Encyclopedia of Evolution.pdf - Online Reading Center

adaptive radiation
they can use them for eating leaves from treetops, but that is
not the original reason that long necks evolved in these animals.
The long neck of the giraffe, therefore, is an adaptation
to combat, and an exaptation for reaching leaves at the tops
of trees.
4. True adaptations. The only true adaptations, according
to many evolutionary scientists, are the genetic changes
that result from natural selection favoring those actual traits.
For each of the preceding eight definitions, the term adaptation
can refer either to the evolutionary process (natural
selection) or to the product. This leads to 16 meanings of adaptation.
As explained above, however, much clarity is achieved
by restricting the use of the term to just two: the process and
product of evolution acting directly on a characteristic.
In most cases, a characteristic is not an adaptation with
just a single function. Most characteristics are adaptations
with numerous evolutionary causes. For example, the glands
within an animal’s epidermis are an adaptation with several
functions, each of which may have provided a separate evolutionary
advantage. Some of the glands produce sweat, which
allows the animal to become cooler as the sweat evaporates.
Sweat also contains dissolved molecules. The body of the animal
uses sweat as one of its methods of disposing of excess or
toxic materials. Molecules in sweat can also serve as chemical
communication between animals. Mammary glands are modified
sweat glands. Sweat glands, therefore, are an adaptation
to at least four different functions.
Once a true adaptation has been identified, an evolutionary
scientist may attempt to test a hypothesis to explain how
and why it evolved. To test a hypothesis, scientists need to
obtain as large a sample of data as possible (see scientific
method). These data must be independent of one another,
rather than repeated measures on the same phenomenon.
Recently evolutionary scientists noticed that testing hypotheses
about adaptation requires observations in which the
adaptation has evolved independently. They refer to these
observations as phylogenetically independent. As an example,
consider small leaves as an adaptation for bushes that live in
dry conditions, and large leaves as an adaptation for bushes
that live in moist conditions. The scientist determines the
average leaf area for each of 12 species that live in different
moisture conditions and finds that there is a positive correlation
between leaf size and moisture. If these 12 species represent
six species from a small-leaved genus that lives in dry
climates, and six species from a large-leaved genus that lives
in moist climates, the number of independent observations is
two, not 12. These 12 species evolved from just two ancestral
species, one with small leaves, one with large leaves. If the
investigator claims that bushes that live in dry climates have
smaller leaves than those in moist climates, the investigator
has only two, not 12, data to test the claim. The 12 species
are not phylogenetically independent of one another. This
phenomenon is called the phylogenetic effect.
Evolutionary biologist Joseph Felsenstein determined a
way around the problem of the phylogenetic effect. The investigator
first identifies pairs of closely related species and then
compares the two members of each pair with one another. If,
within most of the pairs, the species from the drier climate
has smaller leaves, then the investigator has six observations
that test the claim.
An excellent example of testing a hypothesis about adaptation,
incorporating the phylogenetic effect, is the study
by ecologists Angela Moles, David Ackerly, and colleagues.
The hypothesis is that large seed size in plants is an adaptation
to enhance the survival of seedlings in plant species that
live a long time and grow to a large size (see life history,
evolution of). Rather than calculating a simple correlation
between seed size and body size in plants, the investigators
produced a phylogenetic tree (see cladistics) of 12,987
plant species and determined the evolutionary events in which
significant changes in seed size occurred. They found that
increases in seed size occurred along with evolutionary shifts
toward large plant size. These investigators could conclude,
with a great degree of confidence, that large seed size is an
adaptation to large body size in plants.
In order for adaptation to be a useful concept in evolutionary
science, investigators restrict the use of the word to
characteristics that result from the direct effects of natural
selection, and they investigate adaptation using phylogenetically
independent comparisons.
Further Reading
Dawkins, Richard. The Blind Watchmaker: Why the Evidence of
Evolution Reveals a Universe Without Design. New York: Norton,
1996.
Felsenstein, Joseph. “Phylogenies and the comparative method.” The
American Naturalist 125 (1985): 1–15.
Gould, Stephen Jay. The Panda’s Thumb: More Reflections in Natural
History. New York: Norton, 1980.
———, and Richard Lewontin. “The spandrels of San Marco and
the Panglossian paradigm: A critique of the adaptationist programme.”
Proceedings of the Royal Society of London B 205
(1979): 581–598.
———, and Elisabeth Vrba. “Exaptation—a missing term in the science
of form. Paleobiology 8 (1982): 4–15.
Moles, Angela T., et al. “A brief history of seed size.” Science 307
(2005): 576–580.
Sultan, Sonia E. “Phenotypic plasticity for plant development, function,
and life history.” Trends in Plant Science 5 (2000): 363–383.
Van Kleunen, M. and M. Fischer. “Constraints on the evolution of
adaptive phenotypic plasticity in plants.” New Phytologist 166
(2005): 49–60.
adaptive radiation Adaptive radiation is the evolution of
many species from a single, ancestral population. Throughout
the history of life, many species have become extinct, either
by natural selection or simply bad luck (see extinction; mass
extinctions), while others have produced many new species
by adaptive radiation. The net result has been a steady increase
in biodiversity through evolutionary time. Adaptive radiation
occurs because the single ancestral population separates into
distinct populations that do not interbreed, thus allowing separate
directions of evolution to occur in each (see speciation).
The world is full of numerous examples of adaptive radiation.
Nearly every genus that contains more than one species,
or any family that contains more than one genus, can be