Encyclopedia of Evolution.pdf - Online Reading Center

quantitative trait loci Quantitative trait loci (QTLs) are
locations on chromosomes, identified by markers, that have a
statistical association with certain traits. In some cases, a recognizable
trait is caused by a single gene at a single locus on a
chromosome (see DNA [raw material of evolution]; Mendelian
genetics). The examples of such traits in humans are
few and trivial, for example blue vs. brown eyes, and whether
or not the thumb points backward (hitchhiker’s thumb). Most
traits are the product of two or more loci. It would require a
tremendous amount of work to identify these loci. Many evolutionary
biologists who might benefit from such knowledge
do not have the facilities and expertise in molecular genetics
to perform such studies. Instead, they identify markers that
have a statistical association with those loci.
Following the tradition of Gregor Mendel, researchers
cross two genetically different organisms within a species,
or individuals of two closely related species, producing an F 1
generation in which the traits mix together. Then they crossbreed
the members of the F 1 to produce an F 2 generation in
which the traits may separate back out. Using these F 2 individuals,
the researchers seek statistical correlations between
the traits and genetic markers. The markers that are most
strongly correlated with the traits—and there is usually more
than one such marker—are considered QTLs. QTLs indicate
how many loci influence a trait, which of them are most
important, and where they are found on the chromosomes.
In one experiment, evolutionary biologists H. D. Bradshaw
and Douglas Schemske crossed two closely related species
of Mimulus flowers (see hybridization). These two
species grow together in the Sierra Nevada of California but
do not hybridize because they have different pollinators (see
isolating mechanisms). Bees pollinate M. lewisii, while
hummingbirds pollinate M. cardinalis. M. cardinalis flowers
have more red and yellow pigment, produce more nectar, and
are a different shape from M. lewisii flowers. Which floral
characteristic—color, nectar, or shape—was initially responsible
for the evolutionary divergence of these two species? In
Q
the F 2 generation after Bradshaw and Schemske hybridized
the two species, there was a whole range of flower types that
differed in color, nectar production, and shape. Bradshaw and
Schemske also identified 66 QTL markers. They then planted
the F 2 generation of flowers in the field and observed the
responses of pollinators to them. They found that the flowers
visited most by hummingbirds were those that had QTL
markers associated with flower color and nectar production.
The QTL markers associated with flower shape did not have
a strong effect on hummingbird visitation. This demonstrates
that flower color and nectar production (perhaps controlled
by DNA associated with these QTL markers) were responsible
for the initial evolutionary divergence of these two species;
changes in flower shape later reinforced the divergence.
The use of QTLs allows researchers to identify the locations
of DNA sequences that may be important in evolutionary
changes such as speciation. Once they known where to
look, researchers can begin to identify the genes and control
sequences that are responsible for the observed evolutionary
events.
Further Reading
Bradshaw, H. D., Jr., et al. “Quantitative trait loci affecting differences
in floral morphology between two species of monkeyflower
(Mimulus).” Genetics 149 (1998): 367–382.
Schemske, Douglas, and H. D. Bradshaw, Jr. “Pollinator preference
and the evolution of floral traits in monkeyflowers (Mimulus).”
Proceedings of the National Academy of Sciences USA 96 (1999):
11,910–11,915.
Quaternary period The Quaternary period (beginning
about two million years ago) is the current period of Earth
history (in the Cenozoic era; see geological time scale).
It follows the Tertiary period. The Cenozoic era is also
known as the Age of Mammals, because large mammals
dominated many parts of the Earth after the extinction of the
dinosaurs following the Cretaceous extinction. The first