Homology vs. Analogy

Divergent vs. Convergent Evolution

& how they relate to

HOMOLOGOUS Structures

• homologytraits inherited by two different organisms from a common ancestor

Example of Homology: the tetrapod limb

The wing of a dragonfly and the wing of a butterfly are homologous — they were both inherited from an

ancient flying insect

Not all homologies are obvious if they have been adapted for different roles. For example, the chomping

front teeth of a beaver look quite different than the tusks of an elephant. Each is a modification of the basic incisor

tooth structure

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....But they didn’t......so just because an octopus and a lizard have a limb that helps them move does not

mean they are homologous.

Homologies show that Divergent Evolution takes place

Divergent Evolution: A common ancestor evolves into new species, which continue to evolve and become less and less alike over time due to differences in the demands driven by the environment.

Brown Bears and Polar Bears illustrate divergent evolution. Scientists think that a long time ago a group of Brown Bears became geographically separated from the rest. This isolated group acquired new characteristics, such as the ability to eat meat, blubber to keep warm in snowy weather, and a white coat for camouflage. They slowly evolved into Polar Bears

Adaptive Radiation is an example of Divergent evolution on a shorter, smaller time scale

Consider Darwin’s finches...each evolved a new beak to fill a wide range of ecological niches...or ADAPT.

HINT......there must be a recent common ancestor involved if we are to claim that divergent evolution is taking

place, or that two features are homologous.

So what about ANALOGY??

Consider the following example....

Both are extinct animals and both of them have saberteeth...but are they homologous??

(Consider that Australia had separated from the Supercontinent a loooooong time ago)

Thylacosmilus, a marsupial mammal

Smilodon, the saber-toothed cat, which is a placental mammal

•analogysimilarity due to convergent evolution not common ancestry

CONVERGENT Evolution: When two separate groups of animals evolve to have similar structures

They are Analogous!

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.....like the saber teeth.

As they weren't inherited from a common ancestor, the saberteeth 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 saberteeth

from normal length teeth.

Another good example of Analogous structures......

While sugar gliders (marsupials) superficially resemble the placental

flying squirrels of North America, the ability to glide through the air evolved

independently in these unrelated mammals.

So there you have it.....

HOMOLOGY vs. ANALOGY

GAME TIME!Apply your knowledge.....

Homology or Analogy??

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"

Dolphins and Sharks...... both have a streamlined body shape with a triangular fin on the back

They are ANALAGOUS....

Sharks are closely related to rays, and dolphins are closely related to other mammals. Streamlined bodies and fins are traits that dolphins and sharks evolved separately, both as adaptations for swimming.

The leaves of an oak (left) and the leaves of a gingko (right)

they were both inherited from a common ancestor with leaves.

They are homologous

Now it's your turn to study and prepare for your test on TUESDAY!!