Geology Reference
In-Depth Information
of northern populations are larger than those of southern
populations, probably a response to climate. Likewise, organ-
isms that develop resistance to insecticides and pesticides, as
well as plants that adapt to contaminated soils, are examples of
microevolution.
In contrast,
macroevolution
entails changes such as the
origin of a new species or changes at even higher levels in the
classifi cation of organisms, such as the origin of new genera,
families, orders, and classes. The fossil record provides many
good examples of macroevolution—the origin of birds from
reptiles, the evolution of whales from land-dwelling ances-
tors, and many others. Although macroevolution encom-
passes greater changes than microevolution, the cumulative
effects of microevolution are responsible for macroevolu-
tion. They differ only in the degree of change.
are rarely specifi ed. In contrast, a
cladogram
resulting from
cladistic analysis shows the relationships among members of
a
clade
, a group of organisms including its most recent com-
mon ancestor (Figure 18.10b). Thus, a cladogram includes
organisms with shared derived characteristics.
Any number of organisms can be depicted in a cladogram,
but the more shown, the more complex and diffi cult it is to con-
struct. Let's use an example of three animals—bats, dogs, and birds.
◗
Figure 18.11 shows three cladograms, each a different interpreta-
tion of their relationship. Bats and birds fl y, so we might conclude
they are more closely related to each other than to dogs (Figure
18.11a). On the other hand, perhaps birds and dogs are more
closely related to each other than either is to bats (Figure 18.11b).
But if we concentrate on evolutionary novelties, such as hair
and giving birth to live young, we conclude that bats and dogs are
more closely related than either is to birds (Figure 18.11c).
Cladistics and cladograms work well for living organ-
isms, but when applied to fossils, care must be taken to de-
termine what are primitive versus derived characteristics,
especially in groups with poor fossil records. Furthermore,
cladistic analysis depends solely on characteristics inherited
from a common ancestor, so paleontologists must be espe-
cially careful of characteristics that result from convergent
evolution. Nevertheless, cladistics is a powerful tool that has
more clearly elucidated the relationships among many fossil
lineages and is now used extensively by paleontologists.
Cladistics
is a type of biological analysis in which organisms
are grouped together based on derived, as opposed to primi-
tive, characteristics. For instance, all land-dwelling vertebrates
have bone and paired limbs, so these characteristics are primi-
tive and of no use in establishing relationships among them.
Hair and three middle ear bones, on the other hand, are derived
characteristics, sometimes called
evolutionary novelties
, because
they served to differentiate mammals from other vertebrates.
If we consider only mammals, hair and three middle ear bones
are of no further use because all mammals have them, so in
this context they are primitive. However, live birth, as opposed
to laying eggs, is a derived characteristic that serves to distin-
guish most mammals from the egg-laying mammals.
Traditionally, scientists have depicted evolutionary re-
lationships with
phylogenetic trees
, in which the horizontal
axis shows anatomic differences and the vertical axis denotes
time (
Evolutionary changes do not involve all aspects of an organism
simultaneously because selection pressure is greater on some
features than on others. As a result, a key feature we associate
with a descendant group may appear before other features typi-
cal of that group. For example, the oldest known bird had feath-
ers and the typical fused clavicles of birds, but most of its other
features were more like those of small carnivorous dinosaurs
Figure 18.10a). The patterns of ancestor-descendant
relationships are based on shared features, but the ones used
◗
Lungs
Gills
Feathers
Four toes
Placenta
Two openings
behind eye
Hair, mammary
glands
Amniotic egg
Four limbs with five
fingers and toes
Bony internal skeleton
Vertebral column
Change in form
A phylogenetic tree showing the relationships
among various vertebrate animals.
A cladogram showing inferred relationships. Some of the
characteristics used to construct this cladogram are indicated.
a
b
◗
Figure 18.10
Phylogenetic Tree and Cladogram
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