Similarities and differences: understanding homology and analogy

Similarities and differences:
understanding homology and
Inherited Traits
People look like one
another for different
reasons. Two sisters,
for example, might
look alike because
they both inherited
dark eyes and brown
hair from their father.
Environmental Traits
On the other hand,
two people attending
an Elvis impersonators'
convention may look
alike because they are
both wearing
rhinestone studded
suits and long
Biological Similarity
• Biological similarity: It works the same way in
biology. Some traits shared by two living
things were inherited from their ancestor, and
some similarities evolved in other ways. These
are called homologies and analogies.
Homology: traits
inherited by two
different organisms
from a common
Analogy: similarity
due to convergent
evolution (often
occurs when both
lineages face similar
challenges and
selective pressures)
The Tale of the Limb
Look at the following diagrams of front limbs
presented. Each is from a different animal. In
each grouping you see, you should select the
limb that does not belong, and take note of the
similarities that the others share. Cross out the
limb that does not fit with the group. Explain
why you removed that limb in the box beneath
the group.
Set 1:
Set 2
Set 3
The Tetrapod Limb
These four limbs all
belong to tetrapods —
animals with four limbs.
Notice the similarities:
1) They are all built from
many individual bones.
2) They are all spin-offs of
the same basic bone
What animals possess
these homologous limbs?
Inheriting homologies
Whales, lizards,
humans, and birds all
have the same basic
limb layout. But how
did such different
animals wind up with
the same sort of limb?
Inheriting homologies
The answer is that they
inherited it from a
common ancestor, just
as cousins might inherit
the same trait from
their grandfather. This
evolutionary tree shows
the relationships
between different
tetrapod lineages, all of
which evolved from a
single common
This 350 million year old animal, the first tetrapod,
had limbs with one long bone (the humerus)
attached to two other long bones (the radius and
ulna). Its descendants, including whales, lizards,
humans, and birds, as well as many others, inherited
the tetrapod limb from this ancestor. Structures
inherited from a common ancestor are called
homologous structures, or homologies.
Other Homologies
The beaver uses its teeth for
chewing through tree trunks, and
the elephant uses its tusks for a
number of tasks including digging,
peeling bark from trees, and
fighting. But if you examine these
two structures closely, you will see
that each is a modification of the
basic incisor tooth structure. Over
time, evolution adapted each of
these animals' incisors to perform
different functions.
Not all similarity is homology
Not all similarity is homology
In the beginning of this module, you decided
that some legs were "not like the others." The
matching limbs had bones, and you eliminated
the limbs that did not have bones. Since the
octopus, sea star and grasshopper limbs do not
have bones, you concluded that they are
probably not homologous to tetrapod limbs.
Elvis impersonators have many similarities, such
as long black sideburns and rhinestone-studded
suits, that are not inherited. In the same way,
biological similarities are not always due to
inheritance. Sometimes the environment
around the organism plays a role.
Often, two species face a similar problem or
challenge. Evolution may then shape both of them in
similar ways — resulting in analogous structures. In
contrast, the wing of a bird and the wing of a bat are
classified as being analogous structures since their
structures are unrelated to any previous ancestor.
Why Flight?
The most amazing fact about the evolution of
flight is the extent of convergent
evolution between the three main groups that
evolved it (again, the pterosaurs, birds, and
bats). Evolution has no sense of future; the here
and now is the only place where evolution

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