis of
C 4 plants that live in warm regions (such as some grasses),
because the enzyme that removes CO 2 from the air in these
plants is not rubisco. However, C 4 plants have probably
existed only for part of the Cenozoic era (see photosynthesis,
evolution of).
The higher δ 13 C of C 4 plants can also serve as an indicator
of an organism’s diet. Laser studies of layers of enamel
in the teeth of fossils of robust australopithecines indicates
that they seasonally altered their diet between C 3 plants
such as fruits and berries and C 4 plants such as grains. The
most abundant agricultural C 4 plant in the world today is
maize. Many processed foods contain high-fructose corn
syrup, making maize (indirectly) the single greatest source of
Isotopes Used as Climate Indicators
and Bioindicators
Element, common Less common If enriched,
isotope isotope an indicator of:
Carbon, 12C 13C 12C: photosynthesis
13C: C4 photosynthesis
13C: inorganic source
Oxygen, 16O 18O 18O: ocean evaporation,
ice ages
Hydrogen, 1H 2H 2H: ocean evaporation,
ice ages
Nitrogen, 14N 15N 15N: carnivorous diet
Sulfur, 32S 34S 32S: bacterial metabolism
Strontium, 87Sr 86Sr 87Sr: continental erosion