Name: _______________________________________________ Block _________ Date _____________
Similarities and differences: understanding homology and analogy
http://evolution.berkeley.edu/evolibrary/article/0_0_0/similarity_hs_01
In everyday life, people look like one another for different reasons.
Two sisters, for example, might look alike because they both
inherited brown eyes and black hair from their father. On the other
hand, two people attending an Elvis impersonators' convention may
look alike because they are both wearing rhinestone studded suits
and long sideburns. The similarity between the sisters is inherited, but
the similarity between the Elvis impersonators is not.
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 ancestor
Analogy: similarity due to convergent evolution (often occurs when both lineages face similar environmental
challenges and selective pressures)
The tale of the limb
Take a look at the diagrams of front limbs presented below. Each is from a different animal. In each grouping you see, you
should select the limb that doesn't 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.
Explain why you made this choice:
Explain why you made this choice:
Explain why you made this choice:
The Tetrapod limb
The four limbs you should have picked out all belong to tetrapods — animals with four legs. Notice how these tetrapod limbs
are similar to one another: 1) They are all built from many individual bones. 2) They are all spin-offs of the same basic bone
layout: one long bone attached to two other long bones. Here you can see the same bones labeled in these different limbs:
Even though these limbs are similar to one another, the animals they belong to are quite different from one another. What
animals possess these homologous limbs?
List the animals your
think these limbs belong
to:
1 __________________
2__________________
3__________________
4 _________________
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? The answer is that they inherited it from a common ancestor, just as cousins might inherit the same trait from
their grandfather.
This family tree shows several generations
starting with a pair of grandparents. In the
same way, an evolutionary "family" tree
shows relationships over much longer
periods of time.
This evolutionary tree shows the
relationships between different tetrapod
lineages, all of which evolved from a single
common ancestor. 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.
Homologies are everywhere
Once you know what a homology is, you can find them anywhere.
The leaves of an oak (left) and the leaves
of a gingko (right) are homologous —
they were both inherited from a
common ancestor with leaves.
The wing of a dragonfly (left) and the
wing of a butterfly (right) are
homologous — they were both inherited
from an ancient flying insect. Since all
life shares a common ancestor, we find
homologies all over the tree of life
Beyond the obvious
Not all homologies are obvious. If two homologous structures have been adapted for different roles, they may not look very
much alike. For example, the chomping front teeth of a beaver look quite different than the tusks of an elephant.
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. They are homologous
structures, inherited from a common
ancestor with incisor teeth.
Not all similarity is homology
In the game you played at the beginning of this module, you decided that some legs were "not like the others." The matching
limbs in the game have bones, and you eliminated the limbs that don't have bones. Since the octopus, sea star and grasshopper
limbs don't have bones, you concluded that they are probably not homologous to tetrapod limbs.
Homologies are inherited from
common ancestors. The octopus limb
could only be homologous to the
lizard limb if they both inherited the
limb from a common ancestor. This
tree shows how the octopus is related
to tetrapods, and the points in their
evolutionary histories when their
limbs evolved.
Tetrapod and octopus limbs evolved
independently after their point of
common ancestry, so they were not
inherited from a common ancestor.
Therefore, they are not homologous.
The same is true of the grasshopper
leg and the sea star arm. Simple
observation tells us that these limbs
are probably not homologous to the
tetrapod limb, because they have
such different structure. Similar
structures that evolved independently
are called analogous structures, or
analogies.
Analogies
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. For example, look at the skulls below. They belong to
extinct animals, and both of them have saber teeth — long, ferocious canines. Would you guess that these saber teeth are
homologous — inherited from a common ancestor with extra-long saber teeth?
These two skulls certainly look like the
animals could have inherited their
saber teeth from a common ancestor.
And the presence of canine teeth
themselves is definitely a homology,
in that the common ancestor of these
two had canine teeth. However,
despite their similarities, the unusual
length of these teeth is NOT
homologous. One skull belongs
to Thylacosmilus, a marsupial
mammal. The other belongs
to Smilodon, the saber-toothed cat,
which is a placental mammal.
Marsupial and placental mammals are
very different, and diverged from
each other a long time ago on the
evolutionary tree.
Thylacosmilus is more closely related
to other marsupials such as
kangaroos and koalas than it is
to Smilodon. Smilodon is more closely
related to other placentals such as
housecats and elephants than it is
to Thylacosmilus. Saberteeth is not a
common trait in the marsupials
closely related to Thylacosmilus, or
the placentals closely related
to Smilodon. Their common ancestor
certainly had canine teeth, but they
were probably not adapted into fierce
"sabers." As they weren't inherited
from a common ancestor, the saber
teeth
in Smilodon and Thylacosmilus evolved
independently from one another.
That means that one lineage on one
part of the tree of life evolved
saberteeth from normal length teeth,
and a different lineage somewhere
else on the tree also evolved saber
teeth from normal length teeth.
How do analogies evolve?
Often, two species face a similar problem or challenge. Evolution may then shape both of them in similar ways — resulting in
analogous structures. For example, imagine two flower species that are not closely related, but both happen to be pollinated by
the same species of bird. A flower that gets pollinated produces many seeds and has a big advantage over flowers that don't
get pollinated.
If the bird prefers long red flowers, the
flowers' shapes and colors may evolve in
ways that make them more attractive to
the bird. The two flowers might end up
looking the same, even though they
started out looking quite different from
one another.
Explain how flowers would change in
shape and color over time:
__________________________________
__________________________________
__________________________________
__________________________________
__________________________________
__________________________________
__________________________________
Homology or analogy?
You have probably noticed that dolphins and sharks both have a streamlined body shape with a triangular fin on the back and
two side fins. However, the two animals also have many differences.
Sharks
Dolphins
skeleton made of cartilage
skeleton made of bone
use gills to get oxygen from the
water in which they swim
go to the surface and breathe
atmospheric air in through their
blowholes
don't nurse their young
do nurse their young
don't have hair
do have hair — they are born
with hair around their "noses"
They may share the same basic shape, but underneath their skins, sharks and dolphins are very different!
Do you think that sharks' and dolphins' similarities (body shape, fin, and flippers) are homologies or analogies?_______________
Hint 1: Having a streamlined body shape,
fin, and flippers allows aquatic animals
to swim faster. Both sharks and dolphins
swim after prey.
Hint 2: This tree shows the where sharks
and dolphins are positioned on the tree
of life. They are not very closely related
to one another. Is it likely that sharks
and dolphins inherited their body
shapes, fins, and flippers from the same
common ancestor?